Modified bovine serum albumin. VII. Studies on the role of free carboxyl groups of the protein in the interaction with its antibodies

The effect of esterification (methylation) of bovine serum albumin (BSA) on the interaction with its antibodies was studied. Decrease in the number of free carboxyl groups of the antigen by methylation resulted in a parallel decrease in the precipitin reaction. A derivative in which over 50 % of the carboxyl groups were modified did not precipitate at all with certain anti-BSA sera, but did react with these antibodies to give soluble complexes, as was shown by complement fixation and passive cutaneous anaphylaxis tests. When methylated BSA was de-esterified by incubation in alkaline media and reacted with anti-BSA, it was found that the precipitin reaction reappeared to the extent of 60% of the homologous reaction. It would thus appear that the free carboxyl groups of BSA may not be critically involved in the primary reaction with its antibodies, but are essential for the precipitation reaction.