The relative lability of the interchain disulphide bonds of mouse G2a-myeloma protein 5563 was studied as a function of 2-mercaptoethanol concentration. Analysis of partial-reduction mixtures by polyacrylamide-gel electrophoresis and microdensitometry showed that the disulphide bonds between light and heavy chains are much more susceptible to reduction than the bonds between heavy chains. At a low concentration of 2-mercaptoethanol (10mm) the major dissociable products of mouse immunoglobulin G are heavy-chain dimers and free light chains. These findings contrast with the reported behaviour of rabbit immunoglobulin G, for which the lability of inter-heavy-chain bonds was found to exceed that of the bonds linking light and heavy chains (Hong & Nisonoff, 1965); the relative stability of rabbit immunoglobulin G interchain bonds was confirmed in the present study. Examination of human immunoglobulin G and an immunoglobulin G (γ2) of guinea pig showed that at least in the majority of molecules, as with mouse immunoglobulin G, the disulphide bonds between light and heavy chains are more susceptible to reduction than the inter-heavy-chain bonds.
