Mammalian α-Keto Acid Dehydrogenase Complexes

Lipoate succinyltransferase (succinyl-CoA: dihydrolipoate S-succinyltransferase), a core enzyme of the 2-oxoglutarate dehydrogenase complex, has been highly purified. The enzyme has a sedimentation coefficient (s020,w) of 16.4 S and a diffusion coefficient (D020,w) of 1.65 x 10-7 cm2 s-1. An average molecular weight was estimated to be about 1,000,000 from the sedimentation equilibrium data. The content of right-handed α helix in the enzyme molecule was estimated to be about 23% by optical rotatory dispersion and about 22% from circular dichroism spectra. The enzyme was found to contain 8 molecules of protein-bound lipoic acid per mole of enzyme; some other properties are also reported. On polyacrylamide gel electrophoresis in sodium dodecyl sulfate, lipoate succinyltransferase was dissociated into a single subunit with estimated molecular weight of about 48,000. This subunit was also seen on similar gels of the 2-oxoglutarate dehydrogenase complex or its subcomplex (2-oxoglutarate dehydrogenase-lipoate succinyltransferase). The enzyme was also dissociated into subunits with molecular weight 48,000 by gel filtration on Sepharose 6B in 6 m guanidine HCI. An average molecular weight of the dissociated subunit of the enzyme was estimated to be about 41,000 by the sedimentation equilibrium method in 6 m guanidine HCI. The data indicate that 1 molecule of lipoate succinyltransferase consists of approximately 24 similar subunits; only one in three contains lipoic acid.