Two experiments were conducted to reconfirm the essentiality of histidine for maintenance of nitrogen equilibrium and to investigate the effect of dietary histidine levels on overall metabo lism of histidine-
14C in adult roosters. Each of White Leghorn roosters, older than 8 months of age, was given daily 70g of a diet containing 6.8% amino acid mixture for 21 days in Expt. 1 and 7 days in Expt. 2, respectively. Dietary histidine levels were changed from 0 to 0.118% in Expt. 1 and from 0 to 0.70% in Expt. 2. The concentration of hemoglobin, red blood cell and total plasma protein and the hematocrit value were determined. After L-histidine-U-
14C was dosed, the radioactivity in respiratory carbon dioxide and in total excreta was measured for 8 hours and for 24 hours, respectively. Then, the roosters were sacrificed by decapitation and the radioactivity in tissue protein fractions. The results obtained were summarized below: 1) The positive nitrogen balance was not maintained on the histidine-free diet, but 2) no effect of dietary histidine level was found in the weight of body and tissues except testis, the concentration of hemoglobin, red blood cell and total plasma protein and hematocrit value as clinically examined both in Expts. 1 and 2. 3) The recovery percentage of radioactivity in respiratory carbon dioxide and in tissue proteins increased or decreased when a histidine level in diets increased more than 0.118%, but 4) the recovery percentage of radioactivity in excreta was independent of the dietary histidine level.
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