Isolation and properties of multiple forms of histidine decarboxylase from rat gastric mucosa

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Abstract
The heterogeneity of histidine decarboxylase from rat gastric mucosa was studied. The partially purified enzyme was fractionated by preparative isoelectric focusing on a flat-gel bed by using narrow-pH-range carrier ampholytes and a short focusing time. The activity was resolved, with .apprx. 95% recovery, into 3 forms, designated I, II and III, with pI [isoelectric point] values of 5.90, 5.60 and 5.35, respectively. These 3 forms exhibited similar MW, indicating that the forms were not the result of different degrees of polymerization. By preparative refocusing each form refocused as a single peak of enzyme activity with reproducible pI, but a high loss of activity occurred with repeated focusing. Forms I, II and III were purified by the combined use of preparative isoelectric focusing and gel chromatography and other fractionation methods. The active forms could be distinguished by electrophoresis and isoelectric focusing on polyacrylamide gels, and displayed protein heterogeneity. These forms were found in the crude extract and in the partially purified preparations in the presence or absence of proteinase inhibitors. Form II had the highest specific activity, but all 3 forms had the same optimum pH and Km value for histidine.