14CO2 expiration after 14C-histidine administration in normal and uremic men ingesting two levels of histidine.

The rates of histidine degradation were investigated in six normal and three chronically uremic men who were not undergoing dialysis therapy. The nine men were studied in the postabsorptive state after they had ingested one or more of the following diets for the indicated periods of time: a 40 g protein diet providing about 1100 mg/day of histidine for 27 +/- 9 SD days (seven studies), an amino acid diet providing only 65 mg/day of histidine (histidine-deficient diet) for 33 +/- 3 days (eight studies), and an amino acid diet providing about 1125 mg/day of histidine (histidine-replete diet) for 32 +/- 7 days (six studies). Diets are listed in the order of administration; five men received all three diets. After fasting overnight, subjects received an intravenous injection of 25 microCi/70 kg body weight of L-[ring-2-14C]-histidine (eight men) or L-carboxyl-14C]-histidine (one man), and expiration of 14CO2 was measured continuously for 2 h. With both tracers, expiration of 14CO2 fell with ingestion of the histidine-deficient diet (p less than 0.005) and then increased with intake of the histidine-replete diet (p less than 0.05). Free histidine in plasma and muscle also decreased with the histidine-deficient diet and rose with the histidine-replete diet. If other tissue free histidine pools changed similarly and protein turnover was not very different with the three diets, then the 14CO2 expiration data and the estimated specific activity of 14C-histidine indicate that histidine degradation fell markedly with the histidine-deficient diet and then increased with the histidine-replete diet. In uremic patients as compared to normal subjects no differences in the magnitude of 14C-histidine degradation or in the pattern of 14CO2 expiration were observed.