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

Peroxisome proliferator perfluorodecanoic acid alters glutathione and related enzymes

Previously we have shown that treatment with the peroxisome proliferator perfluorodecanoic acid (PFDA) significantly increased hepatic reduced glutathione (GSH) content without altering the activity of selenium-glutathione peroxidase. In this study we examined some potential mechanisms by which PFDA treatment increases GSH levels. Male Sprague-Dawley rats were given a single injection of 0, 8.8, 17.5, and 35 mg PFDA in corn oil per kg body weight. Twelve days later the effects of PFDA on the activities of enzymes associated with GSH synthesis, utilization, and regeneration were assessed. The results showed that in a dose-dependent manner, PFDA treatment significantly decreased the activity of gamma-glutamylcysteine synthetase, while the activities of NADPH-generating enzymes, malic enzyme, glucose-6-phosphate dehydrogenase, and 6-phosphogluconate dehydrogenase were increased. PFDA treatment also dose dependently decreased cytosolic, but not microsomal, glutathione S-transferase activity, and the activity of glutathione reductase was decreased by the highest dose of PFDA. The data obtained suggest that increased hepatic GSH levels following PFDA treatment may result from increased regeneration and/or decreased utilization.

Malonyl-CoA metabolism in cardiac myocytes and its relevance to the control of fatty acid oxidation

1. Viable myocytes were obtained from rat hearts. Oxidation of [1-14C]palmitate by these cells could be decreased by the addition of glucose (5 mM) or lactate (2 mM). In the presence of glucose, insulin decreased and adrenaline increased palmitate oxidation. 2. The myocytes contained activities of ATP citrate-lyase, acetyl-CoA carboxylase and the condensing enzyme of the fatty acid elongation system. No fatty acid synthase activity was demonstrable in myocytes. 3. In rat hearts perfused with 5 mM glucose, malonyl-CoA content was acutely raised by insulin. In the presence of glucose+insulin, perfusion with palmitate or adrenaline decreased the malonyl-CoA content. 4. It is concluded that malonyl-CoA can be synthesized within cardiac myocytes and that the level of this metabolite can be acutely regulated. This is likely to have consequences for the regulation of carnitine palmitoyltransferase in the heart.

Dietary fat effects on hepatic lipid peroxidation and enzymes of H2O2 metabolism and NADPH generation

The purpose of this study was to determine the effects of dietary fat quantity and fatty acid composition on hepatic H2O2-metabolizing systems, activities of NADPH-generating enzymes and lipid peroxidation. One-month-old male C57BL/6J mice were fed one of six diets: (i) 5% fat, rich in 18:2n-6 fatty acid (5% N-6); (ii) 20% fat, rich in 18:3n-3 (N-3); (iii) 20% fat, rich in 18:2n-6 (N-6); (iv) 20% fat, rich in 18:1n-9 (N-9); (v) 20% fat, rich in saturated fatty acids (SAT); and (vi) 20% fat, deficient in essential fatty acids (EFAD); for 11 wk. Comparisons between animal groups receiving different fat quantities showed that activities of glucose-6-phosphate dehydrogenase (G6PDH, EC 1.1.1.49) and malic enzyme (ME, EC 1.1.1.40) and the levels of conjugated dienes were significantly lower in the N-6 than in 5% N-6 group. Conversely, activities of catalase (CAT, EC 1.11.1.6) and selenium-glutathione peroxidase (SeGSHPx, EC 1.11.1.9) were higher in the N-6 than in 5% N-6 group. Among the five dietary groups receiving 20% fat but differing in fatty acid composition, CAT activity was lower in the N-9 group, SeGSHPx activity was lower in the EFAD group, and glutathione reductase (GSSGR, EC 1.6.4.2) activity was higher in the N-6 than in the N-3, N-9, SAT and EFAD group. The EFAD group had much higher levels of total lipids and conjugated dienes, as well as activities of NADPH-generating enzymes, including G6PDH, ME and isocitrate dehydrogenase (EC 1.1.1.42), than the other four high-fat groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Reduction by clofibric acid of serum arachidonic acid in rats. Effect on the acyl composition of renal phospholipids

Alterations induced by p-chlorophenoxyisobutyric acid (clofibric acid) in the composition of phosphatidylcholine and cholesterol esters in serum and their influence on the composition of phosphatidylcholine in the kidney were studied. Rats of different ages responded differently to the drug in terms of the levels of arachidonic acid (20:4) and linoleic acid (18:2) in the phosphatidylcholine and cholesterol esters in the serum. Administration of clofibric acid to 26-week-old rats for 2 weeks caused a marked decreased in the relative level of 20:4 in phosphatidylcholine and cholesterol esters in serum, whereas similar treatment of 6-week-old rats resulted in a reduction of 18:2 and, to a lesser extent, of 20:4 in serum lipids. The decrease in phosphatidylcholine that contained 20:4 in the serum of old rats was mainly due to a decrease in the concentration of stearoyl-arachidonoyl (18:0-20:4) species. The decrease in cholesterol arachidonate in serum caused by the treatment of old rats with clofibric acid seemed to be due to a reduction in the relative level of serum phosphatidylcholine containing 20:4. The marked reduction in serum lipids that contained 20:4 caused a decrease in the relative level of 20:4 in renal phospholipids, in particular, a decrease in the proportion of palmitoyl-arachidonoyl (16:0-20:4) and 18:0-20:4 phosphatidylcholine.

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.

Evidence against cytochrome b5 involvement in liver microsomal fatty acid elongation

This study provides strong evidence against cytochrome b5 participation in the first reduction step-beta-ketoreduction-of rat liver microsomal fatty acid chain elongation. Several lines of evidence led to this conclusion: (a) beta-ketoreductase was not inducible by diet conditions since its activity was the same in microsomes from fasted rats and in rats fed a fat-free diet. Consequently, its activity was appreciable in microsomes from fasted rats. Nevertheless, cytochrome b5 reoxidation rate was not stimulated by adding beta-ketopalmitoyl-CoA to the latter microsomes. This suggests that it is not the activated beta-ketoreductase which stimulates the cytochrome b5 reoxidation rate, but another electron acceptor. (b) The delta 9-desaturase, present in microsomes from rats fed a fat-free diet, was totally inhibited by 4 mM KCN; beta-ketopalmitoyl-CoA or malonyl-CoA stimulated the reoxidation rate of cytochrome b5 but this increase was also inhibited by 4 mM KCN. This suggests that delta 9-desaturase is involved in the stimulation and shows that any inhibitor of delta 9-desaturase, including cytochrome b5 antibodies, may induce elongation inhibition. (c) NADH-dependent beta-ketoreductase activity was partially purified from Triton X-100 solubilised microsomes, in a fraction essentially free of cytochrome b5. Furthermore, when the fraction containing cytochrome b5 and NADH-cytochrome-b5 reductase was added to the fraction containing beta-ketoreductase activity, no increase in beta-ketoreductase activity was observed. Stearoyl-CoA desaturase activity which is also present in microsomes from rats fed a fat-free diet led to the results which have been misinterpreted in the conclusions of previous studies.

Low fatty acid elongation rate in the presence of NADH in the liver endoplasmic reticulum. Overinhibition by BSA at the beta-ketoreductase level

The rate of NADH-dependent palmitoyl-CoA elongation was only 41% of that of NADH-dependent elongation in microsomes from rats fed a fat-free diet, in the absence of BSA. This value was markedly lowered to 5%, when the assay was performed in the presence of BSA. The determination of the intermediate products showed that 93% of the total products accumulated as beta-ketostearate in the presence of BSA and NADH, whereas the accumulated beta-ketostearate was only 25% of the total products in the presence of BSA and NADPH. BSA was shown to be responsible for the low rate of NADH-dependent elongation by inhibiting the beta-ketoreductase in the presence of NADH and, thereby, inducing beta-ketostearate accumulation. These results indicate that NADH is probably not the physiological electron donor to the elongation pathway.

Action of Ebselen on rat hepatic microsomal enzyme-catalyzed fatty acid chain elongation, desaturation, and drug biotransformation

In the previous study, the organoselenium-containing anti-inflammatory agent, Ebselen, was found to disrupt both hepatic microsomal NADH- and NADPH-dependent electron transport chains. In the current investigation, we focus on the action of Ebselen on three separate metabolic reactions, namely, fatty acid chain elongation, desaturation, and drug biotransformation, which utilize reducing equivalents via these microsomal electron transport pathways. Both NADH-dependent and NADPH-dependent chain elongation reactions showed (i) that the condensation step was inhibited by Ebselen; all three substrates, palmitoyl CoA (16:0), palmitoleoyl CoA (16:1), and gamma-linolenyl CoA (18:3), were differentially affected by Ebselen; for example, the apparent Ki's of Ebselen for the condensation of 16:0, 16:1, and 18:3 in the absence of bovine serum albumin (BSA) preincubation were 7, 14, and 34 microM, and those in the presence of BSA preincubation were 35, 62, and 150 microM, respectively, supporting earlier data for multiple condensing enzymes; (ii) that the beta-ketoacyl CoA reductase-catalyzed reaction step which appears to receive electrons, at least in part, from the cytochrome b5 system, was also markedly inhibited by varying Ebselen concentrations; and (iii) that similar results were obtained with the dehydrase and the enoyl CoA reductase. Hence, each of the four component steps was significantly inhibited by Ebselen. Another important fatty acid biotransformation reaction, delta 9 desaturation of stearoyl CoA to oleoyl CoA, was significantly inhibited (90%) by 30 microM Ebselen. This effect appeared to be directly related to the NADH-dependent electron transport chain rather than to a direct action on the desaturase enzyme. Last, Ebselen also inhibited both aminopyrine and benzphetamine N-demethylations, two cytochrome P450-catalyzed reactions, in untreated rats, in rats on a high carbohydrate diet, and in phenobarbital-treated rats.

Comparison between condensation and overall chain elongation of arachidoyl-CoA and arachidonoyl-CoA in swine cerebral microsomes

The condensation and overall elongation products of exogenous arachidoyl-CoA (20:0-CoA) and endogenous fatty acids in swine cerebral microsomes were detected by radio gas chromatography. In addition, the condensation products with malonyl-CoA as substrate were analyzed by radio high-performance liquid chromatography. Three main condensation products were detected; the overall elongation products of exogenous 20:0-CoA were 22:0 and 24:0, and those of endogenous substrates were 18:0, 22:4, and 24:4. The yield was estimated for the conversion of 3-ketoacyl-CoAs to the corresponding saponification products (methyl ketones or R-2-one; e.g., 2-heptadecanone = 17:0-2-one); these products were identified in the preceding paper (S. Yoshida and M. Takeshita (1987) Arch. Biochem. Biophys. 254, 170-179). The extraction of R-2-one by hexane depended on the acyl chain length. The yield of 2-heneicosanone (21:0-2-one) detected by radio gas chromatography was 80% whereas the yields of 17:0-2-one and 2-heneicosatetraenone (21:4-2-one) from the corresponding 3-ketoacyl-CoAs were 56 and 48%, respectively. A quantitative comparison was performed for the condensation and overall elongation activity; it was noticed that the condensation activity for the system which simultaneously produced two elongation products was nearly the same as that of the corresponding overall elongation activity. This result suggests that the condensation step may be at least one of the rate-limiting steps in the overall elongation of very-long-chain fatty acyl-CoA.