Effect of chronic clofibrate feeding on the activities of enzymes involved in glycerolipid synthesis and in peroxisomal metabolism in rat liver

Effects of hypolipidaemics cetaben and clofibrate on mitochondrial and peroxisomal enzymes of rat liver

Clofibrate or cetaben was administered to male rats for 10 days. Peroxisomal and mitochondrial enzymes were assayed in liver subcellular fractions. Clofibrate affected the specific activities of both mitochondrial enzymes (glycerol-3-phosphate dehydrogenase and nicotinamide-linked isocitrate dehydrogenase) and peroxisomal enzymes (fatty acyl-CoA oxidase, glycerone phosphate acyltransferase, urate oxidase, and D-amino-acid oxidase). In contrast, cetaben raised only the peroxisomal enzymes, acyl-CoA oxidase, glycerone-phosphate acyltransferase, D-amino-acid oxidase, catalase, and urate oxidase. Thus, the hypolipidaemic activity of these drugs may be exclusively related to stimulated peroxisomal functioning, while mitochondria play only a minor role.

The fatty acid chain elongation system of mammalian endoplasmic reticulum

Much has been learned about FACES of the endoplasmic reticulum since its discovery in the early 1960s. FACES consists of four component reactions, requires the fatty acid to be activated in the form of a CoA derivative, utilizes reducing equivalents in the form of NADH or NADPH, is induced by a fat-free diet, resides on the cytoplasmic surface of the endoplasmic reticulum, appears to function in concert with the desaturase system and appears to exist in multiple forms (either multiple condensing enzymes connected to a single pathway or multiple pathways). FACES has been found in all tissues investigated, namely, liver, brain, kidney, lung, adrenals, retina, testis, small intestine, blood cells (lymphocytes and neutrophils) and fibroblasts, with one exception--the heart has no measurable activity. Yet, much more needs to be learned. The critical, inducible and rate-limiting condensing enzyme has resisted solubilization and purification; the purification of the other components has met with limited success. We know nothing about the site of synthesis of each component of FACES. How is each component enzyme integrated into the endoplasmic reticulum membrane? Is there a single mRNA directing synthesis of all four components or are there four separate mRNAs? How are elongation and desaturation coordinated? What is (are) the physiological regulator(s) of FACES--ADP, AMP, IP3, G-proteins, phosphorylation, CoA, Ca2+, cAMP, none of these? The molecular biology of FACES is only in the fetal stage of development. We are only scratching the surface--it is an undiscovered country.

Effect of the peroxisomal proliferator di(2-ethylhexyl)phthalate on component reactions of the rat hepatic microsomal fatty acid chain elongation system and on other hepatic lipogenic enzymes

The feeding of 2% di(2-ethylhexyl)phthalate (DEHP) to rats increased the hepatic microsomal elongation of palmitoyl-CoA by about twofold, while those of palmitoleoyl-CoA and gamma-linolenoyl-CoA decreased to 83 and 63%, respectively, of the control values. When component reactions of the elongation pathway were measured, it was observed that only the activity of condensing enzyme was increased by twofold, while those of beta-ketostearoyl-CoA reductase, beta-hydroxypalmitoyl-CoA dehydrase, and trans-2-hexadecenoyl-CoA reductases were not affected. Furthermore, the time course for induction of both condensation and elongation of palmitoyl-CoA was similar. In vitro addition of DEHP had no effect on either condensation or elongation. Thus, these results indicate that the peroxisomal proliferator induces only the condensing enzyme which is the regulatory and rate-limiting step of elongation sequence. The DEHP treatment also markedly enhanced the cytosolic NADPH-generating activities of glucose-6-PO4 dehydrogenase (2.2-fold) and malic enzyme (7.3-fold). Unexpectedly, the activities of fatty acid synthetase and citrate cleavage enzyme were unaffected. These results are discussed in light of the fact that these lipogenic enzymes are coordinately induced by diet or hormones.

Strain dependence of the metabolism of cis- and trans-isomers of 9-octadecenoic acid in perfused liver and cell-free preparation in rats

Hepatic metabolism of cis- and trans-9-octadecenoic acid was compared in various strains of rats and under different nutritional states. In Wistar rats triacylglycerol secretion was consistently higher in livers perfused with the cis isomer than with the trans isomer, while the difference was considerably attenuated in Sprague-Dawley rats. The difference in the hepatic triacylglycerol secretion disappeared when rats were fasted for 2 days. The rate of oxidation of trans fatty acid to ketone bodies was remarkably much higher than the cis isomer in Wistar but not in Sprague-Dawley rats. After fasting, the difference in the ketone body production disappeared in Wistar rats, whereas the oxidation rate was rather lower in the trans isomer than in the cis isomer in Sprague-Dawley rats. In isolated mitochondria, ketogenesis from trans-9-octadecenoic acid was markedly lower than that from the cis counterpart, irrespective of the nutritional states or strains of rats, and correlated well with the substrate specificity of carnitine acyltransferase. The molar concentration of malonyl-CoA to cause 50% inhibition of ketogenesis, the rate of peroxisomal beta-oxidation and the activity of acyl-CoA oxidase were all comparable, irrespective of the substrate sources. The Km value for acyl-CoA oxidase to the trans-acyl-CoA was 2-times higher than that of the cis counterpart in both strains of rats. Thus, peroxisomal as well as mitochondrial fatty acid oxidation systems apparently discriminated between the geometrical differences of the fatty acid substrate.

Modification by clofibric acid of acyl composition of glycerolipids in rat liver. Possible involvement of fatty acid chain elongation and desaturation

Administration of p-chlorophenoxyisobutyric acid (clofibric acid) to rats induced a marked change in acyl composition of hepatic glycerolipids; a considerable increase in the proportion of octadecenoic acid (18:1) was accompanied by a marked decrease in the proportion of octadecadienoic acid (18:2). Among the glycerolipids, the changes in the proportions of 18:1 and 18:2 were the most marked in phosphatidylcholine. The change in the acyl composition of phosphatidylcholine paralleled the change in free fatty acid composition in microsomes. The treatment of rats with clofibric acid resulted in a 2.3-fold increase in activity of microsomal palmitoyl-CoA chain elongation and a 4.8-fold increase in activity of stearoyl-CoA desaturation. The activities of acyl-CoA synthetase, 1-acylglycerophosphate acyltransferase and 1-acylglycerophosphorylcholine acyltransferase in hepatic microsomes were increased approx. 3-, 1.7- and 3.6-times, respectively, by the treatment of rats with clofibric acid. These findings are discussed with respect to the role of fatty acid modification systems in the regulation of acyl composition of phosphatidylcholine.

Selective increase in acylation of 1-acylglycerophosphorylcholine in livers of rats and mice by peroxisome proliferators

Rats were fed a diet containing p-chlorophenoxyisobutyric acid (clofibric acid). Activity of microsomal 1-acylglycerophosphorylcholine (1-acyl-GPC) acyltransferase in liver was increased approx. 3-fold by the treatment with clofibric acid. The treatment of rats with clofibric acid did not increase activity of microsomal 2-acyl-GPC acyltransferase. Feeding a diet containing 2,2'-(decamethylenedithio)diethanol (tiadenol), di(2-ethylhexyl)phthalate or acetylsalicylic acid also resulted in a selective increase in the activity of 1-acyl-GPC acyltransferase in rat liver. Treatment with clofibric acid increased the activity of 1-acyl-GPC acyltransferase in liver of mouse as well as rat, but did not change the activity in liver of guinea-pig. The relative rate of acylation of 1-acyl-GPC with various acyl-CoAs by hepatic microsomes was not changed by the treatment of rats with clofibric acid.

Induction of hepatic long-chain acyl-CoA hydrolase by clofibric acid administration

The induction of hepatic long-chain acyl-CoA hydrolase in the cytosolic fraction by administration of clofibric acid (p-chlorophenoxyisobutyric acid) was compared in rats, mice and guinea-pigs. In rats, two long-chain acyl-CoA hydrolases were induced by the administration of clofibric acid. In mice, only one long-chain acyl-CoA hydrolase was induced, and this hydrolase had properties similar to those of the lower-molecular-weight hydrolase induced in the hepatic cytosol of rats. In hepatic cytosol of guinea-pig, no hydrolase was induced by the administration of clofibric acid.