Protein-quinone interaction: in vitro induction of indirect antiglobulin reactions with methyldopa.

Methyldopa-treated gamma globulin can be demonstrated serologically on either the red cell surface or on latex beads by the indirect antiglobulin reaction. The development of a positive antiglobulin reaction was related to methyldopa concentration and the length and temperature of incubation of methyldopa with protein and could be partially inhibited by the addition of albumin to the incubation mixtures. After more prolonged incubation, antiglobulin positivity also developed with plasma-treated with methyldopa. 14C-methyldopa was covalently bound to gamma globulin. Aggregation of gamma globulin following treatment with methyldopa could be demonstrated by both sedimentation velocity and molecular weight determinations employing low-speed equilibrium centrifugation. Protein aggregation was a function of time, temperature, and methyldopa concentration. Detectability by the antiglobulin reaction, the darkening noted in solutions to which methyldopa or hydroquinone had been added, as well as the aggregation of protein was inhibited by a reducing agent which prevented formation of a quinone from the hydroquinone. Some of the immunologically atypical features of the sensitization of red cells by methyldopa or its structural analogues are explicable by the adherence, in vivo, of chemically modified, nonantibody gamma globulin which renders the red cell directly antiglobulin positive.