Isolation and structural characterization of three isoforms of recombinant consensus alpha interferon

Recombinant DNA-derived consensus alpha interferon was expressed in Escherichia coli and purified. Isoelectric focusing of this purified protein indicated the presence of three isoelectric subforms of pI 6.1, 6.0, and 5.7. These three subforms were preparatively separated by isoelectric focusing using Immobiline polyacrylamide gel and did not exhibit apparent differences in biological activity and tertiary structure. The pI 5.7 subform could also be separated from the pI 6.1 and 6.0 subforms by reverse-phase HPLC. Automated N-terminal amino acid sequence analysis of the pI 6.1 and 6.0 subforms yielded sequences corresponding to the methionyl and des-methionyl forms of the protein, respectively. Sequence analysis of the pI 5.7 subform indicated that its N terminus is blocked. To further determine the structure of the blocking moiety in the pI 5.7 subform, a blocked N-terminal tryptic peptide was isolated from HPLC peptide mapping of the S-carboxymethylated derivative. Results obtained from mass spectroscopic and amino acid analyses of this peptide suggest that it is blocked with an acetyl group at the N-terminal cysteine residue.