Model building and intelligent acquisition with application to protein subcellular location classification.

MOTIVATION: We present a framework and algorithms to intelligently acquire movies of protein subcellular location patterns by learning their models as they are being acquired, and simultaneously determining how many cells to acquire as well as how many frames to acquire per cell. This is motivated by the desire to minimize acquisition time and photobleaching, given the need to build such models for all proteins, in all cell types, under all conditions. Our key innovation is to build models during acquisition rather than as a post-processing step, thus allowing us to intelligently and automatically adapt the acquisition process given the model acquired. RESULTS: We validate our framework on protein subcellular location classification, and show that the combination of model building and intelligent acquisition results in time and storage savings without loss of classification accuracy, or alternatively, higher classification accuracy for the same total acquisition time. AVAILABILITY AND IMPLEMENTATION: The data and software used for this study will be made available upon publication at http://murphylab.web.cmu.edu/software and http://www.andrew.cmu.edu/user/jelenak/Software. CONTACT: jelenak@cmu.edu.

Quantifying the roles of space and stochasticity in computer simulations for cell biology and cellular biochemistry.

Most of the fascinating phenomena studied in cell biology emerge from interactions among highly organized multimolecular structures embedded into complex and frequently dynamic cellular morphologies. For the exploration of such systems, computer simulation has proved to be an invaluable tool, and many researchers in this field have developed sophisticated computational models for application to specific cell biological questions. However, it is often difficult to reconcile conflicting computational results that use different approaches to describe the same phenomenon. To address this issue systematically, we have defined a series of computational test cases ranging from very simple to moderately complex, varying key features of dimensionality, reaction type, reaction speed, crowding, and cell size. We then quantified how explicit spatial and/or stochastic implementations alter outcomes, even when all methods use the same reaction network, rates, and concentrations. For simple cases, we generally find minor differences in solutions of the same problem. However, we observe increasing discordance as the effects of localization, dimensionality reduction, and irreversible enzymatic reactions are combined. We discuss the strengths and limitations of commonly used computational approaches for exploring cell biological questions and provide a framework for decision making by researchers developing new models. As computational power and speed continue to increase at a remarkable rate, the dream of a fully comprehensive computational model of a living cell may be drawing closer to reality, but our analysis demonstrates that it will be crucial to evaluate the accuracy of such models critically and systematically.

Maturation models for endosome and lysosome biogenesis.

The complexity and dynamic nature of the endocytic apparatus of mammalian cells have become increasingly clear over the past ten years. Structures collectively referred to as endosomes are at the crossroads of traffic with the plasma membrane and with the degradative pathway leading to lysosomes. They carry out the sorting and segregation of receptors and ligands, processes that are necessary for nutrient uptake and the maintenance of plasma membrane composition. This article addresses the question of whether endosomes are stable or transient compartments.

A chloroquine-resistant Swiss 3T3 cell line with a defect in late endocytic acidification.

To investigate the role of acidification in cell proliferation, several cell lines resistant to chloroquine were isolated with the expectation that some would express altered endocytic acidification. The preliminary characterization of one of these lines, CHL60-64, is described. In contrast to endocytic mutants described previously, the initial phase of endocytic acidification, as measured by transferrin acidification, is normal in this cell line. However, a difference in subsequent endocytic acidification was observed in CHL60-64. In the parental cells, internalized dextran was fully acidified to approximately pH 5.5 within 1 h. In CHL60-64, the pH in the endocytic compartment was only 6.1 after 1 h and remained as high as 5.8 for at least 4 h. After an 8-h incubation, the pH decreased to 5.5, indicating that the second phase of acidification is only slowed in CHL60-64, and not blocked. Consistent with this retarded acidification, ATP-dependent acidification in vitro (as measured by acridine orange accumulation) was reduced in both the lysosomal fraction and the endosomal fraction isolated from CHL60-64. A decrease in the in vivo rate of acridine orange accumulation after perturbation with amine was also observed. In addition to amine resistance and defective acidification, CHL60-64 was found to be resistant to vacuolation in the presence of chloroquine and ammonium chloride, and was resistant to ouabain. Further studies on this new class of endocytosis mutant, in combination with existing mutants, should help to clarify the mechanisms responsible for the regulation of endocytic acidification.

Endosome pH measured in single cells by dual fluorescence flow cytometry: rapid acidification of insulin to pH 6.

The acidification of various ligands was measured on a cell by cell basis for cell suspensions by correlated dual fluorescence flow cytometry. Mouse 3T3 cells were incubated with a mixture of fluorescein- and rhodamine-conjugated ligands, and the ratio of fluorescein and rhodamine fluorescence was used as a measure of endosome pH. The calibration of this ratio by both fluorometry and flow cytometry is described. Dual parameter histograms of average endosome pH per cell versus amount of internalization were calculated from this data, for samples in the absence and presence of chloroquine added to neutralize acidic cellular vesicles. The kinetics of acidification of insulin were measured and compared with previous results obtained with the chloroquine ratio technique. Rapid acidification of internalized ligand was observed both for insulin, which was mostly internalized via nonspecific pathways, and for alpha 2-macroglobulin, which was mainly internalized by specific receptor-mediated endocytosis. The average pH observed for internalized insulin was less than pH 6 within 10 min after addition of insulin. At 30 min, the average pH began to decrease to approximately pH 5, presumably because of fusion of endosomes with lysosomes.

Apparent endocytosis of fluorescein isothiocyanate-conjugated dextran by Saccharomyces cerevisiae reflects uptake of low molecular weight impurities, not dextran.

Concurrent with Riezman's report (Riezman, H. 1985, Cell. 40:1001-1009) that fluid-phase endocytosis of the small molecule Lucifer yellow occurs in the yeast Saccharomyces cerevisiae, Makarow (Makarow, M. 1985. EMBO [Eur. Mol. Biol. Organ.] J. 4:1861-1866) reported the endocytotic uptake of 70-kD FITC-dextran (FD) and its subsequent compartmentation into the yeast vacuole. Samples of FD synthesized and purified here failed to label yeast vacuoles under conditions that allowed labeling using commercial FD. Chromatography revealed that the commercial FD was heavily contaminated with at least three low molecular weight fluorescent compounds. Dialysis was ineffective for removing the contaminants. After purification (Sephadex G25, ethanol extraction), commercial FD was incapable of labeling vacuoles. Extracts of cells labeled with partially purified FD contained FITC, not FD, based on Sephadex and thin layer chromatography. In either the presence or absence of unlabeled 70-kD dextran, authentic FITC (10 micrograms/ml) was an effective labeling agent for vacuoles. The rapid kinetics (0.28 pmol/min per 10(6) cells at pH 5.5) and the pH dependence of FITC uptake suggest that the mechanism of FITC uptake involves diffusion rather than endocytosis. In view of these results, labeling experiments that use unpurified commercial FD should be interpreted with caution.

A method for incorporating macromolecules into adherent cells.

We describe a simple method for loading exogenous macromolecules into the cytoplasm of mammalian cells adherent to tissue culture dishes. Culture medium was replaced with a thin layer of fluorescently labeled macromolecules, the cells were harvested from the substrate by scraping with a rubber policeman, transferred immediately to ice cold media, washed, and then replated for culture. We refer to the method as "scrape-loading." Viability of cells was 50-60% immediately after scrape-loading and was 90% for those cells remaining after 24 h of culture. About 40% of adherent, well-spread fibroblasts contained fluorescent molecules 18 h after scrape-loading of labeled dextrans, ovalbumin, or immunoglobulin-G. On average, 10(7) dextran molecules (70,000-mol wt) were incorporated into each fibroblast by scrape-loading in 10 mg/ml dextran. The extent of loading depended on the concentration and molecular weight of the dextrans used. A fluorescent analog of actin could also be loaded into fibroblasts where it labeled stress fibers. HeLa cells, a macrophage-like cell line, 1774A.1, and human neutrophils were all successfully loaded with dextran by scraping. The method of scrape-loading should be applicable to a broad range of adherent cell types, and useful for loading of diverse kinds of macromolecules.

Reduction of the membrane fluidity of human breast cancer cells by tamoxifen and 17 beta-estradiol.

The intracellular steady-state levels of methotrexate were previously shown to be reduced in estrogen receptor (ER)-negative human breast cancer MDA-MB-436 cells and ER-positive human breast cancer MCF7 cells following treatment with pharmacologically relevant concentrations of 17 beta-estradiol (E2). We now report that both E2 and tamoxifen (TMX) significantly decreased the fluidity of MCF7 and MDA-MB-436 cellular membranes. With E2 or TMX at concentrations greater than 1 microM, perturbations in membrane fluidity were accompanied by apparently non-ER-mediated cytotoxicity. Alterations in membrane structure may have contributed to the cytotoxicity of high-dose endocrine therapy and to the ability of E2 to inhibit methotrexate transport and cytotoxicity in some human breast cancer cells.

Reversal of tumor-induced biochemical abnormalities by insulin treatment in rats.

In F344 rats bearing transplantable 3-methylcholanthrene (CAS: 56-49-5)-induced sarcomas, plasma concentrations of immunoreactive insulin were decreased following the development of mild or severe anorexia. Plasma levels of immunoreactive glucagon and lactate were elevated in severely anorectic tumor-bearing (TB) rats, while plasma glucose concentrations remained normal. Both groups of TB rats exhibited decreased plasma levels of serine, glutamine, citrulline, and tryptophan and increased concentrations of alanine. Plasma levels of proline and phenylalanine were also elevated in the severely anorectic TB rats. In a second experiment, 7 daily treatments with insulin corrected the anorexia for 6 days and increased body weights of TB rats. Plasma concentrations of lactate and immunoreactive glucagon were decreased, and the abnormal plasma concentrations of glutamine, proline, analine, and phenylalanine were altered toward normal following the insulin treatments. Therefore, these data are consistent with insulin treatments benefiting the TB host by increasing feeding, increasing body weight, reducing tumor glycolysis and metabolism, reducing gluconeogenesis, and reducing host catabolism, while not stimulating tumor growth. Thus insulin therapy may have potential benefits in cancer treatment by shifting glucose metabolism toward the host and away from the tumor.

Variability in the oral bioavailability of all-trans-retinoic acid.

BACKGROUND: Orally administered all-trans-retinoic acid (all-trans-RA) can induce complete remission in a high proportion of patients with acute promyelocytic leukemia. A previous pharmacokinetic study in patients with acute promyelocytic leukemia raised the possibility that the absorption of orally administered all-trans-RA is a saturable process that would have significant clinical impact on dosing strategies. PURPOSE: This study was specifically designed to examine the saturability of all-trans-RA absorption by measuring the effect of doubling the oral dose of all-trans-RA on plasma drug concentration in patients receiving long-term oral therapy. METHODS: Six patients with solid tumors received oral doses of 10-mg gelatin capsules of all-trans-RA. Patients were studied on 2 consecutive days after they received 28 days of all-trans-RA administered as two daily 78-mg/m2 doses. The study assigned the patients to two groups. Three patients took a 156-mg/m2 dose on day 28 and a 78-mg/m2 dose on day 29; the other three patients took the lower dose on day 28 and the double dose on day 29. Blood samples for the determination of all-trans-RA plasma concentration were obtained at 30-minute intervals starting just prior to drug administration and continuing for a total of 7 hours. The plasma concentration of all-trans-RA was measured by high-performance liquid chromatography. RESULTS: Plasma concentrations following an oral dose of all-trans-RA were highly variable, with peak concentrations ranging from 0.07 to 1.2 microM for the 78-mg/m2 dose level. Doubling the dose from 78 to 156 mg/m2 increased plasma concentration in all six patients, but the increase was unpredictable and not related to dose, ranging from less than a 1.2-fold to more than a 10-fold increase. CONCLUSION: The current study does not support the hypothesis that the gastrointestinal absorption of all-trans-RA involves a saturable process but instead suggests that absorption is highly variable among patients. This wide interpatient variability suggests that pharmacokinetic drug monitoring may have an important role in the management of patients receiving all-trans-RA.

Dose-dependent pharmacokinetics of all-trans-retinoic acid.

BACKGROUND: Orally administered all-trans-retinoic acid (all-trans-RA) can induce remission in a high proportion of patients with acute promyelocytic leukemia. PURPOSE: To further define the drug's pharmacokinetics, a study of intravenous all-trans-RA was performed in rhesus monkeys. METHODS: A total of nine monkeys received intravenous bolus injections of all-trans-RA. Three different doses (20, 50, and 100 mg/m2) were each tested in three monkeys. Blood samples for determination of all-trans-RA concentration were obtained prior to drug administration and at 5, 10, 15, 30, 45, 60, 75, 90, 120, 150, 180, 240, 360, and 480 minutes after drug administration. RESULTS: Plasma disappearance of all-trans-RA was characterized by three distinct phases: a brief, initial exponential decline, followed by a relative plateau in the disappearance curve (the duration of which was dose dependent), and finally a terminal exponential decay. This profile is consistent with a capacity-limited (saturable) elimination process. The first-order (terminal) half-life for all-trans-RA averaged 19 minutes, and the mean clearances were 77, 52, and 59 mL/min for the 20-, 50-, and 100-mg/m2 dose groups, respectively. The mean +/- SD Michaelis constant (Km) for the capacity-limited process was 3.2 +/- 1.9 microM. CONCLUSIONS: Peak plasma concentrations following oral administration of 45 mg/m2 all-trans-RA in humans approach the Km for the capacity-limited process; thus, the dose-dependent pharmacokinetics of all-trans-RA described here may occur within the clinically used dosage range.

Biochemical parameters of mercaptopurine activity in patients with acute lymphoblastic leukemia.

Mercaptopurine (MP) is a purine antimetabolite widely used for remission maintenance in the therapy of acute lymphoblastic leukemia. In order to study the biochemical parameters affecting MP activity, leukemic cells were obtained from ten patients with acute lymphoblastic leukemia at the time of diagnosis and from the same patients at the time of their initial marrow relapse. Hypoxanthine phosphoribosyltransferase (HPRT), the enzyme that converts MP to its active, nucleotide metabolite, thioinosine monophosphate; alkaline phosphatase, the primary catabolic enzyme of thioinosine monophosphate; and 5-phosphoribosyl-1-pyrophosphate (PRPP), the cellular ribose-phosphate donor essential for MP activation, were all measured within the patients' leukemic cells. There was marked interpatient variability in the three biochemical parameters studied with a greater than 10-fold range in alkaline phosphatase activity and an approximately 100-fold range in HPRT activity and PRPP levels. Four patients developed changes in biochemical parameters that influence MP activity at the time of relapse. In three of the four patients, alterations in more than one of these three biochemical parameters were noted. Three of four patients had a greater than 50% decrease in intracellular HPRT activity, four of four had a greater than 50% decrease in intracellular PRPP, and two of four had a greater than 9-fold increase in intracellular alkaline phosphatase activity at relapse. Two of four patients demonstrated changes in all three parameters at relapse in the directions that could have resulted in decreased MP sensitivity (i.e., decreased HPRT, decreased PRPP, and increased alkaline phosphatase). There was no correlation between pretreatment values of HPRT, PRPP, and alkaline phosphatase and remission duration. These results indicate that: (a) there is marked variation in HPRT, PRPP, and alkaline phosphatase in patients with acute lymphoblastic leukemia and b) following MP-containing maintenance chemotherapy, some patients develop biochemical changes that may result in decreased sensitivity to MP.

Pharmacokinetics of 9-cis-retinoic acid in the rhesus monkey.

9-cis-Retinoic acid is a naturally occurring biologically active retinoid capable of binding and transactivating both the retinoic acid receptors and the retinoid X receptors. A study was performed to characterize the pharmacokinetics 9-cis-retinoic acid following i.v. bolus administration in the nonhuman primate. Groups of three animals received i.v. bolus doses of 9-cis-retinoic acid of either 50 or 100 mg/m2. Blood and cerebrospinal fluid samples for determination of 9-cis-retinoic acid concentration were obtained prior to and 5, 10, 15, 30, 45, 60, 75, 90, 120, 150, 180, 240, 360, and 480 min following drug administration. The plasma drug concentration profile of 9-cis-retinoic acid was consistent with a first-order elimination process, with a harmonic mean half-life of 31 min, and a mean clearance of 97 ml/min/m2. The pharmacokinetics of 9-cis-retinoic acid were linear over the dose range studied. Plasma concentrations of all-trans-retinoic acid following 9-cis-retinoic acid administration were less than the limit of quantitation (0.1 microM), suggesting that isomerization to all-trans-retinoic acid is not a major metabolic pathway. In contrast to all-trans-retinoic acid, the elimination of 9-cis-retinoic acid did not appear to be capacity limited (saturable). Previous studies in the Rhesus monkey have shown that repeated dosing with all-trans-retinoic acid leads to a reduction of this saturable component of elimination and results in reduced exposure to drug. These studies, in an animal model highly predictive of humans, suggest that declines in plasma concentrations of 9-cis-retinoic acid as a result of its repeat administration at doses up to 100 mg/m2 will not occur.

Effect of cytotoxic drugs on estrogen receptor expression and response to tamoxifen in MCF-7 cells.

We have examined the effects of a 24-h exposure to clinically achievable concentrations of adriamycin, melphalan, 5-fluorouracil, and vincristine on the estrogen binding capacity of MCF-7 human breast cancer cells using a whole cell binding assay. Adriamycin (0.018 to 1.8 microM), melphalan (0.1 to 5 microM), 5-fluorouracil (0.077 to 15.4 microM), and vincristine (0.01 to 1 nM) reduce the estrogen binding capacity in a dose dependent manner. The rate of protein synthesis is reduced following exposure to 5-fluorouracil but not following exposure to adriamycin, melphalan, or vincristine. The rate of cell proliferation, influx of the ligand, and the Kd of remaining estrogen receptor are unaltered following drug exposure. These drugs may, therefore, be inducing a nonspecific reduction in the rate of receptor recycling and/or synthesis. Vincristine (1 nM) abolished estrogen receptor expression but following removal of the drug receptor levels did not reach that expressed in untreated cells for at least 48 h. Prior exposure to vincristine (1 nM) reduced the antiproliferative effects of tamoxifen (2 microM) toward MCF-7 cells.

Intrathecal 6-mercaptopurine: preclinical pharmacology, phase I/II trial, and pharmacokinetic study.

For over 30 years, oral 6-mercaptopurine (6-MP) has been a mainstay of systemic maintenance therapy for acute lymphoblastic leukemia. Despite its efficacy as an antileukemic agent, 6-MP has not been previously administered by the intrathecal (IT) route. In anticipation of a clinical trial of IT 6-MP, preclinical cytotoxicity and pharmacology studies were performed to define a safe, effective dose. The optimal concentration (greater than 1 microM) and duration of exposure (greater than 12 h) to 6-MP required for cytotoxicity were determined in vitro using human leukemia cell lines. The dose required to achieve the desired cerebrospinal fluid concentrations in humans was derived from pharmacokinetic parameters determined in rhesus monkeys. A phase I/II study was then performed in pediatric patients with refractory meningeal leukemia. Nine patients (aged 3.5 to 16 years) with chronic meningeal leukemia (2 to 6 central nervous system relapses) were entered onto the study. All had previously failed, at a minimum, IT methotrexate, IT cytarabine, and cranial (+/- spinal) radiation. A 10-mg IT dose of 6-MP (calculated to produce cytotoxic cerebrospinal fluid levels for 12 h) was administered twice weekly for 4 weeks. There were four complete responses and three partial responses. The duration of complete responses ranged from 7 to 22 weeks. Observed toxicities were not dose limiting and included mild headache (three patients) and minimal nausea (two patients). Pharmacokinetic studies performed in patients confirmed that cerebrospinal fluid concentrations of 6-MP were greater than 1 microM for 12 h. These results indicate that the IT administration of 6-MP is feasible, is not associated with significant toxicity, and has definite activity in patients with refractory meningeal leukemia.

A phase I study of continuous infusion 5-fluorouracil plus calcium leucovorin in combination with N-(phosphonacetyl)-L-aspartate in metastatic gastrointestinal adenocarcinoma.

Preclinical studies suggest that the biochemical effects of N-(phosphonacetyl)-L-aspartate (PALA), an inhibitor of aspartate carbamoyltransferase (ACTase), may increase the metabolic activation of 5-fluorouracil (5-FU) and enhance its cytotoxicity through both RNA- and DNA-directed mechanisms. In this Phase I trial, 22 evaluable patients with adenocarcinoma of the gastrointestinal tract were entered at escalating doses of 5-FU starting at 1150 mg/m2/day given as a concurrent 72-h i.v. infusion with a fixed dose of leucovorin (LCV), 500 mg/m2/day. The dose of 5-FU was escalated within patients according to individual tolerance, and then PALA at 250 mg/m2 was added 24 h prior to the initiation of the 5-FU/LCV infusion of the subsequent cycle. Dose-limiting mucositis and myelosuppression occurred during the initial cycle in 3 of 5 patients treated with 2300 mg/m2/day 5-FU; therefore, the recommended dose of 5-FU with concurrent LCV is 2000 mg/m2/day. Twenty-seven additional patients were then treated with escalating doses of PALA ranging from 375 to 2848 mg/m2, i.v., followed 24 h later by 2000 mg/m2/day 5-FU with high-dose LCV. Dose-limiting mucositis and myelosuppression occurred during the initial cycle in 2 of 3 patients entered at 2848 mg/m2 PALA. Dose-limiting mucositis and skin rash ultimately required both PALA and 5-FU dose reductions in 4 of 6 patients treated with 1899 mg/m2 PALA. Toxicity was similar, however, in patients receiving PALA at doses ranging from 375 to 1266 mg/m2. The mean steady-state plasma concentration of 5-FU at 2000 mg/m2/day was 6.5 +/- 0.9 microM; patients with 5-FU levels > 9 microM had a significantly higher incidence of serious gastrointestinal and hematological toxicity. Compared to each patient's own baseline, a significant trend for decreasing ACTase activity with increasing PALA dose was evident using cytosol isolated from peripheral blood mononuclear cells 24 h after PALA treatment (P2 = 0.01). PALA < or = 844 mg/m2 failed to appreciably inhibit ACTase activity at 24 h in most patients; furthermore, a decrease in ACTase activity by > 50% from baseline was seen in only 29% of cycles. More consistent inhibition of ACTase activity was seen with PALA > or = 1266 mg/m2. Even with the highest PALA doses, however, ACTase activity returned to baseline by 96 h in most patients.(ABSTRACT TRUNCATED AT 400 WORDS)

Time course of induction of metabolism of all-trans-retinoic acid and the up-regulation of cellular retinoic acid-binding protein.

A study of chronic i.v. dosing of all-trans-retinoic acid (all-trans-RA) was performed to determine whether induction of the capacity-limited elimination process for all-trans-RA occurred with long-term drug administration. Because up-regulation of the cellular retinoic acid-binding proteins (CRABP) may act to bind all-trans-RA intracellularly, the amount of CRABP in skin biopsy samples obtained during and following the course of all-trans-RA administration was also determined. Four adult rhesus monkeys received 50 mg/m2 of all-trans-RA by bolus i.v. injection daily for 8 consecutive days and again for one additional dose following a 7-day period without drug. The plasma disappearance curve of all-trans-RA was characterized by a plateau phase, the duration of which decreased during the period of chronic drug administration, followed by a terminal exponential decay phase, which is consistent with a capacity-limited (saturable) elimination process. The Vmax of this process increased from 0.06 mumol/min on the first day to 0.17 mumol/min by the eighth day of all-trans-RA administration, consistent with induction of an enzymatic process. The amount of CRABP measured in skin biopsy specimens was rapidly induced, increasing to approximately 3-fold baseline levels by day 3 of all-trans-RA administration. It remained at this level throughout the period of chronic drug administration but diminished following the 7-day period without drug. These findings suggest that an intermittent schedule of administration for all-trans-RA has potential advantages over a continuous administration schedule. A period of time without drug administration would allow for return of plasma drug clearance toward baseline levels and down-regulation of CRABP, which could result in higher plasma drug concentrations and possibly less cytoplasmic binding of drug.

Pharmacokinetics and metabolism of cyclopentenyl cytosine in nonhuman primates.

The plasma and cerebrospinal fluid pharmacokinetics of cyclopentenyl cytosine (CPE-C) were studied following i.v. bolus and continuous i.v. infusion in male rhesus monkeys. Following an i.v. bolus dose of 100 mg/m2 plasma elimination of CPE-C was biexponential with a mean t1/2 alpha of 8.4 min, a mean t1/2 beta of 36 min, and a total clearance (CLTB) of 662 ml/min/m2, which is 5- to 10-fold higher than clearance rates in rodents and dogs. Less than 20% of the total dose of CPE-C was excreted unchanged in the urine. The remainder was excreted as the inactive deamination product cyclopentenyl uridine (CPE-U). The ratio of the areas under the plasma concentration versus time curves of CPE-U to CPE-C was 7.0 +/- 2.4 following i.v. bolus CPE-C. The cerebrospinal fluid:plasma ratios of CPE-C and CPE-U were 0.08 and 0.30, respectively. Continuous i.v. infusion of CPE-C was compared to continuous infusion of 1-beta-D-arabinofuranosylcytosine in two monkeys. Steady state plasma concentrations, normalized to a dose of 12.5 mg/m2/h of CPE-C and an equimolar dose of 1-beta-D-arabinofuranosylcytosine, were 2.1 and 0.53 microM, respectively. The steady state concentrations of their corresponding uridine metabolites (CPE-U and 1-beta-D-arabinofuranosyluridine) were 8.2 and 15.5 microM. The rapid elimination of CPE-C by deamination in the primate resulted in a much higher CLTB and considerably lower total drug exposure than in rodents and dogs that clear CPE-C at a much lower rate by renal excretion. These significant interspecies differences in the disposition of CPE-C should be considered in the selection of a starting dose and schedule for human trials and suggest that a pharmacologically directed dose escalation scheme should be used in the planned phase I studies.

Interaction of synthetic peptides of apolipoprotein C-II and lipoprotein lipase at monomolecular lipid films.

The triacylglycerol hydrolyase and phospholipase A1 activities of bovine milk lipoprotein lipase toward long-chain fatty acyl ester substrates were investigated with monomolecular lipid films containing trioleoylglycerol and phosphatidylcholine. In a monolayer of egg phosphatidylcholine containing 3 mol% [14C]trioleoylglycerol, and in the presence of apolipoprotein C-II, a 79 amino acid activator protein for lipoprotein lipase, enzyme activity was maximal at a surface pressure of 21-22 mN X m-1 (37 mumol oleic acid released/h per mg enzyme); enzyme activity was enhanced 9-fold by apolipoprotein C-II. At surface pressures between 22 and 30 mN X m-1, lipoprotein lipase activity decreased over a broad range and was nearly zero at 30 mN X m-1. Apolipoprotein C-II and the synthetic fragments of the activator protein containing residues 56-79, 51-79 and 44-79 were equally effective at 20 mN X m-1 in enhancing lipoprotein lipase catalysis. However, at surface pressures between 25 and 29 mN X m-1, only apolipoprotein C-II and the phospholipid-associating fragment containing residues 44-79 enhanced enzyme catalysis. The effect of apolipoprotein C-II and synthetic peptides on the phospholipase A1 activity of lipoprotein lipase was examined in sphingomyelin:cholesterol (2:1) monolayers containing 5 mol% di[14C]myristoylphosphatidylcholine. At 22 mN X m-1, apolipoprotein C-II and the synthetic fragments containing residues 44-79 or 56-79 enhanced lipoprotein lipase activity (70-80 nmol/h per mg enzyme). In contrast to trioleoylglycerol hydrolysis, the synthetic fragments were not as effective as apolipoprotein C-II enhancing enzyme activity towards di[14C]myristoylphosphatidylcholine at higher surface pressures. We conclude that the minimal amino acid sequence of apolipoprotein C-II required for activation of lipoprotein lipase is dependent both on the lipid substrate and the packing density of the monolayer.

Physicochemical and biological properties of glucagon-like polypeptides from porcine colon.

Polypeptide material displaying glucagon-like immunoreactivity was isolated from porcine colon using immunoaffinity chromatography. The immunoreactive material was tightly bound to high molecular weight proteins but was dissociated by 0.1% w/v sodium dodecyl sulphate solution into immunoreactive components of approximate molecular weights 12,000,8000,5000 and 3000. These components reacted at least 50 times more strongly with antibodies specific for the N-terminal region of glucagon than with antibodies specific for the C-terminal region of glucagon. While the 8000 and 3000 dalton fractions were homogeneous, the 12,000 and 5000 dalton fractions were resolved into multiple bands by isoelectric focusing. The 12,000 dalton fraction was devoid of glycogenolytic and lipolytic activity, was not insulin releasing and showed no ability to bind to receptor sites specific for glucagon on hepatic plasma membranes and to active hepatic adenylate cyclase. The 8000 and 5000 dalton components showed weak lipolytic activity. The possible significance of colonic glucagon-like immunoreactivity relative to pancreatic glucagon and immunoreactivity from other tissues is discussed.