Nitric oxide and cell respiration: physiology and pathology

Two enzymes, the soluble guanylate cyclase and cytochrome c oxidase, have been shown to be exquisitely sensitive to nitric oxide (NO) at low physiological concentrations. Activation of the soluble guanylate cyclase by endogenous NO and the consequent increase in the second messenger cyclic GMP are now known to control a variety of biological functions. Cytochrome c oxidase (complex IV, the terminal enzyme of the mitochondrial respiratory chain) is inhibited by NO. We have shown that NO generated by vascular endothelial cells under basal and stimulated conditions modulates the respiration of these cells in response to acute changes in oxygen concentration. This action occurs at the level of complex IV and depends on influx of calcium. Thus, NO plays a physiological role in adjusting the capacity of this enzyme to use oxygen, allowing endothelial cells to adapt to acute changes in their environment. We have, in addition, studied the effect of long-term exposure to NO on different enzymes of the respiratory chain in a variety of cell lines. Our results show that, although NO inhibits complex IV in a way that is always reversible, prolonged exposure to NO results in a gradual and persistent inhibition of complex I that is concomitant with a reduction in the intracellular concentration of reduced glutathione. This inhibition appears to result from S-nitrosylation of critical thiols in the enzyme complex because it can be immediately reversed by exposing the cells to high intensity light or by replenishment of intracellular reduced glutathione. Furthermore, decreasing the concentration of reduced glutathione accelerates the process of persistent inhibition. Our results suggest that, although NO may regulate cell respiration physiologically by its action on complex IV, long-term exposure to NO leads to persistent inhibition of complex I and potentially to cell pathology.