Binding of xanthine oxidase to vascular endothelium - Kinetic characterization and oxidative impairment of nitric oxide-dependent signaling

Concentrations of up to 1.5 milliunits/ml xanthine oxidase (XO) (1.1 mu g/m l) are found circulating in plasma during diverse inflammatory events. The saturable, high affinity binding of extracellular XO to vascular endotheliu m and the effects of cell binding on both XO catalytic activity and differe ntiated vascular cell function are reported herein. Xanthine oxidase purifi ed from bovine cream bound specifically and with high affinity (K-d = 6 nM) at 4 degrees C to bovine aortic endothelial cells, increasing cell XO spec ific activity up to 10-fold, Xanthine oxidase-cell binding was not inhibite d by serum or albumin and was partially inhibited by the addition of hepari n. Pretreatment of endothelial cells with chondroitinase, but not heparinas e or heparitinase, diminished endothelial binding by similar to 50%, sugges ting association with chondroitin sulfate proteoglycans. Analysis of rates of superoxide production by soluble and cell-bound XO revealed that endothe lial binding did not alter the percentage of univalent reduction of oxygen to superoxide. Com parison of the extent of CuZn-SOD inhibition of native a nd succinoylated cytochrome c reduction by cell-bound XO indicated that XO- dependent superoxide production was occurring in a cell compartment inacces sible to CuZn-SOD, This was further supported by the observation of a shift of exogenously added XO from extracellular binding sites to intracellular compartments, as indicated by both protease-reversible cell. binding and im munocytochemical localization studies. Endothelium-bound XO also inhibited nitric oxide-dependent cGMP production by smooth muscle cell co-cultures in an SOD-resistant manner. This data supports the concept that circulating X O can bind to vascular cells, impairing cell function via oxidative mechani sms, and explains how vascular XO activity diminishes vasodilatory response s to acetylcholine in hypercholesterolemic rabbits and atherosclerotic huma ns, The ubiquity of cell-XO binding and endocytosis as a fundamental mechan ism of oxidative tissue injury is also affirmed by the significant extent o f XO binding to human vascular endothelial cells, rat lung type 2 alveolar epthelial cells, and fibroblasts.