1. The synthesis of nicotinamide–adenine dinucleotide from nicotinamide and nicotinic acid was compared over different time scales at both physiological (0.7 μmol/l) and high (0.2–3 mmol/l) substrate concentrations in erythrocytes from three patients with hypoxanthine–guanine phosphoribosyltransferase (hypoxanthine phosphoribosyltransferase, EC deficiency (including one Lesch–Nyhan patient) and from one patient with phosphoribosylpyrophosphate synthetase superactivity. The above disorders are associated with grossly altered erythrocyte nicotinamide-adenine dinucleotide levels.

2. At the physiological substrate concentration and incubation times up to 2 h, nicotinamide proved the most efficient nicotinamide–adenine dinucleotide precursor for erythrocytes from both patients and control subjects. The conversion of nicotinamide to its mononucleotide, but not further metabolism, was impaired in phosphoribosylpyrophosphate synthetase-mutant cells. The Lesch–Nyhan and phosphoribosylpyrophosphate synthetase-mutant cells were unusual in that both showed no further stimulation of nucleotide synthesis at 18 mmol/l Pi compared with 1 mmol/l.

3. At the high substrate concentrations, using 18 mmol/l Pi, nicotinamide was a poor precursor in all instances. Using nicotinic acid, nucleotide formation was 30-fold that from nicotinamide, reaching its maximum at 0.2 mmol/l. Conversion of nicotinic acid to nicotinamide–adenine dinucleotide in the phosphoribosylpyrophosphate synthetase-mutant cells was again grossly impaired.

4. There was no evidence for increased nicotinamide–adenine dinucleotide breakdown in the phosphoribosylpyrophosphate synthetase-mutant cells under any of the above conditions.

5. These results suggest that the differing nicotinamide–adenine dinucleotide levels in the two disorders cannot be related directly to the altered phosphoribosylpyrophosphate levels. The problem appears to be one of decreased synthesis in the phosphoribosylpyrophosphate synthetase-mutant cells, whereas the synthetic capacity in intact hypoxanthine–guanine phosphoribosyltransferase-deficient cells is neither enhanced nor inhibited by the raised nicotinamide–adenine dinucleotide levels.

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