Studies on the succinate dehydrogenating system. interaction of the mitochondrial succinate-ubiquinone reductase with pyridoxal phosphate

The inhibitory effect of pyridoxal phosphate on the Triton X-100 solubilized purified bovine heart succinate-ubiquinone reductase (Choudhry, Z.M., Gavrikova, E.V., Kotlyar, A.B., Tushurashvili, P.R. and Vinogradov, A.D. (1985) FEBS Lett. 182, 171-175) was studied. The kinetics of the enzyme inactivation by pyridoxal phosphate was found to be strongly dependent both qualitatively and quantitatively on the concentration of the protein-detergent complexes. In the diluted system the inactivation of the ubiquinonedepleted enzyme was completely prevented by the saturating concentrations of Q₂, carboxin, thenoiltrifluoroacetone and pentachlorophenol, i.e., by substrate and specific inhibitors of the enzyme. The protective effects of Q₂ and the inhibitors was employed to quantitate the affinities of the ligands to their specific binding sites. Strong difference in the affinity of Q₂ to the reduced and oxidized enzyme was found. When the soluble reconstitutively active succinate dehydrogenase was treated with pyridoxal phosphate, the reactivity of the enzyme towards low ferricyanide concentrations and its reconstitutive activity was significantly protected against aerobic inactivation.