Inhibition by etomoxir of rat liver carnitine octanoyltransferase is produced through the co-ordinate interaction with two histidine residues

Rat peroxisomal carnitine octanoyltransferase (COT), which facilitates the transport of medium-chain fatty acids through the peroxisomal membrane, is irreversibly inhibited by the hypoglycaemia-inducing drug etomoxir. To iden tify the molecular basis of this inhibition, cDNAs encoding full-length wil d-type COT, two different variant point mutants and one variant double muta nt from rat peroxisomal COT were expressed in Saccharomyces cerevisiae, an organism devoid of endogenous COT activity. The recombinant mutated enzymes showed activity towards both carnitine and decanoyl-CoA in the same range as the wild type. Whereas the wild-type version expressed in yeast was inhi bited by etomoxir in an identical manner to COT from rat liver peroxisomes, the activity of the enzyme containing the double mutation H131A/H340A was completely insensitive to etomoxir. Individual point mutations H131A and H3 40A also drastically reduced sensitivity to etomoxir. Taken together, these results indicate that the two histidine residues, H131 and H340, are the s ites responsible for inhibition by etomoxir and that the full inhibitory pr operties of the drug will be shown only if both histidines are intact at th e same time. Our data demonstrate that both etomoxir and malonyl-CoA inhibi t COT by interacting with the same sites.