The utilization of d-amino acids by man; tryptophan and acetyltryptophan.

The recent findings (1, 2) that I( -)-histidine is not required for the maintenance of nitrogen balance in man prompted us to investigate the metabolic fate of d(+)-histidine in the human. The excretion procedure which we have previously employed (3) for determining the degree of utilization of d-amino acids seemed uniquely suited in this instance in which the nitrogen balance technique would not be effective. Five feeding experiments showed that within 9 hours of the administration of 0.01 mole of dl-histidine.HC1.HzO (equivalent to 775 mg. of d(+)-histidine) 692 f 35 mg. more histidine were excreted in the urine than after the feeding of 0.01 mole of I(-)-histidine*HCl.HzO. This difference in histidine output which is equivalent to 89.2 f 4.5 per cent of the d form ingested and the finding that on isolation this excess histidine proved to be of the d configuration suggest that d(f)-histidine is poorly or not at all utilized by man. The analytic procedure for the calorimetric estimation of histidine in the urine which was developed for this work is described in detail.