Surface membrane O-alkyl lipid concentration and metastasizing behavior in transplantable rat mammary carcinomas.

Whole cell and surface membrane O-alkyl and alk-1-enyl lipid concentrations of 11 transplantable rat mammary carcinomas were measured. A correlation was found between surface membrane O-alkyl lipid levels and metastasizing behavior. No relationship was found between alk-1-enyl lipid concentrations and metastasizing behavior. A possible correlation was found between the product of growth rate and rank order of metastasizing behavior versus surface membrane O-alkyl lipid concentration. Growth rate alone did not correlate with surface membrane O-alkyl lipid concentration.

Potentiation of hypoglycemic effect of chlorpropamide and phenfromin by halofenate.

The potentiation of oral hypoglycemic drugs by the antilipemic agent halofenate is reported. Forty-seven diabetic patients were treated for 48 weeks with halofenate, clofibrate, or placebo. Five patients in the halofenate group were taking phenformin plus either chlorpropamide or tolbutamide. Their average initial fasting plasma glucose was 160 mg./dl. All five patients experienced a slow but but substantial fall in fasting plasma glucose. The mean fasting plasma glucose for the five patients after 80 days of halofenate treatment was 63 mg./dl. As oral treatment for diabetes was reduced, the fasting plasma glucose returned to prehalofenate levels. In this study, we did ont detect an effect of halofenate on the fasting plasma glucose of diabetic patients treated with insulin or on the fasting plasma glucose levels of patients treated with diet alone.

Is combination sulfonylurea and insulin therapy useful in NIDDM patients? A metaanalysis.

OBJECTIVE: To assess the efficacy of combination therapy with insulin and sulfonylurea in the treatment of NIDDM. RESEARCH DESIGN AND METHODS: Studies published between January 1966 and January 1991 were identified through a computerized Medline search and by hand searching the bibliographies of identified articles. We identified 17 eligible randomized, controlled trials of combination therapy in NIDDM. These trials had a minimum duration of 8 wk and at least one of three outcome measures (fasting glucose, HbA1, or C-peptide) with SD or SE of the mean reported to do metaanalysis. With standardized forms, three independent reviews abstracted measures of study quality and specific descriptive information about population, intervention, and outcome measurements. RESULTS: We calculated effect size and weighted mean changes of the three outcome measures for control and treatment groups. In the treatment group, the fasting plasma glucose decreased from a mean of 11.4 mM (206 mg/dl) at baseline to a mean of 9.16 mM (165 mg/dl) posttreatment, whereas the control group decreased from (11.3 to 10.8 mM) (204 to 194 mg/dl) (effect size 0.39, P less than 0.0001). For HbA1, the treatment group decreased from a baseline of 11.0 to 10.2% compared to 11.0 and 11.2% in the control group (effect size 0.43, P less than 0.0001). For fasting C-peptide, the treatment group increased from 0.49 to 0.58 nM (1.45 to 1.75 ng/ml) compared with 0.47 and 0.43 (1.42 and 1.30) for the control group (effect size 0.26, P less than 0.017). CONCLUSION: Combined insulin-sulfonylurea therapy leads to modest improvement in glycemic control compared with insulin therapy alone. With combined therapy, lower insulin doses may be used to achieve similar control. Obese patients with higher fasting C-peptides may be more likely to respond than others.

Improved control of non-insulin-dependent diabetes mellitus by combined halofenate and chlorpropamide therapy.

Combined halofenate-chlorpropamide was evaluated for the treatment of NIDDM. Four subjects treated with 500 mg/day chlorpropamide were given 500-1000 mg halofenate daily for 48 wk or longer. Fasting plasma glucose fell from 210 +/- 16 (+/- SEM) (11.67 +/- 0.89 mM) to 107 +/- 10 mg/dl (+/- SEM) (5.94 +/- 0.55 mM), P less than 0.005. Twelve additional subjects were entered into a 16-wk double-blind study testing chlorpropamide plus either placebo or halofenate. In the halofenate group, the mean fasting glucose fell from 227 +/- 27 (+/- SEM) (12.61 +/- 1.50 mM) and reached 107 +/- 19 mg/dl (+/- SEM) (5.94 +/- 1.06 mM) during the fourth month, whereas the placebo groups showed a decrease from 242 +/- 22 (+/- SEM) to 208 +/- 29 mg/dl (+/- SEM) (P less than 0.005). In addition, halofenate reduced the height of postprandial glycemic excursions by lowering fasting plasma glucose. When halofenate was used as the only therapy, reduction in fasting plasma glucose was small [179 +/- 12 reduced to 142 +/- 8 mg/dl (+/- SEM); 9.94 +/- 0.67 mM and 7.89 +/- 0.44 mM], P less than 0.05.

Plasma transport forms of ingested fatty alcohols in the rat.

Previous studies have shown that ingested fatty alcohols are absorbed as fatty acids and fatty acid esters, particularly triglycerides. The present study was carried out to determine whether fatty alcohols are also transported as 0-alkyl glyceryl ethers, alk-1-enyl glyceryl ethers, and as wax esters. Oxidation of fatty alcohols to other lipids was assessed by using a mixture of [1-3H] hexadecanol and [1-14C] hexadecanol of predetermined ratio. The results indicate that the absorption of fatty alcohol, and of its transport forms, parallels the absorption of labeled fatty acids. Six to 25% of plasma radioactivity was present as 1-0-alkyl diacylglyceryl ethers with a smaller proportion of ether lipids in the phospholipid fraction. In addition, 4-13% of the ingested hexadecanol appeared in the plasma as a material having the chromatographic properties of wax ester. Fatty alcohols were not detected in the plasma as alk-1-enyl lipids.

Ehrlich ascites tumor cell surface membranes: an abnormality in ether lipid content.

Most mammalian neoplasms have a defect in ether lipid content manifested by the presence of abnormally large quantities of 0-alkyl glyceryl ethers, in contrast to normal tissues in which the alk-1-enyl structure predominates. These lipids are for the most part structural. The manner in which tumor cell plasma membranes differ from normal may be important, and it has been hitherto unclear whether or not the 0-alkyl lipid abnormality of neoplasms includes the plasma membrane. The present investigation reveals that 0-alkyl lipids are present in the membranes of Ehrlich ascites tumor cells isolated by several different methods. The amount of 0-alkyl lipid, on a weight basis, represents 1-3 percent of the total phospholipids and 1-4 percent of the total aliphatic lipid. These quantities are the same as or greater than the amount of 0-alkyl lipid found in microsomes, mitochondria, and whole cell homogenate. As is generally the case for intact neoplastic tissues, the quantity of 0-alkyl lipids of Ehrlich ascites tumor plasma membrane is greater than the amount of alk-1-enyl lipids.

The formation of tritiated O-alkyl lipid from acyldihydroxyacetone phosphate in the presence of tritiated water.

Previous studies from this laboratory on the mechanism of O-alkyl bond formation using a microsomal system from Tetrahymena pyriformis have shown that O-alkyl lipid synthesized from dihydroxyacetone phosphate has exchanged one hydrogen stereospecifically from the 1-sn position of the glycerol moiety. Indirect evidence suggested that acyldihydroxyacetone phosphate, an intermediate in )-alkyl lipid synthesis, is probably not the locus of the exchange. In the present study in was shown that stable acyldihydroxyacetone phosphate incubated in the presence of tritiated water and Tetrahymena microsomes does not become tritiated. When hexadecanol is added to the system O-alkyl lipid is produced which has incorporated one atom of hydrogen for each mole of hexadecanol at all time periods examined. Experiments in Ehrlich ascites tumor cells have shown that the hydrogen exchange also occurs in a mammalian system. The results indicate that the mechanism of O-alkyl lipid ether bond formation involves a hydrogen exchange and that this exchange occurs after the formation of acyldihydroxyacetone phosphate.

Hydrogen exchange in the formation of dihydroxyacetone phosphate from acyl dihydroxyacetone phosphate in O-alkyl lipid synthesis in Ehrlich ascites tumor cell microsomes.

We have previously presented evidence that acyl dihydroxyacetone phosphate is converted to O-alkyl dihydroxyacetone phosphate via an endiol intermediate which then accepts a fatty alcohol to form O-alkyl dihydroxyacetone phosphate. We have further proposed that, in the absence of fatty alcohol, the endiol derivative of acyl dihydroxyacetone phosphate reacts with water to form dihydroxyacetone phosphate. In support of this hypothesis, we have shown, in an O-alkyl generating system, that the amount of hydrogen released from acyl dihydroxyacetone phosphate in the formation of the endiol is greater than the amount of hydrogen lost from the total lipid present at the end of incubation. The discrepancy is greater in the absence of added hexadecanol. The balance of the hydrogen loss can be accounted for by the formation of a non-lipid substance which was identified as dihydroxyacetone phosphate.

The molar ratio of the two major polypeptide components of human high density lipoprotein.

Human high density lipoprotein (HDL) and its subfractions (HDL2 and HDL3) were separated by ultracentrifugation and the molar ratio of the two major polypeptide chains apo-Gln-I and apo-Gln-II was determined by fluorescence tagging of sodium dodecyl sulfate-denatured proteins combined with polyacrylamide disc gel electrophoresis. Using purified apo-Gln-I and apo-Gln-II standards, it was found that holo HDL, holo HDL2, and holo HDL3 from all plasma samples contained a molar ratio of apo-Gln-I to the disulfide-bound dimer of apo-Gln-II of 2:1, that is a 1:1 ratio in terms of each species of polypeptide chain. The method described is useful for making repeated and rapid measurements on microgram quantities of intact lipoproteins.

Absolute configuration of tritiated O-alkylglycerol synthesized enzymatically from (1,3-3H2, 1,3-14C2)dihydroxyacetone phosphate.

O-Alkyldihydroxyacetone phosphate is synthesized enzymatically from hexadecanol and acyldihydroxyacetone phosphate. In this process there is a hydrogen exchange in which the pro-R hydrogen of C-1 of the sn-glycerol moiety is lost. This hydrogen is replaced by a hydrogen from the medium. In order to obtain additional information on the mechanism of ether bond formation, it would be of interest to know whether or not the hydrogen exchange results in a change of configuration in the product, O-alkyldihydroxyacetone phosphate. By using O-alkylglycerol prepared both chemically and enzymatically from isomerase-treated [1,3-3H2, 1,3-14C2] dihydroxyacetone phosphate and an O-alkylglycerol cleavage enzyme system, it was shown that the hydrogen exchange occurs with retention of configuration of the substituents of C-1 of the sn-glycerol moiety.

Stereochemistry of the acyl dihydroxyacetone phosphate acyl exchange reaction.

The fatty acid of acyl dihydroxyacetone phosphate can be exchanged enzymatically for another fatty acid. It has been shown that this reaction proceeds by cleavage of the oxygen bound to C-1 of the dihydroxyacetone phosphate (DHAP) moiety rather than by the more common cleavage at the acyl to oxygen bond. In the present study, the stereochemistry of this reaction was defined further; using deuterated substrates and fast atom bombardment-mass spectrometry, it was shown that the fatty acid exchange involves the stereospecific labilization of the pro-R hydrogen at C-1 of the DHAP moiety of acyl DHAP. The mechanism of ether bond formation, in which acyl DHAP is converted to O-alkyl DHAP, also proceeds via labilization of the pro-R hydrogen and cleavage of the fatty acid at the C-1 to oxygen bond. In addition, other workers have provided evidence that the enzyme responsible for the exchange reaction is O-alkyl DHAP synthetase. Therefore, the present results support the hypothesis that the acyl exchange is the reverse reaction of the first step in O-alkyl DHAP synthesis; in both of these reactions the pro-R hydrogen of C-1 of the DHAP moiety of acyl DHAP and the fatty acid moiety are labilized with cleavage of the fatty acid at the DHAP C-1 to oxygen bond.

The mechanism of ether bond formation in O-alkyl lipid synthesis.

O-Alkyl dihydroxyacetone phosphate is formed enzymatically from acyl dihydroxyacetone phosphate and a long chain fatty alcohol. This reaction is accompanied by stereospecific exchange of the pro-R hydrogen of carbon 1 (carbon 1 of all compounds corresponds to carbon 1 of sn-glycerol) of the dihydroxyacetone phosphate moiety with retention of configuration. In the present investigation, data are provided to show that the initial loss of hydrogen from carbon 1 of acyl dihydroxyacetone phosphate does not depend on the presence of the fatty alcohol. In addition, the occurrence of a Schiff base between enzyme and acyl dihydroxyacetone phosphate, comparable to the fructose-1,6-diphosphate aldolase reaction, could not be demonstrated. It is concluded that the formation of 1-O-alkyl dihydroxyacetone phosphate via the formation of intermediate 1-O-acyl endiol and 1-O-alkyl endiol is a likely mechanism.

The rate of formation of surface membrane ether lipids in Ehrlich ascites tumor cells: kinetic considerations.

A previous investigation has shown that O-alkyl phospholipids are present in the surface membrane of Ehrlich ascites tumor cells. In the present investigation it was shown that 90% or more of [1-3H]hexadecanol injected intraperitoneally into mice bearing Ehrlich ascites tumors is taken up by the neoplastic cells in less than 15 min. Near maximum formation of surface membrane O-alkyl phospholipids requires approximately 8 h. The rate of accumulation of O-alkyl phospholipids is very similar both for the whole cell and for the surface membrane. Further examination of the data revealed that the conversion of hexadecanol into O-alkyl glycerophospholipids can be described by a simple model in which O-alkyl lipids appear at a single rate constant of 0.25 to 0.35 per hour and disappear at a rate of 0.02 per hour or less. These rate constants were obtained initially by stochastic analysis and validated both by deterministic methods and by compartmental analysis using the SAAM computer program. The method of kinetic analysis described may find broader application in providing comparative rate constants for the in vivo turnover of O-alkyl lipids in both normal and neoplastic tissues. The advantage of a stochastic approach is that kinetic data may be obtained with fewer assumptions relating to pool structure or specific models.

O-alkyl lipid synthesis: the mechanism of the acyl dihydroxyacetone phosphate fatty acid exchange reaction.

We have previously provided evidence for a mechanism by which acyl DHAP is converted enzymatically to O-alkyl DHAP. This mechanism involves, in part, the formation of an endiol of acyl DHAP, loss of the fatty acid by splitting of the DHAP carbon-1 to oxygen bond and the gain of a long chain fatty alcohol. It has been shown that acyl DHAP can exchange its fatty acid for one in the medium, presumably by the mediation of O-alkyl DHAP synthase. In the present investigation we have shown that the fatty acid which is gained by acyl DHAP in the exchange process retains both carboxyl oxygens, as predicted by our postulated mechanism. This reaction is exceptional because the usual action of acyl hydrolases is to cleave at the oxygen to acyl bond.

The mechanism of ether bond formation in O-alkyl lipid synthesis in Ehrlich ascites tumor. Unusual cleavage of the fatty acid moiety of acyl dihydroxyacetone phosphate.

We have previously presented evidence for the formation of 1-O-alkyl dihydroxyacetone-P from acyl dihydroxyacetone-P via the initial formation of an intermediate 1-O-acyl endiol of acyl dihydroxyacetone-P. This reaction involves a stereospecific exchange of the pro-R hydrogen of the acyl dihydroxyacetone-P moiety without change in configuration. The fatty acid is replaced by a long chain fatty alcohol which retains the oxygen of the primary carbinol. In the absence of fatty alcohol, water substitutes and the product is dihydroxyacetone-P which has also exchanged the pro-R hydrogen with a hydrogen from the medium. An absolute requirement of the proposed mechanism is that the loss of the fatty acid must proceed via an unusual cleavage of the dihydroxyacetone-P C-1 to oxygen bond instead of the usual cleavage at the fatty acid acyl to oxygen bond. In the present investigation, we have synthesized hexadecanoyl dihydroxyacetone-P containing oxygen-18 exclusively at the dihydroxyacetone-P C-1 oxygen. Using this substrate, we have shown that cleavage of hexadecanoyl dihydroxyacetone-P at the C-1 to oxygen bond is linked to O-alkyl dihydroxyacetone-P synthesis. Inhibition of O-alkyl lipid synthesis by means of magnesium or NADPH inhibited the unusual cleavage. At the same time, we have shown that there was hydrolysis of acyl dihydroxyacetone-P which proceeded by the usual mechanism and which was not related to synthesis of O-alkyl dihydroxyacetone-P.

Mechanism of avian estrogen-induced hypertriglyceridemia: evidence for overproduction of triglyceride.

Relying on methods other than the determination of turnover rate of triglyceride from the curve of plasma triglyceride radioactivity after administration of labeled precursor, we have confirmed that the endogenous hypertriglyceridemia induced by estrogenization of the chick is accompanied by increased production of triglyceride. Chicks estrogenized with diethylstilbestrol became grossly hypertriglyceridemic and had elevated levels of plasma free fatty acid. Within 5 min of administration of labeled palmitate, estrogenized hypertriglyceridemic birds converted approximately 10 times more plasma free fatty acid to hepatic triglyceride than did controls. In addition, 2 hr after intraperitoneal injection of [14-C]acetate or [U-14-C]glucose, the specific activity of very low density lipoprotein triglyceride (VLDL-TG) of estrogenized birds reached or exceeded that of the untreated controls, and the rapid enrichment of the vastly expanded plasma VLDL-TG pool with labeled triglyceride further indicated that increased production of triglyceride occurs with estrogenization. Furthermore, [14-C]acetate incorporation into VLDL-TG was calculated to be 1.6 and 6.6% of the injected dose in estrogenized birds compared with 0.1 and 0.2% in untreated birds. Increased production of plasma VLDL-TG was confirmed by a kinetic study of VLDL-TG metabolism, employing reinjected, endogenously prepared [14-C]triglyceride-labeled VLDL. The fractional turnover rate of VLDL-TG in estrogenized hypertriglyceridemic birds was substantially less than that in untreated controls (0.32 plus or minus 0.03 vs 0.71 plus or minus 0.03/hr), but the total turnover rate was nearly 50 times greater (244 plus or minus 52 vs. 5 plus or minus 1 mg/hr).

A novel approach to structure proof of glyceryl ether-containing glycerophospholipids. Base-catalyzed methanolysis of platelet-activating factor (AGEPC) at 60 degrees C.

A novel reaction was explored in which synthetic platelet-activating factor, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine (AGEPC), upon treatment with 1 N NaOH in methanol at 60 degrees C for 20 min, sequentially released the acetyl group, then the choline moiety with concomitant formation of the monomethyl ester of 1-O-alkyl-glycero-phosphoric acid. A mechanism is proposed in which a transient cyclic phosphate intermediate is formed and then attacked by a CH3O moiety to yield a mixture of the sn-2 and sn-3 methyl esters. Proof of structure of the monomethyl ester derivative was achieved through the use of thin-layer chromatography, aluminum oxide chromatography, and examination of the trimethylsilyl derivative of the monomethyl ester by gas-liquid chromatography-mass spectrometry. Replacement of the acyl group on the 2 position with an ethyl or methyl residue completely prevented any attack by 1 N NaOH in methanol at 60 degrees C. Sphingomyelin was not attacked and only acetate removal was noted with 1-O-alkyl-2-acetyl-sn-glycero-3-phosphoethanolamine under similar conditions. The significance of these findings as they relate to the influence of substituents on the chemical and biological reactivity of AGEPC is discussed.