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J. Biol. Chem., Vol. 255, Issue 21, 10248-10255, 11, 1980

The effects of ornithine on mitochondrial carbamyl phosphate synthesis

NS Cohen, CW Cheung and L Raijman

When carbamyl phosphate is synthesized by isolated liver mitochondria in the absence of ornithine, the following is observed. 1. Carbamyl phosphate synthesis is progressively inhibited during the first 2 min of incubation, after which it reaches a steady rate which is 8% of that observed with ornithine. 2. Within 2 min, carbamyl phosphate accumulates in the matrix to very high levels (16 nmol/microliter) which are inhibitory to carbamyl phosphate synthetase; afterwards the levels decline, and by 15 min they are essentially the same as those found in the presence of ornithine (3 to 4 nmol/microliter). 3. The decrease in matrix carbamyl phosphate is the result of decreased synthesis and of leakage from mitochondria; 90% of the total carbamyl phosphate is in the medium after 10 min. 4. Addition of ornithine after 5 to 15 min results in rates of carbamyl phosphate synthesis essentially identical with those found when ornithine is added at the start of the incubation. 5. ATP synthesis appears to be somewhat inhibited. 6. When the accumulation of carbamyl phosphate in the medium is prevented by converting that compound to carbamyl aspartate, the total synthesis of carbamyl phosphate is not increased. Three factors appears to be involved in the low capacity of mitochondria to synthesize carbamyl phosphate in the absence of ornithine: a. inhibition of carbamyl phosphate synthetase by matrix carbamyl phosphate early in incubations; b. slight inhibition of ATP synthesis throughout the incubations; c. quantitatively most important, a very low activity of carbamyl phosphate synthetase even when matrix carbamyl phosphate is low and ATP is not limiting.
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