Regulation of nicotinamide–adenine dinucleotide synthesis in erythrocytes of patients with hypoxanthine–guanine phosphoribosyltransferase deficiency and a patient with phosphoribosylpyrophosphate synthetase superactivity

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Abstract
The synthesis of nicotinamide-adenine dinucleotide from nicotinamide and nicotinic acid was compared over different time scales at both physiological (0.7 .mu.mol/l) and high (0.2-3 mmol/l) substrate concentrations in erythrocytes from three patients with hypoxanthine-quanine phosphoribosyltransferase (hypoxanthine phosphoribosyltransferase, EC 2.4.2.8) 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. 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. 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. There was no evidence for increased nicotinamide-adenine dinucleotide breakdown in the phosphoribosylpyrophosphate synthetase-mutant cells under any of the above conditions. 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.