Metabolism of L-Histidine, Carnosine, and Anserine in Skipjack Tuna

The effects of some physiological conditions on the levels of L-histidine, carnosine, and anserine were examined in skipjack tuna, Katsuwonus pelamis. Also examined were the turnover rates of L-[U-14C]histidine in skipjack blood and skeletal muscle, the metabolic interrelationship among several organs, and interconversion between histidine and the dipeptides. In white muscle of control tuna, these compounds in total occurred at a mean ± SD concentration of 147.4 ± 3.5 μmol/g muscle (n = 5) (histidine, 93.5 ± 9.2; carnosine, 2.90 ± 1.66; anserine, 51.1 ± 10.2 μmol/g muscle), but in red muscle the total concentration was only 21.3 ± 2.1 μmol/g muscle. Exhaustive burst swimming did not lead to any large changes in these concentrations, whereas starvation markedly affected them. In white muscle, histidine decreased to 30% and 4% of control levels after 5 and 12 days of starvation, respectively; carnosine increased 3.7- and 8.6-fold, while anserine decreased in the initial stage but recovered to control levels during sustained starvation. A similar pattern also was observed in red muscle. The metabolic half-life of 14C-labeled histidine was 0.72 ± 0.124 h (n = 3) in skipjack blood, and 66 and 52.5 h in white and red muscle, respectively. The label was incorporated into muscle carnosine and anserine, though the incorporation was fairly slow. The label was transported rapidly from blood into liver and muscles and also from white muscle into red.