Enzyme-Activation by Antibody

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Abstract
Antibodies that activate ribonuclease occur in fractions of rabbit IgG eluted at pH 8 from DEAE Sephadex A-50 between ionic strength 0.05 and 0.15. The fraction eluted at the lowest ionic strength (P1) activates the enzyme two- to threefold. Fractions eluted at higher ionic strength (P4 to P7) inhibit enzyme at relatively low antibody concentration, but become less inhibitory at high antibody concentrations. Activation and decreased inhibitory power at high antibody concentrations are observed with cyclic cytidylic and cyclic uridylic but not with ribonucleic acid as substrate. In fractions, such as P5, the zone of activation can be shifted when part of the antibody is removed by precipitation with ribonuclease. The resulting supernatants can activate ribonuclease (from 100% to 120%) at low concentrations of antibody. The interaction of inhibiting and activating antibody was studied in terms of the neutralizing capacity of mixtures of enzyme and various proportions of activating and inhibiting antibodies. In antigen excess, the enhancing capacity of activating antibody was reduced by the admixture of inhibiting antibody. In antibody excess, the neutralizing capacity of inhibiting antibody was reduced by the admixture of activating antibody. The latter observation was not confined to experiments with cylic cytidylic acid but was also made when ribonucleic acid served as substrate. This was remarkable since enzymic hydrolysis of nucleic acid was not activated by isolated P1 or P5. These findings are analyzed in terms of relative concentrations and relative binding constants of inhibiting and activating antibodies and in terms of competition for overlapping determinants of the enzyme.

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