Mitochondrial turnover in animal cells. Half-lives of mitochondria and mitochondrial subfractions of rat liver based on [14C]bicarbonate incorporation.

The half-lives of proteins of mitochondria and a well characterized set of mitochondrial subfractions have been estimated using two methods based on incorporation of a minimally reutilized amino acid precursor, [‘‘Clbicarbonate. The apparent average rate constants of protein degradation were measured on rat liver homogenate, mitochondria, and distinct outer compartment, inner membrane, and matrix compartments. The half-lives were found to be 3.3 days for liver homogenate, 3.8 days for mitochondria, 2.8 days for outer compartment, 3.8 days for inner membrane, and 3.9 days for matrix. The value obtained for inner membrane is not significantly different from that of matrix, but both are significantly greater than outer compartment. In an alternative method, [14C]bicarbonate and r3H] leucine were used together to compare the relative rates of degradation of these subcompartments. The double label procedure yielded relative half-lives which were again in the order inner membrane equal to matrix and greater than outer compartment. When the absolute values of these rates were extrapolated from the information obtained in the double and single precursor studies, the extrapolated values agreed well with those obtained by direct measurement. The half-lives measured here in mitochondria and mitochondrial subfractions were based on [l*C]bicarbonate metabolism and were lower than values reported with highly reutilized precursors. When bicarbonate was used as the first precursor in a double label study, it gave results consistent with single label incorporation studies. Results presented here are consistent with a model of mitochondrial turnover in animal cells in which outer compartment is degraded independently of the inner compartment. The latter, consisting of inner membrane and matrix, may be degraded as a unit.