Copurification of bovine milk xanthine oxidase and immunoglobulin

Xanthine oxidase, isolated from bovine milk, exhibited an A280:A450 nm ratio of 5.0. This ratio is reported to be indicative of highly purified enzyme preparations. Serum from a rabbit hyperimmunized against this enzyme fraction exhibited two precipitation lines when incubated with the protein in agarose double diffusion plates. Serum albumin, beta-lactoglobulin, alpha-lactalbumin, lactoferrin, casein, chymosin, and immunoglobulin were tested for reactivity. The second antigen was identified as bovine immunoglobulin. Commercial preparations of xanthine oxidase also contained immunoglobulin as a contaminant. IgG and IgA were present in Sigma (Grade III) fractions and IgM was identified in Boehringer Mannheim preparations. Immunofluorescent studies indicated that xanthine oxidase antiserum reacted with the capillary endothelium of bovine heart. Absorption of this antiserum with bovine IgG abrogated this reaction. These findings may explain apparent discrepancies between reported immunohistological association of xanthine oxidase in heart capillary endothelial cells and the absence of detectable enzymatic activity.