The succinate dehydrogenase ofEscherichia coli: subunit composition of the Triton X-100-solubilized antigen

Remarkably little is known about the molecular properties of succinate dehydrogenase (EC 1.3.99.1) in Escherichia coll. A number of workers have solubilized this membrane-bound respiratory enzyme [1-4] and have estimated its Mr-value by gel filtration. Values of 100000 M r have been quoted for the cytochrome-free enzyme [1] and 150000 M r for a succinate dehydrogenase cytochrome b 1 complex [4]. Unfortunately, problems related to purification have hindered resolution and analysis of the individual components present in the solubilized enzyme/enzyme complex. The genes encoding succinate dehydrogenase in E. coli have not been cloned yet, although analysis of succinate dehydrogenase (sdh) amber mutants has identified a 67000 M r inner membrane protein as a probable subunit [5]. We have adopted an immunochemical approach to the analysis of E. coli succinate dehydrogenase and in the previous communication we reported on the resolution by crossed immunoelectrophoresis (CIE) of two inner membrane immunogens possessing succinate dehydrogenase activity [6]. The major antigen (No. 22 of the CIE reference profile) was shown to possess both covalently bound flavin and nonheme iron, and was absent in several sdh mutants [6,7]. Available evidence suggested strongly that this enzyme antigen was associated with the aerobic respiratory chain of the organism. The minor immunogen displaying succinate dehydrogenase activity also possessed covalently bound flavin and was thought to correspond to fumarate reductase (EC 1.3.1.6). In this communication we determine the subunit composition of the two enzyme-active antigens using methods of precipitate excision and analysis [8].