Influence of Zinc and Vitamin D on Plasma Amino Acids and Liver Xanthine Oxidase in Rats12

Plasma amino acids and liver xanthine oxidase activity were determined in rats fed a control diet with and without calciferol or fed diets providing 9% casein with and without added zinc and calciferol. Reduction of dietary protein resulted in a significant decrease in xanthine oxidase activity, concentrations of plasma threonine, methionine, isoleucine, leucine and phenylalanine, and in elevated histidine levels. Addition of calciferol to diets containing added zinc decreased the levels of plasma isoleucine, leucine, threonine and lysine. Lysine in plasma was also decreased by calciferol supplementation of the low protein diet and by the addition of zinc to this diet. Xanthine oxidase activity was lowest in animals fed the low protein diet without calciferol and zinc. The apparent interaction between zinc and vitamin D closely approached significance at the 5% level. The results indicated that xanthine oxidase activity in animals fed low protein decreased with increased plasma lysine, which was observed in animals maintained without dietary sources of zinc and calciferol.