Histidine decarboxylase from rat gastric mucosa: heterogeneity and enzyme forms modification

Rat gastric mucosal histidine decarboxylase is shown to exist in the crude extract as three active charged forms which are separable by isoelectric focusing. The distribution of enzyme activity in the three forms is independent of the homogenizing medium and of the isoelectric focusing procedure indicating that the heterogeneity does not arise during isolation. Multiple forms correspond to histidine decarboxylase and are related neither to 3,4-dihydroxyphenylalanine decarboxylase nor to the result of aggregation. This charge difference between the enzyme forms changes according to the time of storage and to the temperature, leading to the generation of less negatively charged species. The conversion cannot be attributed to proteolytic degradation nor to differences in stability between forms. The data indicate that these alternative charged states may really result from an in vivo post-translational modification of the enzyme.