Abstract
The effect of sodium nitrite on the respiratory chain of porcine skeletal muscle mitochondria was investigated in vitro under conditions relevant to the meat curing process, that is, at pH 6.0 under reducing (anaerobic) conditions with sodium nitrite (3 mM, ~200 ppm). Other concentrations of nitrite were used to include the range relevant to curing and beyond. Improvements were made to the extraction of mitochondria by alterations to the homogenisation procedure and the extraction medium. Observations of respiratory chain cytochromes were made using differential spectrophotometry. For observations at -160°C, where the cytochrome absorption bands are sharpened and intensified, low temperature cells were designed and constructed. The redox potential for the reduction of nitrite to nitric oxide is close to that for the reduction of oxygen to hydroxyl ions and attention was therefore focussed upon the terminal portion of the respiratory chain. At low concentrations, nitrite exerted a cyanide-sensitive oxidation of the respiratory chain cytochromes which was gradually inhibited by increasing concentrations of nitrite. At concentrations above 3 mM, nitrite produced a direct, non-enzymic oxidation of the cytochromes which superseded the inhibitory effect. Model systems studies, which used physiologically active ferrocytochrome c produced by reduction of the Terri (oxidised) form with H2/Pd, confirmed and extended the in vitro experiments and demonstrated the "inhibitory zone" of nitrite under aerobic conditions. Nitrite also inhibited the oxygen uptake of a mitochondrial suspension (measured polarographically). This effect corresponded to the inhibitory action of nitrite on cytochrome oxidation and is probably the result of inhibition of cytochrome oxidase by formation of a complex between reduced cytochrome a3 and the nitrosyl radical. The chronic or acute administration of nitrite in vivo did not affect the drug metabolising enzymes of rat liver. In addition, skeletal muscle mitochondria did not appear to influence either the synthesis or breakdown of nitrosamines in vitro.