Studies on succinate dehydrogenase

• 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.