Uptake and processing of the precursor for rat liver ornithine transcarbamylase by isolated mitochondria. Inhibition by uncouplers

Abstract

The precursor for rat liver ornithine transcarbamylase (EC 2.1.3.3) synthesized in vitro was converted to an apparently mature form of the enzyme by isolated rat liver mitochondria. The processed product was recovered in the sedimented mitochondria and was not extracted from the mitochondria with 1 M KCl. The processed product could be extracted with digitonin. The concentration of digitonin required was higher than that for the intermembrane space enzyme, adenylate kinase (EC 2.7.4.3), but lower than that for endogenous ornithine transcarbamylase, which is localized in the matrix space. The processed product sedimented on a sucrose gradient with an S20, omega value of 6.7 S, which is close to that of the mature enzyme (6.0 S), and assembly to the active trimer appeared to occur. The processing of the precursor by the isolated mitochondria was strongly inhibited by both 0.1 mM dinitrophenol and 1 microM carbonyl cyanide p-trifluoromethoxyphenylhydrazone. However, neither KCN nor NaN3 markedly inhibited the processing. Rat kidney mitochondria, which lack ornithine transcarbamylase, could also import and process the precursor to the mature form of the enzyme. The results indicate that the precursor for ornithine transcarbamylase is transported deeply into the isolated mitochondria, although not completely to the matrix space, in association with the proteolytic processing to the mature enzyme. The transport-processing system was found to be sensitive to uncouplers and to be common to several mitochondrial matrix proteins.