Metabolic aspects of peroxisomal beta-oxidation.
In the course of the last decade peroxisomal beta-oxidation has emerged as a metabolic process indispensable to normal physiology. Peroxisomes beta-oxidize fatty acids, dicarboxylic acids, prostaglandins and various fatty acid analogues. Other compounds possessing an alkyl-group of six to eight carbon atoms (many substituted fatty acids) are initially omega-oxidized in endoplasmic reticulum. The resulting carboxyalkyl-groups are subsequently chain-shortened by beta-oxidation in peroxisomes. Peroxisomal beta-oxidation is therefore, in contrast to mitochondrial beta-oxidation, characterized by a very broad substrate-specificity. Acyl-CoA oxidases initiate the cycle of beta-oxidation of acyl-CoA esters. The next steps involve the bi(tri)functional enzyme, which possesses active sites for enoyl-CoA hydratase-, beta-hydroxyacyl-CoA dehydrogenase- and for delta 2, delta 5 enoyl-CoA isomerase activity. The beta-oxidation sequence is completed by a beta-ketoacyl-CoA thiolase. The peroxisomes also contain a 2,4-dienoyl-CoA reductase, which is required for beta-oxidation of unsaturated fatty acids. The peroxisomal beta-hydroxyacyl-CoA epimerase activity is due to the combined action of two enoyl-CoA hydratases. (For a recent review of the enzymology of beta-oxidation enzymes see Ref. 225.) The broad specificity of peroxisomal beta-oxidation is in part due to the presence of at least two acyl-CoA oxidases, one of which, the trihydroxy-5 beta-cholestanoyl-CoA (THCA-CoA) oxidase, is responsible for the initial dehydrogenation of the omega-oxidized cholesterol side-chain, initially hydroxylated in mitochondria. Shortening of this side-chain results in formation of bile acids and of propionyl-CoA. In relation to its mitochondrial counterpart, peroxisomal beta-oxidation in rat liver is characterized by a high extent of induction following exposure of rats to a variety of amphipathic compounds possessing a carboxylic-, or sulphonic acid group. In rats some high fat diets cause induction of peroxisomal fatty acid beta-oxidation and of trihydroxy-5 beta-cholestanoyl-CoA oxidase. Induction involves increased rates of synthesis of the appropriate mRNA molecules. Increased half-lives of mRNA- and enzyme molecules may also be involved. Recent findings of the involvement of a member of the steroid hormone receptor superfamily during induction, suggest that induction of peroxisomal beta-oxidation represents another regulatory phenomenon controlled by nuclear receptor proteins. This will likely be an area of intense future research. Chain-shortening of fatty acids, rather than their complete beta-oxidation, is the prominent feature of peroxisomal beta-oxidation.(ABSTRACT TRUNCATED AT 400 WORDS)
Osmundsen
H
H
Department of Physiology and Biochemistry, Dental School, University of Oslo, Norway.
Bremer
J
J
Pedersen
J I
JI
D000818:Animals; D002784:Cholesterol / Q000378:metabolism; D003998:Dicarboxylic Acids / Q000378:metabolism; D005227:Fatty Acids / Q000378:metabolism; D006801:Humans; D008830:Microbodies / Q000378:metabolism*; D010084:Oxidation-Reduction
D003998:Dicarboxylic Acids; D005227:Fatty Acids; D002784:Cholesterol