Metabolism of exogenous histamine in rainbow trout (Oncorhynchus mykiss)

Information on the metabolism of exogenous histamine in fish is of much concern regarding the effect of dietary histamine on fish. Histamine catabolic enzymes, diamine oxidase and histamine N-methyl transferase were measured in the tissues of rainbow trout. Diamine oxidase was detected in the stomach, pylorus caeca and intestine. Histamine N-methyl transferase was detected only in the liver.A change in the contents of histamine and its metabolites was observed in the tissues of rainbow trout after oral administration of histamine. A large amount of imidazole acetic acid was observed in the serum, kidney, liver and muscle. On the other hand, 1-methyl histamine was detected only in the liver. Histamine and its metabolites, imidazole acetic acid and 1-methyl histamine were metabolized and diminished within 48 hr in all tissues.These results showed that histamine was metabolized by two metabolic routes in rainbow trout. One is the main pathway producing imidazole acetic acid by intestinal diamine oxidase and the other is the complementary pathway producing 1-methyl histamine by liver N-methyl-transferase.