D-3-Phosphoglycerate dehydrogenase from chicken liver. II. Chemical and physical properties

The molecular weight of chicken liver D-3-phosphglycerate dehydrogenase, as measured by sedimentation equilibrium analysis, was found to be 165,000, consistent with the measured sedimentation coefficient of 8.13 S. Sedimentation equilibrium studies in 6 M guanidine hydrochloride and gel electrophoresis in sodium dodecyl sulfate gave molecular weights of 40,000 to 43,000, indicating that the protein is composed of 4 subunits of equal molecular weight. Fractionation of the soluble tryptic peptides of the S-[14C]carboxymethylated enzyme by ion exchange chromatography and unidirectional paper electrophoresis indicated a maximum of 40 major peptides, which is approximately one-fourth of the number of tryptic peptides expected from the lysine plus arginine content of 157 of the tetramer. This observation suggests that the subunits possess identical or very similar amino acid sequences. The purified enzyme is a basic protein with an isoelectric point of 8.95. Amino acid analysis and titration with p-hydroxymercuribenzoate indicated that the 12 half-cystinyl residues present/subunit exist in the free sulfhydryl form. NH2-terminal analysis in an automatic protein sequenator indicated that the polypeptide chains of the protein are blocked.