JBC, Vol. 250, Issue 19, 7759-7765, Oct, 1975
Inactivation of normal beta-D-galactosidase by antibodies to defective forms of the enzyme
R. A. Roth and B. Rotman
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.
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