Kinetics of neutralization of bacteriophage f2 by rabbit gamma-G-antibodies.

The neutralization of bacteriophage f2 by intact gammaG-immunoglobulin or fragments is first order with respect to both bacteriophage and antibody. Minimum values for the rate constants are of the order of 10(7)M(-1) sec(-1). The temperature dependence of the rates corresponds to the activation parameters: DeltaHdouble dagger = 6.7 kcal mole(-1) and DeltaSdouble dagger = -4 cal deg(-1) mole(-1) (gammaG-immunoglobulin); DeltaHdouble dagger = 8.0 kcal mole(-1) and DeltaSdouble dagger = -0.9 cal deg(-1) mole(-1) (5S pepsin fragment); and DeltaHdouble dagger = 13.3 kcal mole(-1) and DeltaSdouble dagger = 12 cal deg(-1) mole(-1) (3.5S fragment). The kinetic observations are consistent with the view that the binding of a single antibody molecule can bring about phage neutralization. There are two ways in which a single antibody molecule can affect neutralization: (1) binding at or near some critical site on the phage may block its function, (2) binding may disturb the general architectural design of the protein shell of the phage. Although the rate of neutralization varied directly with antibody size, consideration of the activation parameters speaks against the dependence of neutralization on simple steric factors. Addition of antibodies directed against rabbit gammaG-immunoglobulin or the 5S pepsin fragment caused approximately a threefold neutralization enhancement. This enhancement may result from the detection of a class of infectious bacteriophage antibody complexes.