The prosthetic group of myeloperoxidase.

In general, when hemoproteins are treated with acidified acetone, the protein is precipitated, while the heme remains in solution. Exceptions are cytochrome c, lactoperoxidase, and myeloperoxidase; the linkages between heme and protein in these instances being covalent. While the nature of the linkages in cytochrome c and lactoperoxidase have been characterized, that present in myeloperoxidase remains unknown. Previous studies have, however, indicated that the linkage is not the heine c type [1] and that low yields of the prosthetic group are obtained with reagents which readily cleave ester bonds such as methanolic HC1 [2] and HI in acetic acid [3]. In this report we consider the possibility of an amide bond. To differentiate between an ester and an amide linkage in myeloperoxidase, we have investigated the lability of the bond toward sodium methoxide. In comparing our results with those Morrison [4] obtained with lactoperoxidase (ester bond), we conclude that the bond in myeloperoxidase is significantly more stable, and therefore amide. In addition, examination of the products of methoxide and hydrazine cleavage show that the derivatives of myeloperoxidase heme obtained by such hydrolysis are very closely related to protoheme IX. A preliminary account of portions of this study has been reported [5].