Two mechanisms produce tissue-specific inhibition of fatty acid oxidation by oxfenicine

Oxfenicine [S-2-(4-hydroxyphenyl)glycine] is transaminated in heart and liver to 4-hydroxyphenylglyoxylate, an inhibitor of fatty acid oxidation shown in this study to act at the level of carnitine palmitoyltransferase I (EC 2.3.1.21). Oxfenicine was an effective inhibitor of fatty acid oxidation in heart, but not in liver. Tissue specificity of oxfenicine inhibition of fatty acid oxidation was due to greater oxfenicine transaminase activity in heart and to greater sensitivity of heart carnitine palmitoyltransferase I to inhibition by 4-hydroxyphenylglyoxylate [I50 (concentration giving 50% inhibition) of 11 and 510 microM for the enzymes of heart and liver mitochondria, respectively]. Branched-chain-amino-acid aminotransferase (isoenzyme I, EC 2.6.1.42) was responsible for the transamination of oxfenicine in heart. A positive correlation was found between the capacity of various tissues to transaminate oxfenicine and the known content of branched-chain-amino-acid aminotransferase in these tissues. Out of three observed liver oxfenicine aminotransferase activities, one may correspond to asparagine aminotransferase, but the major activity could not be identified by partial purification and characterization. As reported previously for malonyl-CoA inhibition of carnitine palmitoyltransferase I, 4-hydroxyphenylglyoxylate inhibition of this Enzyme was found to be very pH-dependent. In striking contrast with the kinetics of malonyl-CoA inhibition, 4-hydroxyphenylglyoxylate inhibition was not affected by oleoyl-CoA concentration, but was partially reversed by increasing carnitine concentrations.