The synthesis of histamine is not the only way of histidine metabolism. In the fish putrefaction process, an even more important role is assigned to catabolic formation of glutamate catalyzed by most of the bacteria. The first step of this metabolism is the separation of ammonia from histidine by histidine ammonia lyase (HAL) or histidine with the formation of urocanic acid. The latter has been found in sufficiently higher concentration than histamine (4.74 and 0.19 mg per 100 mL of mackerel respectively) after storage of fish for 8 days at 0๐C. Urocanic acid, side by side with histamine, may be one of the causative agents of scombroid poisoning. Biogenic amines like putrescine or cadaverine are candidates for these agents as well.