Evidence for the existence of a pristanoyl-CoA oxidase gene in man

Abstract

In the rat, 2-methyl branched fatty acids and the bile acid intermediates di- and tri-hydroxycoprostanic acids are desaturated by pristanoyl-CoA oxidase and trihydroxycoprostanoyl-CoA oxidase respectively. In the human, these compounds are oxidized by a single enzyme, branched-chain acyl-CoA oxidase, which according to its amino acid sequence is the human homologue of rat trihydroxycoprostanoyl-CoA oxidase. Pristanoyl-CoA oxidase is apparently absent from human tissues as indicated by immunoblot analysis [Van Veldhoven, Van Rompuy, Fransen, de Béthune and Mannaerts (1994) Eur. J. Biochem. 222, 795-801] and Northern-blot analysis [Vanhooren, Fransen, de Béthune, Baumgart, Baes, Torrekens, Van Leuven, Mannaerts and Van Veldhoven (1996) Eur. J. Biochem. 239, 302-309] of human tissues. In this paper we present evidence, however, that at least the gene for pristanoyl-CoA oxidase is present in the human. A human liver cDNA encoding a protein of 700 amino acids, showing 75% amino acid identity with rat pristanoyl-CoA oxidase and harbouring a peroxisomal C-terminal-targeting signal (SKL), was isolated. Bacterial expression of the cDNA resulted in a fusion protein that was cross-reactive with antibodies directed against rat pristanoyl-CoA oxidase and the C-terminal SKL sequence. Screening of a genomic library with the isolated cDNA as a probe resulted in a genomic clone in which four introns were localized. By means of fluorescence in situ hybridization the gene for human pristanoyl-CoA oxidase was mapped at chromosome position 4p15.3. We conclude that a gene for pristanoyl-CoA oxidase is present in the human genome. The gene appears to be expressed to such a low extent in liver that its mRNA cannot be detected by routine Northern-blot analysis and that its product remains undetected by standard immunoblotting or by enzyme activity measurements. We speculate that the gene may be expressed under special (e.g. certain developmental stages) conditions or in certain specialized tissues not examined thus far.