Significance of xanthine oxidase in capillary endothelial cells

Antibodies to xanthine oxidase from bovine milk lipid globules localize the antigen in capillary endothelial cells of many tissues including liver, heart, lung and kidney, but not in other epithelial, endothelial or mesenchymal cell types. The antigen from bovine capillaries was purified by immunoaffinity chromatography and shown by chemical, enzymatic and immunological methods to be indistinguishable from milk xanthine oxidase. Using an ultrasensitive radioimmunoassay, concentrations of this protein were found to be 1 000-10 000-fold higher in capillary endothelial cells than in other cells studied except mammary epithelial cells which were also rich in xanthine oxidase. Similar results were obtained with human cells and tissues. In the cytoplasm of capillary endothelial cells, xanthine oxidase was present as a dehydrogenase which was rapidly converted to the O-2-radical-producing oxidase form after release by cell disrupture. This conversion was partly prevented by addition of thiol reagents. Free xanthine oxidase was not detected in human serum, even from patients with extensive capillary lesions. However, specific-apparently constitutive--antibodies (IgG) were present at high concentrations (1-8% of total IgG) in the sera of all individuals tested. A role of these specific antibodies in the removal of the potentially hazardous oxidase form of xanthine oxidase is discussed.