Effects of age and calorie restriction on tryptophan nitration, protein content, and activity of succinyl-CoA:3-ketoacid CoA transferase in rat kidney mitochondria

This study examined the protein targets of nitration and the consequent impact on protein function in rat kidney mitochondria at 4, 13, 19, and 24 months of age. Succinyl-CoA transferase (SCOT), a rate-limiting enzyme in the degradation of ketone bodies, was the most intensely reactive protein against anti-3-nitrotyrosine antibody in rat kidney mitochondria. However, subsequent mass spectrometric and amino acid analyses of purified SCOT indicated that tryptophan 372, rather than a tyrosine residue, was the actual site of simultaneous additions of nitro and hydroxy groups. This finding suggests that identification of nitrated tyrosine residues based solely on reactivity with anti-3-nitrotyrosine antibody can be potentially misleading. Between 4 and 24 months of age, the amounts of SCOT protein and catalytic activity, expressed per milligram of mitochondrial proteins, decreased by 55 and 45%, respectively. SCOT, and particularly its nitrated carboxy-terminal region, was relatively more susceptible to in vitro proteolysis than other randomly selected kidney mitochondrial proteins. The age-related decreases in SCOT protein amount and catalytic activity were prevented by a relatively long-term 40% reduction in the amount of food intake. Loss of SCOT protein in the aged rats may attenuate the capacity of kidney mitochondria to utilize ketone bodies for energy production.