Mechanism of the stimulation of branched chain oxoacid oxidation in liver by carnitine.

The oxidation of 4-methyl-2-oxopentanoate (alpha-ketoisocaproate) by rat liver mitochondria was shown to be independent of exogenous coenzyme A and exogenous NAD+. Carnitine stimulated the oxidation of 4-methyl-2-oxopentanoate in rat liver homogenates and mitochondria; octanoate, DL-octanoylcarnitine, 4-pentenoate, and 3-methylbutyrate (isovalerate) were inhibitory, and 2-bromopalmitate had no effect. Addition of carnitine was found to increase the export from the mitochondria of acylcarnitines derived from 4-methyl-2-oxopentanoate in the presence of absence of 4-pentenoate but not in the presence of octanoylcarnitine. It is concluded that the branched chain oxoacid dehydrogenase is localized on the inner surface of the inner mitochondrial membrane and that it is regulated in part by the intramitochondrial levels of branched chain fatty acyl-CoA esters and/or free CoA.