Effect of Histidine Intake and Hepatic Histidase Activity on the Metabolism of Histidine in Vivo

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Abstract
The metabolic fate of histidine was investigated in young male rats fed diets containing from 0 to 0.6% of L-histidine and 14.7% of a mixture of other L-amino acids. Liver histidase activity was not influenced by dietary histidine content. Tissue histidine concentration increased gradually, then more sharply, as dietary histidine content was increased from 0 to 0.325% then it plateaued. During ad libitum consumption of these diets containing L-[U-14C]histidine, 14CO2 production remained low until dietary histidine content exceeded 0.25%, then it increased linearly until with 0.6% of dietary histidine 25% of the absorbed dose was oxidized. The proportion of absorbed 14C incorporated into tissue proteins was high when dietary histidine content was low and decreased as dietary histidine content increased. Histidine requirement estimated from the inflection point of the oxidation curve was in good agreement with values determined from growth response and urinary nitrogen excretion. Liver histidase activity increased almost linearly with increasing dietary casein content. Rats fed 12% casein oxidized more of a load of L-[ring-2-14C]histidine (500 mg/100 g rat) to CO2 than those fed 8% casein. Plasma histidine clearance rate after intraperitoneal injection of 50 mg histidine/100 g rat increased as the casein content of the diet was increased from 4% to 8%, 12% and 40%.

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