The overall aim is to extend our ongoing work on peroxisomal bile acid synthesis and to characterize the peroxisomal HMG-CoA reductase. We have recently shown that rat liver peroxisomes are capable of metabolizing 26-hydroxycholesterol to a C-24 bile acid, 3Beta-hydroxy-5-cholenoic acid. We have also just demonstrated that peroxisomes contain a hydroxylase active toward 3Alpha, 7Alpha, 12Alpha,-trihydroxy-5Beta-cholestane, the major substrate of 26-hydroxylation in bile acid synthesis. By the use of cell fractionation methods, we have recently confirmed the immunoelectron microscopy data of Keller et al. that rat liver peroxisomes contain HMG-CoA reductase, the rate-limiting enzyme in the biosynthesis of cholesterol, and have extended the findings to suggest independent regulation of the microsomal and peroxisomal enzymes. Studies include determining if enzymes of peroxisomal fatty acid oxidation are responsible for peroxisomal side-chain oxidation of cholesterol. We will also characterize the peroxisomal hydroxylase with regards to cytochrome P-450 activity, the position of hydroxylation, optimal co-factor requirements, and substrate specificity. Other studies are directed towards the investigation of the significance of peroxisomal bile acid synthesis in vivo. Studies on peroxisomal HMG-CoA reductase will include determining the molecular weight of the enzyme and whether it can be regulated via a phosphorylation/dephosphorylation mechanism. Finally, we will test for the presence of other peroxisomal enzymes which may participate in the biosynthesis of HMG-CoA, cholesterol, and dolichol. The metabolism of bile acids as well as the regulation of cholesterol synthesis is of fundamental importance to our understanding of cholesterol metabolism, arterial disease, and gallstone formation. In addition, there are peroxisomal deficiency diseases such as Zellweger's Syndrome, Retsum's disease, and cerebrotendinous xanthomatosis, all of which cause death in early infancy and include abnormalities in cholesterol and fatty acid metabolism. Thus, clarification of the role of peroxisomes in cholesterol oxidation will lead to a greater understanding of lipid-related diseases and peroxisomal metabolic disorders.
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