The major causative agent of scombroid poisoning is histamine formed by bacterial decarboxylation of histidine. The authors reported previously that histamine was exclusively formed by the psychrotrophic halophilic bacteria Photobacterium phosphoreum in scombroid fish during storage at or below 10°C. Moreover, histamine-forming ability was affected by two histidine decarboxylases: constitutive and inducible enzymes. This article reports the effect of various growth and reaction conditions, such as temperature, pH, and NaCl concentration, on the activity of two histidine decarboxylases that were isolated and separated by gel chromatography from cell-free extracts of P. phosphoreum. The histidine decarboxylase activity of the cell-free extracts was highest in 7°C culture; in 5% NaCl, culture growth was inhibited, and growth was best in the culture grown at pH 6.0. Moreover, percent activity of the constitutive and inducible enzymes was highest for the inducible enzyme in cultures grown at 7°C and pH 7.5 and in 5% NaCl. The temperature and pH dependences of histidine decarboxylase differed between the constitutive and inducible enzymes; that is, the activity of histidine decarboxylases was optimum at 30°C and pH6.5 for the inducible enzyme and 40°C and pH 6.0 for the constitutive enzyme. The differences in the temperature and pH dependences between the two enzymes extended the activity range of histidine decarboxylase under reaction conditions. On the other hand, histidine decarboxylase activity was optimum in 0% NaCl for the two enzymes. Additionally, the effects of reaction temperature, pH, and NaCl concentration on the constitutive enzyme activity of the cell-free extracts were almost the same as those on the whole histidine decarboxylase activity of the cell-free extracts, suggesting that the constitutive enzyme activity reflected the whole histidine decarboxylase activity.

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