Source of the hepatic microsomal trans-2-enoyl CoA hydratase bifunctional protein: endoplasmic reticulum or peroxisomes

Hepatocarcinogenic potential of di(2-ethylhexyl)phthalate in rodents and its implications on human risk

The plasticizer di(2-ethylhexyl) phthalate (DEHP), to which humans are extensively exposed, was found to be hepatocarcinogenic in rats and mice. DEHP is potentially set free from objects made of synthetic materials (e.g., those used in medicine). Chronically, the greatest amounts are transferred to persons undergoing hemodialysis (up to 3.1 mg/kg b.w. per day) who would thus be considered the individuals most endangered by tumorigenesis. Although toxicokinetics seem to play a certain unclear role in the course of DEHP-related toxicity, toxicodynamic factors appear more decisive. DEHP is a representative of "peroxisome proliferators" (PP), a distinct group of substances that, in rodents, do not only induce peroxisomes but also specific enzymes in other organelles, organ growth, and DNA synthesis. The cluster of the characteristic effects of PP is generally, although perhaps not quite appropriately summarized as "peroxisome proliferation," and is strongest in the liver. The lowest observed effect level (LOEL) and the no observed effect level (NOEL) of peroxisome proliferation in the rat, as determined by the induction of specific enzymes (peroxisomal beta-oxidation, carnitine-acetyl-transferase, cytochrome P-452), DNA synthesis, and hepatomegaly, may be assumed as 50 and 25 mg/kg b.w. per day, respectively. DEHP and other carcinogenic PP are neither genotoxic nor tumor initiators, but they appear to be tumor promoters, also implicating a threshold level for the carcinogenic effect. Although a causal relationship between a particular effect of peroxisome proliferation and hepatocarcinogenesis is as yet unknown, peroxisome proliferation as a whole phenomenon appears to be associated with the potential of tumor induction, as shown by comparison of the relative strength of individual PP and by comparison of species and organ specificities. Likewise, LOEL and NOEL of rodent carcinogenesis, that is, 300 and 50 to 100 mg/kg b.w. per day, respectively, are above but not too far from the corresponding values for the investigated parameters of peroxisome proliferation. Thus, with respect to dose alone, worst-case exposure in hemodialysis patients is at least 16-fold below the LOEL of any characterized PP-specific effect of DEHP and approximately 100-fold below that of DEHP-related tumorigenesis. Also, primates are less responsive to PP than rats with respect to the investigated biochemical and morphological parameters. If this lower primate responsiveness is extrapolated to estimate carcinogenicity in humans, we might thus arrive at an even larger safety margin than when based on exposure alone. Doses of PP hypolipidemics that had clearly induced several indicators of peroxisome proliferation in rats did not cause any clear-cut enhancements in the peroxisomes of patients, even though most of these hypolipidemics were considerably stronger PP than DEHP. Thus, an actual threat to humans by DEHP seems rather unlikely. Accordingly, hepatocarcinogenesis was neither enhanced in workers exposed to DEHP nor in patients treated with hypolipidemics.

Evidence that beta-hydroxyacyl-CoA dehydrase purified from rat liver microsomes is of peroxisomal origin

The present study provides strong evidence that the previously isolated hepatic microsomal beta-hydroxyacyl-CoA dehydrase (EC 4.2.1.17), believed to be a component of the fatty acid chain-elongation system, is derived, not from the endoplasmic reticulum, but rather from the peroxisomes. The isolated dehydrase was purified over 3000-fold and showed optimal enzymic activity toward beta-hydroxyacyl-CoAs or trans-2-enoyl-CoAs with carbon chain lengths of 8-10. The purified preparation (VDH) displayed a pH optimum at 7.5 with beta-hydroxydecanoyl-CoA, and at 6.0 with beta-hydroxystearoyl-CoA. Competitive-inhibition studies suggested that VDH contained dehydrase isoforms, and SDS/PAGE showed three major bands at 47, 71 and 78 kDa, all of which reacted to antibody raised to the purified preparation. Immunocytochemical studies with anti-rabbit IgG to VDH unequivocally demonstrated gold particles randomly distributed throughout the peroxisomal matrix of liver sections from both untreated and di-(2-ethylhexyl) phthalate-treated rats. No labelling was associated with endoplasmic reticulum or with the microsomal fraction. Substrate-specificity studies and the use of antibodies to VDH and to the peroxisomal trifunctional protein indicated that VDH and the latter are separate enzymes. On the other hand, the VDH possesses biochemical characteristics similar to those of the D-beta-hydroxyacyl-CoA dehydrase recently isolated from rat liver peroxisomes [Li, Smeland & Schulz (1990) J. Biol. Chem. 265, 13629-13634; Hiltunen, Palosaari & Kunau (1989) J. Biol. Chem. 264, 13536-13540]. Neither enzyme utilizes crotonoyl-CoA or cis-2-enoyl-CoA as substrates, but both enzymes convert trans-2-enoyl substrates into the D-isomer only. In addition, the VDH also contained beta-oxoacyl-CoA reductase (beta-hydroxyacyl-CoA dehydrogenase) activity, which co-purified with the dehydrase.

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.

Isolation of peroxisomal enoyl-CoA hydratase in rainbow trout and immunochemical identification with the bifunctional enzyme

Peroxisomes are the sites for β-oxidation of long-chain fatty acids. The peroxisomal bifunctional enzyme (PBE) enoyl-CoA hydratase, 3-hydroxyacyl-CoA dehydrogenase catalyzes the second and third reactions of the β-oxidation system. Originally termed PPA-80 for peroxisome-proliferation associated 80,000 MW polypeptide, PBE levels are monitored to measure peroxisome proliferation in rodents and other species. The quantity of a 79,000 MW polypeptide in the light mitochondrial fraction of the liver, as analyzed by SDS-PAGE, increases when rainbow trout are exposed to peroxisome proliferating agents. This correlates with increases in acyl-CoA oxidase activity and peroxisome volume density. In the present study, peroxisomal enoyl-CoA hydratase was purified from trout liver and analyzed by immunoblotting with anti-PBE. A positive reaction with the 79,000 MW polypeptide band was observed providing strong evidence that this is the bifunctional enzyme.

Hepatic peroxisomal and microsomal enzyme induction by citral and linalool in rats

The short-term effects of oral administration of citral and linalool to rats have been compared. Male Wistar rats were given, by gastric intubation, 1.5 g citral or linalool/kg body weight/day for 5 days. Citral caused peroxisome proliferation as indicated by induction of cyanide-insensitive palmitoyl-CoA oxidation and bifunctional enzyme; levels of microsomal cytochrome P-450 IVA1 were also raised. Linalool caused induction of the peroxisomal enzymes but not of cytochrome P-450 IVA1, indicating that it possesses activity somewhat different from that of citral. These results suggest that the mechanisms of peroxisome proliferation may be independent of induction of cytochrome P-450 IVA1.

Acyl-CoA oxidase activity and peroxisomal fatty acid oxidation in rat tissues

Acyl-CoA oxidase, the first enzyme of the peroxisomal beta-oxidation, was proved to be rate-limiting for this process in homogenates of rat liver, kidney, adrenal gland, heart and skeletal muscle. Acyl-CoA oxidase activity, based on H2O2-dependent leuko-dichlorofluorescein oxidation in tissue extract, was compared with radiochemically assayed peroxisomal beta-oxidation rates. Dichlorofluorescein production was a valid measure of peroxisomal fatty acid oxidation only in liver and kidney, but not in adrenal gland, heart or skeletal muscle. Production of 14C-labeled acid-soluble products from 1-14C-labeled fatty acids in the presence of antimycin-rotenone appears to be a more accurate and sensitive estimate of peroxisomal beta-oxidation than the acyl-CoA oxidase activity on base of H2O2 production. Chain-length specificity of acyl-CoA oxidase changed with the acyl-CoA concentrations used. Below 80 microM, palmitoyl-CoA showed the highest activity of the measured substrates in rat liver extract. No indications were obtained for the presence in rat liver of more forms of acyl-CoA oxidase with different chain-length specificity.