INTERHEAVY DISULFIDE BRIDGE IN IMMUNOGLOBULIN-G (IGC) REACTING WITH DITHIONITROBENZOATE - A UNIQUE FEATURE IN SERUM-PROTEINS

Immunoglobulin G (IgG) of many species contains 'labile' disulfide bon ds (SS), which within 24 h undergo a disulfide exchange with dithioni trobenzoic acid (NBSSBN). Aims of the present study were to detect dir ectly this type of SS groups by means of C-14-labelled NBSSBN, and to investigate its possible presence in other serum proteins. NBSSBN rea cts during the first 30 min of incubation with free SH groups, and the reafter with the sulfur atoms of SS groups. The latter reaction reach es equilibrium after 24 h. The total of thionitrobenzoate residues (NB S) bound to IgG is called 'Sigma S' and represents both SH and SS. Th e results can be summarized as follows: (1) The measurement of the bin ding of C-14-NBSSBN gave identical BS values with IgG from humans and mice, as compared to the detection with the unlabelled reagent, which is based on the photometric determination of liberated NBS anions; whe reas with IgG from rats some differences were observed which were ascr ibed to different batches of animals investigated; (2) experiments wit h electrophoretically separated serum proteins revealed only the gamma -globulins strongly binding C-14-NBSSBN in addition to the 30 min reac tion, which indicates that SS is confined to the gamma-globulin fract ion; and (3) the significant decrease of Sigma S in association with m alignant tumors in man and animal models, which was previously describ ed to be due to a specific alteration of the IgG subclass pattern, was likewise detected with the radiometrical method. Previous studies hav e identified SS as one of the two inter-heavy disulfide bridges in Ig G1, and possible implications of this group in specific functions of I gG1 are discussed. (C) 1997 Elsevier Science B.V.