Inhibitory effect of K-76 monocarboxylic acid, an anticomplementary agent, on the C3b inactivator system.

K-76COOH caused dose-dependent inhibition of the degradation, by C3b inactivator (C3bINA) and beta 1H, of membrane-bound C3b on EAC1-3b and of free C3b in the fluid phase, i.e., cleavage of the C3b alpha' peptide chain. K-76COOH primarily attacked C3bINA but not beta 1H or C3b. K-76COOH inhibited the suppression of immune adherence reactivity and the manifestation of conglutination reactivity of EAC1-3b cells by C3bINA and beta 1H. The drug also inhibited the reaction between conglutinin and EAC1-3b' cells derived from EAC1-3b by treatment with C3bINA and beta 1H. EAC1-3b cells did not form rosettes with either Daudi or Raji lymphoblastoid cells. Treatment with C3bINA and beta 1H rendered EAC1-3b cells reactive with Daudi cells, and this change was inhibited by K-76COOH. EAC1-3b cells became able to form rosettes with Raji cells after addition of beta 1H. This rosette formation was enhanced by further addition of C3bINA, and this enhancement was also suppressed by K-76COOH.