Occurrence of a novel acetylated amino acid, Nα-acetylhistidine, in skeletal muscle of freshwater fish and other ectothermic vertebrates

The occurrence of Nα-acetylhistidine (NAH) in skeletal muscle of 91 species of freshwater fish and 9 species of other ectothermic vertebrates was investigated, with consideration of phylogenetic relationships. Of the 91 freshwater fish species examined, 13 species (7 cichlids, 5 anabantids, and 1 catfish) contained considerable amounts (> 1 µmol/g) of NAH in their skeletal muscles. The highest level (10.37 µmol/g) of NAH was found in the tissue of Betta splendens (Siamese fighting fish). Moreover, the NAH contents in the tissues of Trichogaster trichopterus (three spot gourami), Kryptopterus bicirrhis (glass catfish), Oreochromis niloticus (Nile tilapia), Mikrogeophagus ramirezi (ram cichlid) and Parachromis managuensis (Guapote tigre) were 3.17–6.16 µmol/g. The skeletal muscle of amphibians (5 species) and reptiles (4 species) had a low level (< 0.25 µmol/g) of NAH. The present findings clearly demonstrate NAH as the fifth imidazole-related compound, in addition to histidine, carnosine, anserine and ophidine (balenine), recognized as a major non-protein nitrogenous constituent in the skeletal muscle of vertebrate animals.