Inactivation of normal beta-D-galactosidase by antibodies to defective forms of the enzyme.

A counterpart of the antibody-mediated activation of genetically defective enzymes is reported here. Antibodies elicited by certain mutant forms of beta-D-galactosidase (EC 3.2.1.23) of Escherichia coli were found to inactivate the normal form of the enzyme. (Antibodies elicited by normal beta-D-galactosidase do not affect the enzyme's catalytic activity.) We present evidence that the inactivating antibodies are directed against one or a few determinants of the enzyme. The level of inactivation caused by the antibodies was independent of temperature below 25 degrees and increased with temperature above 25 degrees. The inactivation was proportional to the concentration of antiserum until a maximum level of 50% inactivation was reached. Antibodies capable of inactivating up to 87% of the activity were obtained after the antiserum was partially absorbed in an affinity column. This antibody preparation showed a 10-fold enrichment of inactivating antibodies over other antibodies direct against the enzyme. The antibody-mediated inactivation caused a reduction in the Vmax of beta-D-galactosidase without affecting the apparent Km of the enzyme. In contrast to antibodies to normal beta-D-galactosidase, inactivating antibodies changed the response of the enzyme to cations. To explain these results, we present a model in which there is a temperature-dependent equilibrium between two active forms of beta-D-galactosidase. Inactivation results from a conformational change induced by the binding of inactivating antibodies to only one of these two forms.