Conversion of inactive glycosylation inhibiting factor (GIF) peptide to bioactive derivatives by modification of its sulfhydryl group (S)

Experiments were carried out to test the hypothesis that bioactivity of glycosylation inhibiting factor (GIF) depends on its conformational structure. E.coli-derived recombinant human (rh) GIF contains three cysteine residues, i.e., Cys57, 60 and 81 in the sequence. All SH groups in the cysteine residues are free, and the molecules showed no bioactivity. Carboxymethylation of the SH group of CyseO resulted in the generation of bioactivity, although the activity of the derivative was 10 fold less than that of Ts hybridoma-derived GIF. Treatment of the inactive rhGIF with 0.25 mM ethylmercaptothiosalicylate (EMTS) or 5,5'dithiobis (2nitrobenzoic acid) resulted in the formation of highly bioactive derivatives, of which bioactivity was comparable to that of Tsderived bioactive GIF, and the activity of the derivatives was lost by treatment with dithiothreitol. Treatment of inactive GIF in the cytosol of mammalian cells with EMTS also generated bioactivity. Bioactive GIF derivative could be chromatographically separated from inactive GIF. Chemical analysis indicated that modification of a single SH group was responsible for the generation of bioactivity, and supported the hypothesis that the bioactivity of the molecule depend on its conformation.