Production of membrane whorls in rat liver by some inhibitors of protein synthesis.

Inhibitors of protein synthesis capable of differential effects on nascent peptide synthesis on membrane-bound and free polyribosomes were employed to investigate the structure and function of cellular membranes of liver. The formation of membranous whorls in the cytoplasm and distension of nuclear membranes were induced by inhibitors of protein synthesis (i.e., cycloheximide and emetine) which predominantly interfere with nascent peptide synthesis on membrane-bound polyribosomes in situ. Other inhibitors of protein synthesis such as puromycin and fusidic acid, which inhibit nascent peptide synthesis on both free and membrane-bound polyribosomes, and chloramphenicol, which inhibits mitochondrial protein synthesis, did not induce these alterations. Cycloheximide, puromycin, and chloramphenicol produce some common cellular lesions as reflected by similar alterations in morphology, such as swelling of mitochondria, degranulation of rough endoplasmic reticulum, and aggregation of free ribosomes. The process of whorl formation in the cytoplasm, the incorporation of [(3)H]leucine and of [(3)H]choline into endoplasmic reticulum and the total NADPH-cytochrome c reductase activity of the endoplasmic reticulum were determined. During maximum formation of membranous whorls, [(3)H]leucine incorporation into cytoplasmic membranes was inhibited, while [(3)H]choline incorporation into these structures was increased; maximum inhibition of protein synthesis and stimulation of choline incorporation into endoplasmic reticulum, however, preceded whorl formation. Cycloheximide decreased the activity of NADPH-cytochrome c reductase of rough endoplasmic reticulum, but increased NADPH-cytochrome c reductase activity of smooth endoplasmic reticulum. In addition, cycloheximide decreased the content of hemoprotein in both the microsomal and mitochondrial fractions of rat liver, and the activities of mixed function oxidase and of oxidative phosphorylation were impaired to different degrees. Succinate-stimulated microsomal oxidation was also inhibited. The possible mechanisms involved in the formation of membranous whorls, as well as their functions, are discussed.

Hepatic organelle interaction. IV. Mechanism of succinate enhancement of formaldehyde accumulation from endoplasmic reticulum N-dealkylations.

Further evidence for organelle interaction during drug metabolism by the liver is presented. The apparent stimulation by succinate of formaldehyde accumulation in the medium, which was reported to occur with liver slices and homogenates as well as with mitochondria plus microsomes, has been shown to be the result of succinate inhibition of mitochondrial aldehyde dehydrogenase. The mechanism of succinate inhibition is shown to be by reverse electron transport, and an increase in the NADH to NAD+ ratio in the mitochondria; the aldehyde dehydrogenase requires the oxidized form of the pyridine nucleotide as its cofactor. Studies on in vitro N-demethylation by liver microsomes and endoplasmic reticulum segments which cosediment with the mitochondria indicate that formaldehyde produced by the mixed function oxidase is handled differently from formaldehyde added to the medium. The latter is mainly retained in the medium containing 5 mM semicarbazide, while the generated formaldehyde is more than 50% consumed by the mitochondria. Electron microscopy has indicated that the microsomes and the endoplasmic reticulum fragments have a tendency to align themselves close to the mitochondria when present in the same medium. Consequently, it is possible that formaldehyde released to the medium adjacent to the mitochondria, as by N-demethylation, would be exposed to semicarbazide for shorter periods than that added directly to the medium. In agreement with this suggestion, complexing of formaldehyde with semicarbazide was observed spectroscopically not to be an extremely rapid reaction even at 37 degrees C. This is believed to be the reason for the greater extent of consumption of formaldehyde generated by the endoplasmic reticulum.

CYP1B1, a developmental gene with a potential role in glaucoma therapy.

The association of CYP1B1 gene alterations in primary congenital glaucoma individuals has been known for about a decade. Recent evidence has shown the involvement of CYP1B1 mutations in a number of forms of glaucoma and anterior segment disorders. This suggests a wide role for CYP1B1 in ocular physiology. Histochemical studies of eyes from individuals with primary congenital glaucoma revealed abnormalities in the anterior chamber angle, the region between the cornea and the iris, containing the trabecular meshwork. The cells of the trabecular meshwork serve as a filter to allow drainage of the aqueous humour, the fluid formed by the ciliary body that fills the anterior chamber. Mutations in CYP1B1 that affect its activity have frequently been shown to influence development of the trabecular meshwork, and it is thought that CYP1B1, a monooxygenase, acts to form or degrade some endobiotic compound that is necessary for proper development of the filtering structures. The rapidly developing area of stem cell research suggests a potential therapeutic approach for glaucomas resulting from deleterious mutations in CYP1B1, that is, the transfer of stem cells, differentiated to a specific lineage, containing wild-type CYP1B1 to specific regions of the eye, where they will develop into normal cells of that region and rectify the defect.

4-methyl-5-amino-1-formylisoquinoline thiosemicarbazone, a second-generation antineoplastic agent of the alpha-(N)-heterocyclic carboxaldehyde thiosemicarbazone series.

4-Methyl-5-amino-1-formylisoquinoline thiosemicarbazone (MAIQ-1) was studied to determine its potential for clinical trail as a second-generation antineoplastic agent of the alpha-(N)-heterocyclic carboxaldehyde thiosemicarbazone class. MAIQ-1 was shown to be among the most potent known inhibitors of the major target for the expression of antineoplastic activity by this class of agents, the enzyme ribonucleoside diphosphate reductase, requiring only 0.06 micronM for 50% inhibition. This potency at the enzymatic level was consistent with its antineoplastic activity against the murine neoplasms Sarcoma 180, Leukemia L1210, Leukemia P388, and the B16 melanoma. The acetylation of the 5-amino group of the model substrate 5-amino-1,4-dimethylisoquinoline was lower than that of 5-amino-1-methylisoquinoline when incubated with acetyl-coenzyme A and rat liver homogenate. This finding suggests that the presence of the 4-methyl function offers steric hinderance to enzymatic substitution of the adjacent 5-amino group. In vivo metabolism of MAIQ-1 in mice, studied with [3'-14C]MAIQ-1 showed that relatively slow excretion of this agent occurred, since the cumulative urinary excretion of radioactivity was only 35% in 48 HR. About 51% of excreted urinary radioactivity was present in chromatograms in an area corresponding to the iron chelate of MAIQ-1, and only a minor quantity of material migrating like acetylated MAIQ-1 was present in urine, a finding consistent with enzymatic data with liver homogenates. The results indicate that MAIQ-1 has the antineoplastic activity, enzyme inhibitory potency, and relative resistance to metabolic inactivation required of an agent of this class for clinical trials.

Composition changes in hepatic microsomal cytochrome P-450 during onset of streptozocin-induced diabetes and during insulin treatment.

Monospecific polyclonal antibodies to five constitutive hepatic microsomal cytochromes P-450 were prepared. These antibodies were used to monitor alterations in the content of the enzymes in livers of diabetic male rats. Within 3 wk of onset of streptozocin-induced diabetes, immunodetectable levels of RLM3 and RLM5 were decreased by 85 and greater than 95%, respectively. Insulin treatment for 1 wk reversed the decline in these isozymes and restored RLM3 to 60% and RLM5 to 53% of levels found in the untreated rat. After a 2nd wk of therapy, these levels were returned to 86 and 92%, respectively. In contrast, the levels of RLM5b and RLM6 were elevated in diabetes 1.7- and 8-fold, respectively. Insulin treatment for 1 wk only slightly decreased the levels of RLM5b but completely reduced RLM6 levels to those seen in age-matched untreated rats. After the 2nd wk of insulin treatment, the level of RLM5b was almost completely restored to normal, with no additional change in the RLM6 level. The level of a fifth enzyme, RLM5a, was not markedly altered by diabetes or by insulin treatment. The results suggest there are at least three types of responses by constituents of the cytochrome P-450 population to diabetes: no change in the microsomal content, a rapid increase when insulin level declines and restoration when insulin is supplied, and a rapid decline when insulin level declines and a restoration by insulin treatment.

Effect of two mutations of human CYP1B1, G61E and R469W, on stability and endogenous steroid substrate metabolism.

CYP1B1 is linked to normal eye development by the disease phenotype, primary congenital glaucoma (PCG). CYP1B1 mRNA was expressed in a number of human fetal tissue cDNA libraries, supporting the suggestion of its involvement in tissue development. Highest expression levels were found in thymus and kidney, followed by spleen. A considerably lower level was observed in lung, cardiac and skeletal muscle. No expression was detected in liver or brain. CYP1B1 is able to metabolize steroid hormones. Testosterone was a poor substrate and activity with progesterone was 6-fold higher, but estradiol was the preferred substrate, exhibiting a greater metabolite profile with CYP1B1 than with CYP1A2. Major metabolites were A-ring hydroxylations (75-80%). Others were 15alpha-, 6alpha-, 16alpha- and 6beta-hydroxy metabolites. Two CYP1B1 mutations found in families with the PCG phenotype in which incomplete penetrance is seen were expressed in Escherichia coli. G61E, a hinge region mutation, and R469W, a heme region mutation, were shown to code for holoenzymes. G61E had greatly diminished stability, while the R469W holoenzyme, if anything, was stabilized. Both mutants showed compromised catalytic activity. The extents of isomeric site activity diminution were not proportional, resulting in alterations in the metabolite profiles. The results suggest that if a metabolite of CYP1B1 or elimination of a metabolite by CYP1B1 is necessary for normal embryonic or fetal tissue development, the appearance of these two mutations could result in developmental abnormalities. The altered activities of the mutants and ability of CYP1B1 to respond to external challenge may be the basis for the observed incomplete penetrance.

Immunocytochemical staining of the RLM6 form of cytochrome P-450 in oligodendrocytes and myelin of rat brain.

We used immunocytochemical staining to localize the RLM6 form of cytochrome P-450 in rat brain. Immunofluorescence staining in vibratome sections was positive in cells that resembled oligodendrocytes, which are the cells that synthesize and maintain myelin. Double immunofluorescence staining with anti-RLM6, plus mouse monoclonal antibodies (MAb) against 2',3'-cyclic nucleotide-3'-phosphohydrolase or galactocerebrosides, showed localization of each of these oligodendrocyte "markers" in the same cells as RLM6. In vibratome sections from brains of adult rats there was faint RLM6 immunostaining in some of the myelinated fibers as well as in oligodendrocytes. In paraffin sections from adult rat brains, myelinated tracts were RLM6 positive, as were oligodendrocytes and myelinated fibers in the gray matter. Oligodendrocytes were also shown to contain glucose-6-phosphate dehydrogenase. We suggest that RLM6, which is constitutive to liver, is also constitutive to brain and, via the acetone monooxygenase reaction, which also utilizes NADPH, may contribute to the conversion of ketone bodies to substrates that could provide energy for the synthesis and maintenance of myelin.

Cytochrome P-450 alterations in the BB/Wor spontaneously diabetic rat.

The hepatic content of two individual cytochrome P-450 enzymes was analyzed in spontaneously diabetic (BB/Wor) male rats. The major male-specific form, RLM5, was found to be slightly decreased in livers of male rats shortly after the onset of diabetes. In contrast, the level of RLM6 was elevated in livers of diabetic rats that had not received insulin and had become ketotic. These results confirm that the changes in some individual forms of cytochrome P-450 after chemical (streptozotocin) induction of diabetes are also seen in the spontaneously diabetic animal. The earlier observed alterations in cytochrome P-450 are therefore due to the state of diabetes and not to inductive or depressive effects of streptozotocin.

Relationship between malondialdehyde production and arachidonate consumption during NADPH-supported microsomal lipid peroxidation.

Fatty acid concentrations and malondialdehyde formation were determined before and during NADPH-supported lipid peroxidation in liver microsomes from rat, mouse, guinea pig and rabbit. In agreement with earlier reports, malondialdehyde production was greatest for rat, followed by mouse, and much less for guinea pig and rabbit. The microsomal content of total unsaturated fatty acids (18:1, 18:2, 18:3, 20:3, 20:4, 20:5, 22:5, 22:6) was approximately the same for rat and mouse and was lower in guinea pig and rabbit. Lipid peroxidation caused a time-dependent decrease in the polyunsaturated fatty acids, particularly 20:4 and 22:6, for all species. These decreases were most pronounced for rat and mouse. Alterations in the dietary regime for rat produced marked changes in microsomal fatty acid content as reported by others, and also caused changes in the rates of malondialdehyde production and polyunsaturated fatty acid consumption during lipid peroxidation. A comparison between the rates of malondialdehyde production and the rates of individual unsaturated fatty acid consumption was performed for each animal species and for rats fed different diets. A linear relationship was found between malondialdehyde production and 20:4 disappearance in individual microsomal preparations and in different species. A similar relationship was seen for the initial microsomal concentration of 20:4 and the initial rate of malondialdehyde formation. Other unsaturated fatty acids did not exhibit linear relationships. Various microsomal mixed-function oxidase variables were measured for the different species. No direct relationship between these values and malondialdehyde production was found.

Monoclonal antibodies to rat liver cytochrome P-450 2c/RLM5 that regiospecifically inhibit steroid metabolism.

Hybridomas were formed from myeloma cells and spleen cells derived from BALB/c female mice immunized with purified liver microsomal cytochrome P-450 2c/RLM5 (P-450 gene IIC11) isolated from untreated adult male rats. Six hybridoma clones produced monoclonal antibodies (MAbs) of the IgM(kappa) type. All the MAbs bound strongly to P-450 2c/RLM5 when measured by radioimmunoassay, and four of the six specifically immunoprecipitated P-450 2c/RLM5 in an Ouchterlony double-immunodiffusion test. These four MAbs also bound but did not immunoprecipitate P-450 RLM3. The MAbs that precipitated P-450 2c/RLM5 neither bound nor precipitated P-450 PB-B (gene IIB1) and P-450 BNF-B (gene IA1) of rats or P-450 LM2 and P-450 LM4 of rabbits. In contrast, mouse polyclonal anti-P-450 2c/RLM5 antibody strongly immunoprecipitated P-450 RLM3 as well as P-450 2c/RLM5 and to a lesser extent P-450 PB-B and P-450 LM2. The MAbs that precipitated P-450 2c/RLM5 also inhibited by more than 90% androstenedione 16 alpha-hydroxylase activity of untreated rat microsomes, but did not inhibit microsomal 6 beta- or 7 alpha-hydroxylation. In addition, complete inhibition of both androstenedione 16 alpha-hydroxylation and testosterone 16 alpha-hydroxylation was observed in a reconstituted system with P-450 2c/RLM5. Androstenedione 6 beta-hydroxylation catalyzed by P-450 2c/RLM5 was also inhibited, whereas P-450 3-catalyzed 7 alpha-hydroxylation was not inhibited by the MAbs. P-450 2c/RLM5 catalyzed 2 alpha-, 16 alpha- and 6 beta-hydroxylation of progesterone in a reconstituted system were also inhibited by the MAb by 60-80%. These MAbs should prove useful for "reaction phenotyping," i.e. for defining the contribution of microsomal P-450 2c/RLM5 to the oxidative metabolism of endogenous steroids and other P-450 substrates in animal and human tissues.

Enhanced expression of CYP1B1 in Escherichia coli.

Conditions for the optimal expression of the human CYP1B1 hemoprotein in Escherichia coli have been investigated. CYP1B1 cDNA was prepared from a retinal cDNA template and used to generate cDNA fragments with modified 5'-sequences reported to allow enhanced expression in E. coli DH5alpha. Plasmids were constructed, using the pCWori+ expression vector and were used to examine necessity for thiamine, delta-aminolevulinic acid (ALA), and IPTG. The optimal shaking speed in an orbital incubator was 150 rpm at 30 degrees C. Higher speeds resulted in increased cell death and lower speeds resulted in lower expression of cytochrome P450. IPTG was necessary for this expression system, which makes use of the lac repressor, but levels above 0.5 mM were without additional benefit. We were able to show thiamine to be unnecessary in this expression system, although included by others expressing CYP1B1. ALA has been reported to enhance expression of several different forms of cytochrome P450. We examined the dependence of CYP1B1 expression on ALA. The expression proved to be highly dependent upon this heme precursor, with levels of CYP1B1 increasing approximately 20-fold, to 920 nmol/l in the presence of up to 2.5 mM ALA. The question of whether heme synthesis and apoprotein synthesis were coupled was then investigated. It could be shown that although heme synthesis was not limiting (CYP101 holoenzyme expression in the absence of ALA was four times higher than the ALA-supported CYP1B1 holoenzyme expression), it was necessary for optimal expression of CYP1B1. CYP1B1 protein synthesis appears to be coupled to heme precursor availability, as seen by SDS-PAGE, because in the absence of heme precursor apocytochrome P450 1B1 does not accumulate.

Inhibition of nadph-driven microsomal lipid peroxidation by cytosol factor(s). Effect of a fat-free, high carbohydrate diet.

Male Swiss mice were given free access to a fat-free, high carbohydrate diet. The liver cytosol fraction from these mice contained a heat-sensitive factor that markedly inhibited microsomal, ferric pyrophosphate stimulated, NADPH-driven lipid peroxidation. The diet-induced factor was apparently incorporated into the microsomes after 12 days of continuous feeding, since lipid peroxidation by these microsomes was strongly diminished. The factor disappeared from the cytosol after 24 h of fasting and reappeared after refeeding the mice with the fat-free, high carbohydrate diet.

Kinetic studies of benzpyrene and hydroxybenzpyrene metabolism.

The kinetics of benzypyrene (BP) metabolism were examined in liver microsomes, and in accordance with the results of Hansen and Fouts [9] exhibited curcilinear Lineweaver-Burk plots. The problem was exacerbated in microsomes of 3-methylcholanthrene (MC-ms) treated rats. The Km for BP, measuring hydroxybenzypyrene (OHBP) appearance was about 0.3 muM in MC-treated adult rats and about 1.0 muM in untreated rats. These values were obtained using a substrate range of 0.2-2.0 muM benzpyrene, 20 mug of microsomal protein/ml and a 3 min assay time. With longer assay times and with higher microsomal protein concentrations curvilinear reciprocal plots were obtained. This was found to be due to a combination of three factors, namely non-specific binding of BP to the microsomes, rapid depletion of substrate, and further metabolism of hydroxy products. At 100 mug microsomal protein/ml about 50% of added BP was non-specifically bound to the microsomes in the range of 0.2-2.0 muM BP. Addition of albumin to the medium (1 mg/ml) greatly enhanced the BP hydroxylase activity but only slightly increased the amount of BP remaining in the medium after sedimentation of the microsomes by centrifugation. 3-OHBP, one of the phenolic products of BP metabolism was found to be metabolized to a non-fluorescent products(s); the Km for this compound was similar to that for BP. Differences were seen in the Vmax rates of BP disappearance and OHBP appearance. Disappearance of BP is several fold faster than OHBP appearance and has a larger Km. The latter may be due to the need to use higher amounts of protein and to allow depletion of enough substrate to make measurements significantly reproducible or the higher Km may reflect a composite value for different routes of BP metabolism.

Roles of cytochrome p450 in development.

Cytochrome P450 (CYP) forms are ubiquitous in nature, appearing in almost all phyla, with many forms appearing in any organism. About 50 different forms have been identified in man, and some of these are found in the embryo, some showing temporal dependence. Many of the forms of cytochrome P450 present in one species have homologues in other species. For example, CYP1A2 is present in many species, including man, rabbits, rodents, fish and fowl. The amino acid sequence identity of these homologues is often in excess of 70%. CYP26, too, has more than 61% identity in amino acid sequence between fish, fowl and mammals. In view of the high degree of conservation of sequence as well as of enzymatic activities, it is only reasonable to assume that such strong conservation of sequence also reflects a conservation of function. Since the 'xenobiotic metabolizing' enzymes predate the production of the many xenobiotics they are known to metabolize, perhaps it is reasonable to consider endobiotics as natural substrates for their metabolism. Of the identified forms of cytochrome P450 that are present in embryonic tissue, we consider the possibility that they serve the organism in support of morphogenesis of the embryonic tissue. These forms may either function to generate morphogenic molecules or to keep regions free of them, thereby creating temporal and spatial regions of morphogen action and supporting region-specific changes in cells. One known morphogen, retinoic acid, has the enzymes retinal dehydrogenase (RALDH) and CYP26 maintaining its actions, the former responsible for its generation and the latter for its elimination. Another form of cytochrome P450, CYP1B1 appears also to be involved in differentiation of tissue, with its absence resulting in primary congenital glaucoma. However, the nature of the morphogen it may maintain still remains to be elucidated.