Abstract
A comparison of the properties of histidine decarboxylase (EC 4.1.1.22) from the pylorus of the stomach and the fetus of rats indicated similarities in kinetic parameters, such as Km values and pH optima. Certain acidic compounds, carbonyl reagents, and substrate analogues inhibited the enzymes competitively and displayed indentical inhibitor dissociation constants. Heat inactivation studies employing partially purified preparations indicated indistinguishable rates of loss of activity. The results indicate that, except for a difference in the degree of inhibition by bisulfite, the properties of the pyloric and fetal enzymes were indistinguishable by kinetic criteria.
Gel filtration experiments, however, permitted partial resolution of the pyloric and fetal histidine decarboxylases. An additional species of enzyme emerged in cochromatography experiments involving the pyloric and fetal enzymes and with preparations derived from pylorus that had been aged for 2 months at -15°. The observed differences in elution volume of the pyloric and fetal enzymes suggest that the two species are discrete catalysts.
A partially purified histidine decarboxylase from the fundic portion of rat stomach was characterized. Kinetic studies indicated that it represents a second histidine decarboxylase, distinct from aromatic L-amino acid decarboxylase. The resistance of fundic histidine decarboxylase to known histidine decarboxylase inhibitors is discussed in relation to the difficulties encountered in attempting to inhibit histamine formation in vivo.
ACKNOWLEDGMENTS The authors are indebted to Mr. Gerald Lightkep for providing rat fetuses. Histamine assays were performed by Mrs. Louise A. Walker.
- Copyright ©, 1970, by Academic Press Inc.
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