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Proteolytic dimers of porcine muscle lactate dehydrogenase: characterization, folding, and reconstitution of the truncated and nicked polypeptide chain.

Lactate dehydrogenase from porcine skeletal muscle is a "dimer of dimers" that is stabilized in its tetrameric state by an N-terminal "arm" of approximately 20 amino acid residues. Due to the low dissociation constant of the tetramer, the dimer is inaccessible to direct analysis. Limited proteolysis during reconstitution (after dissociation at pH 2.3) yields stable "dimers". As suggested by affinity chromatography, these inactive dimers contain the dinucleotide fold of native LDH. In the presence of structure-making ions, approximately 40% activity is restored in the dimeric state [Girg, R., Jaenicke, R., & Rudolph, R. (1983) Biochem. Int. 7, 443-444]. The cleavage yields about equal amounts of three fragments, F 34, F 21, and F 14 (Mr 33.5K, 21.4K, and 13.5K, respectively). F 34 represents the intact chain lacking the N-terminal 10-11 amino acid residues; its C-terminus is heterogeneous, varying in the range between residues 326 +/- 5. F 21 contains residues 11/12 to 200 +/- 3; F 14 is a mixture of three subfragments: residues 11/12 to approximately 133, 38 to approximately 163, and 208 to approximately 327. After solubilization in 6 M guanidine hydrochloride, F 34 can be reconstituted to partially active dimers. Reactivation is determined by slow subunit refolding with subsequent diffusion-controlled dimerization, in accordance with the monomer-dimer transition in the reconstitution mechanism of the intact tetramer. Reconstitution of F 21 and F 14 is concentration dependent and leads to partially active "nicked dimers", indicating that separate domains are able to reassociate correctly to yield the native subunit arrangement.