The biosynthesis and destruction of anserine and carnosine in the rat were investigated in vivo using radioactive β-alanine, histidine and methylhistidine. In the normal rat, the incorporation of 14C-histidine and 14C-β-alanine into carnosine was found to proceed at significant rates, but their incorporation into anserine was hardly detectable. Radioactive anserine arising from 3H-Nπ-methylhistidine was detected in gastrocnemius muscle of the rat pretreated with β-alanine. Neither anserine nor carnosine biosynthesis was found in liver, but was found in gastrocnemius muscle. At 8 hr after the administration of a single dose of 14C-histidine or 14C-β-alanine, the incorporation of radioactivity into carnosine attained a plateau, and then maintained the level for the investigated period. Incorporation of 14C-histidine into carnosine was increased about 2-fold when rats were injected in advance with f-alanine. The half-lives of histidine and β-alanine were 0.67 and 0.41 hr in liver, and 3.6 and 2.3 hr in gastrocnemius muscle, respectively. β-Alanine and histidine in rat gastrocnemius muscle disappeared at the rates of 39 and 29 nmol/wet tissue (g)/hr, respectively. The half-life of carnosine, as was determined from the decrease in carnosine contents in the gastrocnemius muscle of a rat fed a histidine-free diet, was 29 days. The rate constant of carnosine biosynthesis in rat gastrocnemius muscle was 0.321 μmol/DNA (mg)/day, that is, 0.286 μmol/wet tissue (g)/day.
