Mitochondria recycle nitrite back to the bioregulator nitric monoxide

Nitric monoxide (NO) exerts a great variety of physiological functions. L-A rginine supplies amino groups which are transformed to NO in various NO-syn thase-active isoenzyme complexes. NO-synthesis is stimulated under various conditions increasing the tissue of stable NO-metabolites. The major oxidat ion product found is nitrite. Elevated nitrite levels were reported to exis t in a variety of diseases including HIV, reperfusion injury and hypovolemi c shock. Denitrifying bacteria such as Paracoccus denitrificans have a memb rane bound set of cytochromes (cyt cd(1), cyt be) which were shown to be in volved in nitrite reduction activities. Mammalian mitochondria have similar cytochromes which form part of the respiratory chain. Like in bacteria qui nols are used as reductants of these types of cytochromes. The observation of one-e(-) divergence from this redox-couple to external dioxygen made us to study whether this site of the respiratory chain may also recycle nitrit e back to its bioactive form NO. Thus, the aim of the present study was the refore to confirm the existence of a reductive pathway which reestablishes the existence of the bioregulator NO from its main metabolite NO2-. Our res ults show that respiring mitochondria readily reduce added nitrite to NO wh ich was made visible by nitrosylation of deoxyhemoglobin. The adduct gives characteristic triplet-ESR-signals, Using inhibitors of the respiratory cha in for chemical sequestration of respiratory segments we were able to ident ify the site where nitrite is reduced. The results confirm the ubiquinone/c yt bc(1) couple as the reductant site where nitrite is recycled. The high a ffinity of NO to the heme-iron of cytochrome oxidase will result in an impa irment of mitochondrial energy-production. "Nitrite tolerance" of angina pe ctoris patients using NO-donors may be explained in that way.