Occurrence of S-(1,2-dicarboxyethyl)-cysteine at position 77 in mutant human lysozyme secreted by Saccharomyces cerevisiae.

A mutant human lysozyme P110, in which Val110 was replaced with Pro, was secreted by Saccharomyces cerevisiae; modification of the cysteine residue at position 77 was found in a purified mutant protein (P110-B) upon primary structure analysis. A peptide fragment containing 15 amino acid residues from Thr70 to Leu84 was obtained by proteolytic digestion of the protein and subsequently isolated by reverse-phase HPLC. This fragment was analyzed by high-resolution fast-atom-bombardment (FAB) mass spectrometry, which showed that 1,2-dicarboxyethyl group was attached to the thiol group of Cys77. This modification was confirmed by comparing it with a sample of chemically synthesized S-(1,2-dicarboxyethyl)-L-cysteine. It was found that the modification caused a disruption of the disulfide bond Cys77-Cys95 in the mutant molecule. These observations, plus structural considerations, suggest that Cys77 and Cys95 either remain uncrosslinked or the disulfide bond Cys77-Cys95, once formed, is opened during the final step in the folding of human lysozyme in vivo.