Conjugation of horseradish peroxidase to staphylococcal protein A with benzoquinone, glutaraldehyde, or periodate as cross-linking reagents

Horseradish peroxidase was conjugated to Staphylococcal protein A by three different two-step procedures using an increasing excess of peroxidase in the second step reaction. The yield of conjugated protein A was analyzed by SDS-polyacrylamide gel electrophoresis. Conjugation of peroxidase to protein A with benzoquinone or glutaraldehyde as cross-linking reagents at a 3- to 4-fold molar excess of peroxidase resulted in a high yield of coupled protein A with conjugates of low molecular size. Conjugation of peroxidase to protein A by the periodate method resulted in a high yield of coupled protein A with polymeric conjugates of large molecular size. Based on these results, conjugates produced with glutaraldehyde as cross-linking reagents were further analyzed. The capacity of the conjugates to precipitate human immunoglobulin evaluated by radial immunodiffusion was found to be reduced to about 50% of that of native protein A. Conjugates produced with glutaraldehyde as cross-linking reagent retained 70% of the enzyme activity of native peroxidase.