THE INHIBITION OF CYTOCHROME-C-OXIDASE BY NITRIC-OXIDE USING S-NITROSOGLUTATHIONE

The effect of the free radical nitric oxide (NO) on the activity of is olated cytochrome c oxidase was investigated by using ferrocytochrome c as an electron donor, and the system SNOG/DTT, which produces a stea dy-state NO concentration similar to that expected to be found in vivo . The initial electron entry into the heme a/Cu-a center and the initi al rate of the electron transfer between the two hemes were not affect ed by the presence of NO. Under oar conditions, the rate of inhibition of cytochrome c oxidase was found to be dependent both on the SNOG (N O concentration) and on the ferrocytochrome c concentration (electron entry rate). The data confirm that NO binds exclusively at the binucle ar center, and that the NO binding in these conditions requires the pr esence of an intermediate populated only in turnover. Accordingly, we found that the rate of inhibition is directly related to the electron entry rate. In addition, a residual activity seems to be present in cy tochrome c oxidase in the presence of nitric oxide, suggesting that NO can act as an electron acceptor to cytochrome c oxidase in the presen ce of oxygen. (C) 1997 Elsevier Science Inc.