INFLUENCE OF MONOVALENT CATIONS AND HYDROSTATIC PRESSURE ON BETA-GALACTOSIDASE ACTIVITY

Monovalent cation activation of β-galactosidase (β-d-galactoside galacto-hydrolase, EC 3.2.1.23) was investigated utilizing o-nitrophenyl-β-d-galactopyranoside, p-nitrophenyl-β-d-galactopyranoside and lactose as substrates. With these substrates the affinity of β-galactosidase for Na+ was found to be higher than for K+. Activation of p-nitrophenyl-β-d-galactopyranoside hydrolysis by K+ was inhibited by Na+, while the activation of o-nitrophenyl-β-d-galactopyranoside hydrolysis by Na+ was stimulated by K+. Hydrostatic pressure stimulated the rate of o-nitrophenyl-β-d-galactopyranoside, p-nitrophenyl-β-d-galactopyranoside and lactose hydrolysis in the presence of saturating levels of Na+ and substrate. In contrast, activation by K+ with these substrates was consistently depressed by the application of hydrostatic pressure. These results are interpreted on the bais of volume change of an activated enzyme-substrate complex. It was concluded that a decrease in the volume of the complex occurs in the presence of NA+, while the converse occurs in the presence of K+, and that the mechanism of hydrolysis is different in the case of Na+-activated as opposed to K+-activated substrate hydrolysis. © 1969.