Role of Hemin in the Acid Aggregation of Horseradish Peroxidase

Expulsion of hemin from horseradish peroxidase at pH 2.1 is accompained by aggregation of the protein. The process is completely reversible after short periods in acid and is partially reversible after long periods (48 hours). On gel filtration of peroxidase immediately after acidification, hemin and the protein elute together. Apoperoxidase (pH 2.1) elutes much later (about the same elution volume as neutral enzyme) than peroxidase (pH 2.1) indicating a hemin requirement for the aggregation. Analytical sedimentation velocity and sucrose density gradient sedimentation of peroxidase immediately after acidification reveal a 7.8S component with both 400-nm and 280-nm absorbance and a 4S component with 280-nm abosrbance. Sedimentation velocity studies of hemin in the presence of bovine serum albumin or rabbit immunoglobulin G reveal hemin aggregation without hemin precipitation or protein aggregation. A molecular weight of about 80,000 for the peroxidase-hemin aggregate has been determined by membrane osmometry.