Studies on succinate dehydrogenase

Abstract

Abstract 1. 1. Succinate dehydrogenase (succinate: (acceptor) oxidoreductase, EC 1.3.99.1) reacts immediately with competitive inhibitors to form spectrally detectable enzyme-inhibitor complexes, which are converted to succinate-reduced enzyme on the subsequent addition of succinate. 2. 2. The enzyme-inhibitor complexes formed with malonate, fumarate, maleate and methylene succinate differ from that with oxaloacetate in spectral characteristics, the latter showing a wide, diffuse band from 500 to 750 mμ. 3. 3. Pyrophosphate, a strong competitive inhibitor of succinate dehydrogenase, but which is not in any obvious way structurally related to succinate, has no effect on the spectrum. 4. 4. Dissociation constants of the enzyme-inhibitor complexes determined by spectral titration are in good agreement with kinetically determined inhibitor constants. 5. 5. The changes in spectrum associated with the formation of the enzyme-inhibitor complexes are explainable in terms of changes in polarity near the flavin prosthetic group and by the formation of charge-transfer complexes between the electron-donating inhibitor and the electron-accepting enzyme. 6. 6. The apparent incomplete reoxidation by fumarate of dithionite-reduced enzyme, reported by previous workers, can be explained by the formation of the complex between oxidized enzyme and fumarate.