Structure of pyruvate dehydrogenase complex. Comparison between freeze-etching and negative staining.

Pyruvate dehydrogenase complex (pyruvate : lipoate oxidoreductase (decarboxylating and acceptor-acetylating), EC 1.2.4.1), from pig heart, was studied by spray freeze etching and negative staining. From freeze etching experiments an average particle weight of 7-10(6) was estimated. Negative staining after glutaraldehyde fixation and freeze etching of unfixed and prefixed enzyme solutions yielded no significant difference in particle dimensions: the majority of the isometric complex molecules measured approximately 400 A in diameter. Tantalum tungsten shadowed freeze etch replicas indicated that the surface of the complex is built up of globular units. The relative positions of these units are in good agreement with the model still under discussion.