1. An enzyme present in raw fish that destroys thiamin and may produce thiamin deficiency in animals on a diet largely composed of raw fish. 2. A hydrolase cleaving thiamin into a pyrimidine moiety (2-methyl-4-amino-5-hydroxymethylpyrimidine) and a thiazole moiety (4-methyl-5-(2'-hydroxyethyl)thiazole); the pyrimidine moiety may appear in the urine as pyramin.