Purification properties and subunit composition of pig heart lipoate acetyltransferase

Lipoate acetyltransferase [acetyl-CoA: dihydrolipoate S-acetyl-transferase, EC 2.3.1.12], the core enzyme of the pyruvate dehydrogenase complex, has been highly purified by gel chromatography on Sepharose 6B and sucrose density gradient centrifugation in the presence of potassium iodide. The native enzyme has a sedimentation coefficient (S020,W) of 26.7S and a diffusion coefficient (D020,W) of 1.25 x 10(-7) cm2.-sec-1. The weight-average molecular weight was estimated to be 1.8 million from the sedimentation equilibrium data. The content of right-handed alpha helix in the enzyme molecule was estimated to be about 25% by optical rotatory dispersion and about 22% from the circular dichroism spectra. The enzyme was found to contain about 23 moles of protein-bound lipoic acid per mole of enzyme; some other properties are also reported. Lipoate acetyltransferase dissociated to yield a single subunit with a molecular weight of 74,000 as estimated by polyacrylamide gel electrophoresis in sodium dodecyl sulfate and by gel filtration on Bio-Gel in 6 M guanidine-HCl. The molecular weight was also estimated to be 74,000 from sedimentation equilibrium data in 6 M guanidine-HCl] containing 0.1 M 2-mercaptoethanol. Evidence is presented that 1 molecule of lipoate acetyltransferase apparently consists of 24 very similar subunits, each of which contains NH2-terminal alanine. Each subunit contains 1 molecule of covalently bound lipoic acid.