Studies on the sub-units of triose phosphate isomerase.

The sub-unit structure of rabbit muscle triose phosphate isomerase was studied by determination of the number of unique cysteine peptides. Alkylation of the thiol groups with radioactive iodoacetate in the presence of guanidine hydrochloride gave the S-carboxy[(14)C]methyl derivative of the protein. This was digested with trypsin, and the radioactive peptides were fractionated by ion-exchange chromatography; four main radioactive peaks were obtained, one of which contained two radioactive peptides. Peptide ;maps' of the tryptic digest showed five main spots. The relationship between the members of both sets of five peptides was established. The radioactive peptides were characterized, and the results indicated the presence of five unique cysteine residues in the protein. Since there are approximately ten thiol groups/molecule, there are two closely related or identical sub-units. Studies of the terminal residues bear out this suggestion; only one kind of N-terminal residue (alanine) and one kind of C-terminal residue (glutamine) were detected. These results are in accord with the evidence from crystallography.