Relative rates of turnover of subunits of mitochondrial proteins.

1. The double-isotope concept [Arias, Doyle & Schimke (1969) J. Biol. Chem. 244, 3303--3315] for the measurement of protein turnover was used to estimate the turnover rates of protein subunits from rat liver submitochondrial fractions resolved by means of polyacrylamide-gel electrophoresis in the presence of sodium dodecyl sulphate. NaH14CO3 and [5-3H]arginine were used as first and second precursors respectively. 2. Marked heterogeneity of protein subunit turnover rates is seen for protein subunits from water-soluble, salt-soluble and Tween 20-soluble mitochondrial proteins. 3. Much lower heterogeneity is seen in the turnover of protein subunits in Triton X-100-soluble material not binding to DEAE-cellulose at low ionic strength. The relative rates of turnover of proteins in this fraction are lower than for proteins in any other submitochondrial fraction. This fraction contains the integral membrane proteins. 4. Incorporation of [3H]arginine into subunits of the cytochrome oxidase complex is greatest for subunits with molecular weights in excess of 20000. 5. No correlation is seen between protein subunit size and the rate of turnover of the protein subunits in any of the submitochondrial fractions.