Isolation and characterization of five neutral isoenzymes of horseradish peroxidase.

Five components of neutral horseradish peroxidase were isolated and purified by means of column chromatography, and designated B1, B2, B3, C1, and C2, respectively. All the components contained 2 atoms of calcium and 16.8-to-21.0% as much carbohydrate as in the enzyme molecule. They were very similar to one another with respect to physicochemical and chemical properties such as molecular weight, molar absorption coefficient, rate constants of the catalytic reaction and dissociation of cyanide compound, but were dissimilar with respect to isoelectric point. Values of the isoelectric points determined from column isoelectric focusing at 20 degrees C were 5.75 (B1), 7.15 (B2), 7.10 (B3), 9.40 (C1), and 9.63 (C2). However, these values varied significantly depending upon the method and conditions of the focusing. The acid-alkaline titration curves of components B2 and C1 were flat in the pH region of 6 to 9. The facts suggest that a slight difference in the number of ionized groups of the components causes a large difference in the isoelectric points.