DISULFIDE LINKAGES IN THE IN-VITRO REFOLDED INTERMEDIATES OF RECOMBINANT HUMAN MACROPHAGE-COLONY-STIMULATING FACTOR - ANALYSIS OF THE SULFHYDRYL ALKYLATION OF FREE CYSTEINE RESIDUES BY FAST-ATOM-BOMBARDMENT MASS-SPECTROMETRY

Fast-atom bombardment mass spectrometry was used to follow the time co urse of disulfide bond formation during in vitro refolding of recombin ant human macrophage-colony-stimulating factor. The content of iodoace tamide-alkylated half-cystines in proteolytic peptides of trapped refo lding intermediates collected at 0, 6, 17, 24, and 72 hr was determine d under reducing conditions. Size exclusion highperformance liquid chr omatography analyses of the collected alkylated samples indicate that aggregated monomer proceeded through a nonaggregated monomer to an int ermediate dimer and finally to the fully folded and active dimer. Unde r-alkylation was first detected by fast-atom bombardment mass spectrom etry in 17-hr samples at Cys(157) and Cys(159),nd this corresponded to the first sample containing dimer. Analyses of intermediates from sub sequent time points indicated a decrease in alkylated sulfhydryls, and at 72 hr no alkylated peptide was detected. Early samples containing only monomer showed no evidence of disulfide bonds, and the occurrence of disulfide shuffling at the monomer stage could be ruled out under the highly reducing conditions used for refolding. Biological activity was not detectable in early samples but increased to 3.6% after 24 hr of refolding and to 86% of maximum at the 72-hr time point.