Ornithine carbamoyltransferase of rat liver mitochondrial matrix (subunit Mr = 36,000) is synthesized extra-mitochondrially as a larger precursor (subunit Mr = 39,400) which is transported into mitochondria, in association with post-translational proteolytic processing. Rat liver mitochondria convert the precursor to the mature
enzyme as well as to a 37,000-Mr product, a possible intermediate of the processing [Mori, M., Miura, S., Tatibana, M., and Cohen, P.P. (1980) Proc. Natl Acad. Sci. USA, 77, 7044-7048]. A
protease responsible for the conversion of the precursor to the 37,000-Mr product was purified 140-fold from the matrix fraction of rat liver mitochondria. The
protease had an estimated Mr of 108,000 and an apparent pI of 5.5. Mature
ornithine carbamoyltransferase (0.5 microgram) did not inhibit the cleavage of the precursor by the
protease and presumably the latter cleaves a specific site on the extrapeptide of the
carbamoyltransferase precursor. The
protease was inhibited by
metal-chelating
reagents such as
EDTA,
o-phenanthroline and
zincon and by a high concentration (1 mM) of
leupeptin. It did not cleave several of the
protein and
peptide substrates tested including the precursor of mitochondrial
carbamoyl-phosphate synthetase I. Apparently the same
protease activity is widely distributed among mitochondria of rat kidney, spleen, heart and
ascites tumor cells, all of which lack
ornithine carbamoyltransferase. A possible physiological role of the
protease in the processing of the
mitochondrial protein precursors is discussed.