Serum catalase: reversibly formed charge isoform of erythrocyte catalase

The different electrophoretic mobilities of erythrocyte and serum catalase (EC 1.11.1.6) were confirmed and the causes responsible for their differences were examined. The presence of a catalase-binding protein in serum that could form a complex with erythrocyte catalase was excluded by incubating serum proteins with erythrocyte catalase. No new unequivocal catalase bands representing a catalase-binding protein were detected. The erythrocyte and serum catalase proved to be charge isoforms: their molecular masses, estimated by gel permeation chromatography or polyacrylamide gel electrophoresis in a nondenaturing system, were very similar, whereas their electrophoretic mobilities were different. Assay of serum catalase by gel permeation and hydrophobic chromatography yielded a product with the same electrophoretic mobility as that of erythrocyte catalase. Different dilution of erythrocyte catalase with human sera led to a gradual decrease of its mobility, 20-fold or greater dilution yielding the same results as for serum catalase. Similarly, when serum catalase was diluted 20-fold or more with 60 mmol/L phosphate buffer, it migrated similarly to erythrocyte catalase. I detected no effect of dialyzable serum ligands, NADPH, or protection of SH groups on the electrophoretic mobility of either catalase isoform. I conclude that formation of charge isoforms of catalase is caused by a reversible, conformational modification due to matrix effect of serum.