Abstract
The metabolic fate of the carbon skeleton of L-(U-14C)-histidine has been investigated in growing rats fed diets containing different percentages of protein calories (0, 5, 10, 15 and 30 PC%) at 410 kcal of metabolizable energy per 100g diet.
The incorporation of 14C into body protein at 12 hr after injection of 14C-histidine was about 70% of the dose in rats fed 0 to 10 PC% diets, though the value was reduced in rats fed higher PC% diets. The expired 14CO2 production was depressed in the low protein groups, and it showed an inverse pattern to that of 14C incorporation into body protein. Urinary excretion of 14C was about 20% of the dose in all dietary groups. The activities of hepatic histidase, urocanase and histidine-pyruvate aminotransferase were increased in the 30 PC% group.
These results indicate that the metabolic response of histidine to dietary protein changes around 10 PC%, where growth rate and body protein retention reached approximate plateaus.
The nutritional significance of the metabolism of histidine has been discussed and compared with that of leucine, alanine and serine reported previously.
