Heterogeneity in anticatalytic activity of antibody specific for horseradish peroxidase

Rabbit antibodies specific for horseradish peroxidase are heterogeneous in their ability to inhibit enzyme activity. Heterogeneity was demonstrated by fractionation of the total antiperoxidase pool by differential ammonium sulfate precipitation and differential elution of antibody from enzyme affinity columns. Both fractionation methods yielded antibody subpopulations that differed in anticatalytic activity. Some antibody subpopulations decreased enzyme activity almost completely at low molar ratios of antibody to peroxidase. Other subpopulations were not effective inhibitors even at great molar excess. Admixture experiments demonstrated that inefficient antibody pools decreased the anticatalytic effect of highly inhibitory antibody. The degree of inhibition observed with unfractionated antiserum is a reflection of the interaction of various antibody subpopulations with the enzyme. No correlation was found between the immunoglobulin class of antiperoxidase and anticatalytic efficiency in analyses of numerous antisera. The determinant specificity of an antiperoxidase molecule determines its anticatalytic ability. A peptide fragment (mol. wt 22,500) of peroxidase prepared by partial tryptic digestion bount 60 to 70% of the total antiperoxidase in a number of antisera. However, the peptide did not bind inhibitory antibodies.