Respiratory chain
In the insect stage the reducing equivalents that are generated by the citric-acid cycle and the pyruvate dehydrogenase complex are oxidised by the respiratory chain. There has been some doubt as to the presence of a functional complex I (NADH dehydrogenase) which was borne out by the lack of inhibition of respiration by rotenone, a potent inhibitor of NADH dehydrogenase activity. However it has now been demonstrated that NADH dehydrogenase can be inhibited by rotenone although at much higher concentration (Beattie and Howton, 1996). The ubiquinone (UQ9) that is reduced by the complexes I (NADH dehydrogenase) and II (SDH) and by the FAD-linked glycerol-3-phosphate dehydrogenase, is mainly oxidised by a cytochrome-containing respiratory chain which consists of complexes similar to the complexes III and IV of the classical mammalian respiratory chain. The cytochromes a,b and c have all been detected. As a consequence respiration in these cells is sensitive to cyanide, but due to residual alternative oxidase activity (see above) a considerable amount of cyanide-insensitive respiration may remain present and this can be inhibited by SHAM. The function for this residual alternative oxidase is not clear. It may serve for the oxidation of excess of reducing equivalents which otherwise would lead to the production of reactive oxygen species and so help in protecting the cell against oxidative stress, as has recently been shown for the plant alternative oxidase (Popov et al., 1997).