The Fab region of IgG2 human myeloma proteins does not bear the streptococcal protein G-specific determinant

Seven out of ten Fab (F(ab'),(2)/Fab') preparations derived from purified human myeloma IgG showed a substantial binding to protein G-Sepharose. Subclass analysis revealed that the 7 protein G-reactive Fabs included 3 IgG1, 2 IgG3 and 2 IgG4 Fabs, whereas the remaining 3 which were not adsorbed were IgG2 Fab. Incubation of protein G-Sepharose with non-saturating amounts of 4 Fab preparations, representative of all IgG subclasses, showed that gamma 1, gamma 3 and gamma 4 Fabs adsorbed from 26 to 28.3%, whereas 80% of gamma 2 Fab was left in the supernatant after adsorption. These results indicate that human IgG2 lack PG-specific Fab-associated reactive site(s).