The effects of endogenous production of NO., catalysed by the mitochon
drial nitric oxide synthase (NOS), on mitochondrial metabolism were st
udied. The respiratory rates of intact mitochondria in State 4 were de
creased by 40% and 28% with succinate and malate-glutamate, respective
ly, in the presence of L-arginine (L-Arg); conversely, the O-2 uptake
with N-G-methyl-L-arginine (NMMA), a competitive inhibitor of NOS, was
increased. The production of NO. and the inhibition of the respirator
y rates were dependent on the metabolic state in which mitochondria we
re maintained: NO. production was probably supported by mitochondrial
NADPH, the latter maintained by the energy-dependent transhydrogenase.
In addition to the decline in the respiratory rate, an inhibition of
ATP synthesis was also observed (40-50%) following supplementation wit
h L-Arg. The dependence of the respiratory rates of mitochondria in St
ate 3 and cytochrome oxidase activities on O-2 concentrations with eit
her L-Arg or NMMA indicated that both processes were competitively inh
ibited by NO. at the cytochrome oxidase level. This inhibition can be
explained by the interaction of NO. with cytochrome oxidase at the bin
uclear centre. The role of NO. as a physiological modulator of cytochr
ome oxidase is discussed in terms of cellular metabolism.