Verification by mass spectrometry of the primary structure of human interleukin-2

Proteolytic digests of interleukin-2 from a human leukemic T-cell line produced by Escherichia coli carrying a recombinant DNA were analyzed by fast atom bombardment mass spectrometry. The mass values of intense signals observed in the mass spectrum were consistent with peptides predicted from the nucleotide sequence of cDNA for human interleukin-2, an indication that the protein with the predicted amino acid sequence was produced by E. coli. BrCN and proteolytic digests of interleukin-2 obtained from cultured cells were also examined by fast atom bombardment mass spectrometry. The observed mass values were identical with those from interleukin-2 from E. coli except for that of the NH2-terminal sequence, in which the Thr residue at position 3 was bound to a sugar moiety. The mass spectra of the digests of the two interleukin-2 preparations and synthetic peptides with sequences from 117 to 128 and 121 to 128 predicted from the nucleotide sequence of cDNA for a human interleukin-2 indicated that Cys residues at positions 58 and 105 are linked by a disulfide bond and that the Cys residue at position 125 is free.