Comparison between the metabolic effects of tryptophan and histidine deficiencies in the rat

Rats fed a tryptophan-free diet for 2 days showed less weight loss but more nitrogen loss than those fed a histidine-free diet. The metabolic nature of the body weight loss and of urinary nitrogen loss in rats fed the tryptophan-free diet was investigated in comparison with those fed the histidine-free diet. More body water was retained in rats fed the tryptophan-free diet than in those fed the histidine-free diet. This suggests that the lesser weight loss associated with tryptophan deficiency than with histidine deficiency may be due to the greater water content in rats fed the tryptophan-free diet. The tryptophan contents of the muscle, plasma, and liver decreased rapidly in rats fed the tryptophan-free diet, whereas the histidine contents of these tissues did not decrease as much as in rats fed the histidine-free diet. The conversion of radioactivity of (U-14-C)phenylalanine into respiratory CO-2 and liver and muscle proteins of rats fed the tryptophan- and histidine-free diets were compared. The relative recovery ratio of 14-C into respiratory CO-2 was found to be higher in tryptophan deficiency than in histidine deficiency, and the recovery ratio in muscle was lower in tryptophan deficiency than in histidine deficiency. These results suggest that the greater nitrogen loss seen in tryptophan deficiency than in histidine deficiency may be due to reduced synthesis of muscle protein, presumably due to the low free-tryptophan content in muscle resulting from the tryptophan-free diet.