EFFECT OF NITRIC-OXIDE AND CELL REDOX STATUS ON THE REGULATION OF ENDOTHELIAL-CELL XANTHINE DEHYDROGENASE

We have previously reported that endothelial cell (EC) xanthine dehydrogenase/xanthine oxidase (XD/XO) activity correlates inversely with the O-2 tension to which the cells are exposed. Whether this effect is related to the production of reactive O-2 species is unclear. We exposed bovine pulmonary artery EC to various conditions that altered the redox status of the cells: 1) hypoxia (3% O-2) add normoxia (20% O-2); 2) menadione (MEN), known to generate O-2 radicals; 3) catalase (CAT) and reduced glutathione (GSH), which detoxify H2O2; and 4) various NO-generating systems. Changes in intracellular XO and XO+XD activities were correlated with rates of extracellular H2O2 release from the same cells. Conditions that decreased extracellular H2O2 release (hypoxia, CAT, and GSH) produced significant and parallel increases in intracellular XO and XO+XD activities in a time-dependent fashion. MEN treatment increased extracellular release of H2O2 and subsequently reduced intracellular XO and XO+XD activities. NO-generating agents did not change extracellular release of H2O2 but significantly reduced XO and XO+XD activities. The latter effect was prevented by reduced hemoglobin. Scavengers of hydroxyl radicals reversed the inhibition of XO and XO+XD activities produced by MEN but not that produced by NO. While NO significantly inhibited XD/XO activity from rat epididymal fat pad, it did not affect XD/XO mRNA expression in these cells. We conclude that intracellular XD/XO activity is sensitive to changes in oxidant-generating and protective systems. Inhibition of XD/XO activity by NO may be mediated through direct binding of NO to the enzyme iron-sulfur moiety or to its sulfhydryl groups. The finding of an inhibition of XD/XO by NO provides an important and novel regulatory function for this molecule.