In order to estimate the conversion efficiency of tryptophan-nicotinamide in freshwater fishes, carp and crucian, the liver 3-hydroxyanthranilate 3,4-dioxygenase (3-HAO) and aminocarboxymuconate-semialdehyde decarboxylase (ACMSD) activities were measured. The activity ratios of 3-HAO/ACMSD in carp and crucian were both around 5. These values were extremely low compared with that of rat (650), which can synsthesize a significant amount of nicotinamide from tryptophan. The activities of quinolinate phosphoribosyltransferase in carp and crucian were below the limit of detection. From these results, it was suggested that fishes cannot biosynthesize nicotinamide from tryptophan.
