Effects of chronic ethanol feeding on the metabolism of tryptophan and nicotinamide in rats.

Oxidation of tryptophan and urinary excretion of N1-methylnicotinamide were studied in rats fed a modified DeCarli-Lieber liquid diet of low (1.1 mg/liter or 5 mg/kg dry diet) and adequate (5.5 mg/liter or 25 mg/kg dry diet) nicotinamide content with and without ethanol replacing carbohydrate to 36% of total calories. Protein was restricted to 12% of calories so that the tryptophan content was about 0.16% of dry weight. Rats were fed for 30 or 40 days and feeding was restricted to 7 or 12 hours per day. N1-Methylnicotinamide was measured in urine collected during the last 2 days of feeding. At the end of the experimental period, fasted rats were injected intraperitoneally with 0.8 muCi of L-[ring-2-14C]tryptophan (0.12 or 0.11 muCi/mg tryptophan) or 0.7 muCi of [14C]formate (0.44 muCi/mg formate) per 100 g body weight. Expired CO2 was collected every 30 minutes or hourly for 5 or 6 hours. N1-Methylnicotinamide excretion was increased in ethanol-fed rats (P less than 0.05). The percent of injected [14C]tryptophan recovered as 14CO2 was increased in rats fed ethanol in a low niacin diet as compared to controls (P less than 0.01). Recovery of 14CO2 from [14C]formate oxidation was not affected by ethanol feeding. These findings suggest that the initial oxidation of tryptophan by tryptophan 2,3-dioxygenase is enhanced following chronic ethanol feeding.