Human salivary peroxidase and bovine lactoperoxidase are cross-reactive

Peroxidases are abundant in nature, and the primary function of mammalian peroxidases is to catalyze the peroxidation of halides and pseudohalides. Previous studies have shown that antibodies raised against bovine lactoperoxidase moderately cross-react with human salivary peroxidase, a feature that has been used in the present study to examine epitopes common to the antigen and human salivary peroxidase. Polyclonal antibodies against a highly purified preparation of bovine lactoperoxidase were raised in rabbits, and their properties were examined. In double-immunodiffusion experiments, the two enzymes showed partial identity, and in competitive radioimmunoassay and enzyme-linked immunosorbent assay, lactoperoxidase replaced the labeled and coated antigen, while salivary peroxidase did not. However, salivary peroxidase from human and rat saliva samples and the purified enzyme in its non-reduced, reduced, and de-glycosylated forms were recognized by these antibodies, as analyzed by Western blot analysis and immunodetection. The major activity of these antibodies was directed against the protein core of the antigen. Immunodetection of the peptide fragments of bovine lactoperoxidase and human salivary peroxidase revealed structural differences in the two enzymes. These antibodies also precipitated an in vitro translation product from rat-parotid-gland cell lysate that, on SDS-PAGE, compared favorably with the expected molecular weight of a de-glycosylated peroxidase. The antibodies partly inhibited the enzyme activity of salivary peroxidase and the peroxidase in rat parotid gland lysate, but the enzyme activity of lactoperoxidase was not affected by addition of anti-lactoperoxidase IgG between 25 and 400 micrograms/mL. The enzyme activity remained unchanged in all samples when pre-immune IgG was used.