1. The pathways of disposition of waste nitrogen were studied in a male infant with neonatal citrullinaemia during 3 months of normal growth on a protein-restricted diet supplemented by a mixture of amino acids and nitrogen-free analogues of amino acids.

2. The rate of excretion of urea averaged one-sixteenth, and the rate of excretion of total urinary nitrogen one quarter, of that reported in normal infants retaining the same amount of nitrogen per kg. Thus the efficiency of retention of dietary nitrogen for growth in this infant was very high. Plasma urea varied from 0·35 to 1·30 mmol/l.

3. The small amount of urea formed was apparently derived entirely from dietary arginine via arginase, as indicated by the observation that urea excretion and plasma urea were linear functions of arginine intake with intercepts of zero or less. These relationships imply that urea synthesis would cease at zero arginine intake. This hypothesis could not be verified because arginine intakes below 2·2 mmol day−1 kg−1 led to hyperammonaemia.

4. The high intake of arginine was required chiefly to replenish ornithine skeletons lost as citrulline in the urine (0·7 mmol day−1 kg−1). Smaller amounts of arginine were estimated to be required for protein synthesis and for creatine synthesis. The rate at which ornithine was utilized in reactions other than citrulline synthesis appeared to be excessively high.

5. Citrulline was the principal end-product of nitrogen metabolism, accounting for 51% of urinary nitrogen.

6. The results suggest that normal growth can be obtained in the virtual absence of the ability to dispose of waste nitrogen as urea.

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