1. The biochemical response to an intravenous alanine load of 0.25 g/kg was studied in nine adult female relatives of children with ornithine carbamoyltransferase deficiency. Six were classified as affected by partial deficiency and three as unaffected.

2. The plasma ammonium concentration showed no change after the alanine load in the unaffected group, but marked increases occurred in all but one of the affected group. The maximum rate of urea synthesis after the alanine load was decreased by 37% (P = 0.02) and delayed by 43% (P = 0.02) in the affected group.

3. In the affected group a low rate of urea synthesis was associated with high urinary orotate excretion, high maximum plasma ammonium concentration and delay in the time taken to reach the maximum rate of urea synthesis (Kendall concordance W = 0.55, P < 0.05).

4. The effects of a higher dose of alanine and of oral protein were compared. The alanine load of 0.25 g of alanine/kg body weight was shown to provide an adequate stimulus to urea synthesis with a more rapid return of ammonium concentration to the pre-load level than with the protein load.

5. The implication of these results in determining the distribution of flux control of urea synthesis, the discrepancy between them and predicted results and the necessary modifications to quantitative simulations are discussed.

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