Abstract

Mitochondria comprise one of the main sites of intracellularly produced oxidative stress. The recent discovery of an enzyme, mitochondrial nitric oxide synthase (mtNOS), has put a new perspective on studies of oxidative stress. This enzyme catalyzes the reaction of L-arginine to L-citrulline and nitric oxide (NO). NO, once thought of only as a noxious gas, is now known to have numerous biological functions. One of the most prominent is its tendency to react with ROS to form RNOS (Reactive Nitrogen and Oxygen Species). These RNOS have been indicated in several forms of oxidative stress, including tyrosine nitration, lipid peroxidation, and mitochondrial swelling. The procedures in the present experiment utilized a rat liver mitochondria model to investigate mtNOS activity. mtNOS was selectively stimulated and/or inhibited while mitochondria were either not respiring, respiring only through respiratory chain Complex I, or respiring only through respiratory chain Complex II. The results indicate that mtNOS may be associated with Complex I in such a way that when Complex I is active, mtNOS is able to function properly. However, when Complex I is not active, mtNOS is not able to carry out its normal functions. The results also show that lipid peroxidation and tyrosine nitration cannot both be caused by peroxynitrite, as was previously assumed. It is more likely that lipid peroxidation is caused by peroxynitrite and that tyrosine nitration is caused by another type of radical, one that will be called a ‘nitrating radical’. These findings could have relevance for future studies of apoptosis, including those related to cancer treatment and Alzheimer’s disease or other degenerative diseases.