Activity of carbamoyl-phosphate synthetase (ammonia) in isolated rat-liver mitochondria: cycling of carbamoyl phosphate in the absence of ornithine.

1. When NH3 was added to isolated rat-liver mitochondria incubated with succinate and bicarbonate, oxidation of succinate was stimulated to a greater extent than could be accounted for by the net formation of carbamoyl phosphate. 2. Measurement of the rate of incorporation of [14C]bicarbonate into carbamoyl phosphate, after the mitochondria had been preincubated with NH3 and unlabelled bicarbonate, revealed that flux through carbamoyl-phosphate synthetase (ammonia) was much greater than the net formation of carbamoyl phosphate indicated. 3. It is concluded that part of the carbamoyl phosphate produced in the absence of ornithine is degraded. About 20% of the degradation can be accounted for by non-enzymatic reactions of carbamoyl phosphate outside the mitochondria. It is proposed that the remainder of the degradation of carbamoyl phosphate occurs by partial reversal of the reaction of carbamoyl-phosphate synthetase.