Correlation between the cellular level of long-chain acyl-coa, peroxisomal β-oxidation, and palmitoyl-CoA hydrolase activity in rat liver. Are the two enzyme systems regulated by a substrate-induced mechanism?

Abstract

Data obtained in earlier studies with rats fed diets containing high doses of peroxisome proliferators (niadenate, tiadenol, clofibrate, or nitotinic acid) are used to look for a quantitative relationship between peroxisomal β-oxidation, palmitoyl-CoA hydrolase, palmitoyl-CoA synthetase and carnitine palmitoyltransferase activities, and the cellular concentration of their substrate and reaction products. The order of the hyperlipidemic drugs with regard to their effect on CoA derivatives and enzyme activities was niadenate > tiadenol > clofibrate > nicotinic acid. Linear regression analysis of long-chain acyl-CoA content versus palmitoyl-CoA hydrolase and peroxisomal β-oxidation activity showed highly significant linear correlations both in the total liver homogenate and in the peroxisome-enriched fractions. A dose-response curve of tiadenol showed that carnitine palmitoyltransferase and palmitoyl-CoA synthetase activities and the ratio of long-chain acyl-CoA to free CoASH in total homogenate rose at low doses before detectable changes occurred in the peroxisomal β-oxidation and palmitoyl-CoA hydrolase activity. A plot of this ratio parallelled the palmitoyl-CoA synthetase activity. The specific activity of microsomally localized carnitine palmitoyltransferase was low and unchanged up to a dose where no enhanced peroxisomal β-oxidation was observed, but over this dose the activity increased considerably so that the specific activity of the enzyme in the mitochondrial and microsomal fractions became comparable. The mitochondrial palmitoyl-CoA synthetase activity decreased gradually. The correlations may be interpreted as reflecting a common regulation mechanism for palmitoyl-CoA hydrolase and peroxisomal β-oxidation enzymes, i.e., the cellular level of long-chain acyl-CoA acting as the metabolic message for peroxisomal proliferation resulting in induction of peroxisomal β-oxidation and palmitoyl-CoA hydrolase activity. The findings are discussed with regard to their possible consequences for mitochondrial fatty acid oxidation and the conversion of long-chain acyl- l-carnitine to acyl-CoA derivatives.