Quantification and localisation of SH-groups in human blood serum proteins

The thiol groups of human blood serum proteins were determined after 24 hours interaction with dithionitrobenzoic-acid (DTNB) to an average of 538 +/- 60 mumol/l serum. After treatment of the serum with [35S]DTNB, autoradiograms of the protein elpherograms revealed two main peaks: The first with 63% of total activity, in the albumin region, corresponding to 0.60 SH/mol, the second with 23% of total activity, in the gamma-globulin range, corresponding to 2.2 SH/mol. After 30 minutes incubation with DTNB, or with p-chloromercuribenzoate (CMB), in freshly prepared pools of IgG only 0.2 SH/mol were found which is the expected value already known from the literature. Autoradiograms taken from serum protein elpherograms after interaction with [14C]CMB only show the main SH-peak in the albumin range. Thus it is concluded that the SH-peak in the gamma-globulin region after 24 hours incubation with [35S]DTNB is due to one highly labile S-S-bond which easily undergoes a disulfide exchange with DTNB.