Abstract

Phlorizin stimulated the K⁺-dependent phosphatase activity (measured with p-nitrophenyl phosphate) but inhibited the (Na⁺ + K⁺)-dependent adenosine triphosphatase activity in an enzyme preparation obtained from rat brain. The concentration of phlorizin required for half-maximal stimulation of the phosphatase was 0.05 mM, whereas the K_i for the ATPase was 0.06 mM. Of a series of hydroxylated aromatic compounds, phloretin alone also stimulated the phosphatase.

With the phosphatase, 0.03 mM phlorizin decreased the concentration of KCl required for half-maximal velocity (K_0.5) from 1.92 to 1.17 mM but had little effect on V_max. Na⁺ inhibited the phosphatase, and phlorizin increased the K_i for NaCl, from 6 to 12 mM. With the ATPase, 0.1 mM phlorizin similarly decreased the K_0.5 for KCl, from 0.74 to 0.48 mM, and increased the K_0.5 for NaCl, from 5.0 to 10.5 mM. The positive cooperative allosteric response to Na⁺, as indicated by the Hill plot with n > 1, was also converted to a negative cooperative response. In addition, phlorizin inhibited the Na⁺-dependent phosphorylation of the enzyme.

These data indicate that phlorizin reacts similarly with the cation-dependent ATPase and phosphatase, serving as a heterotropic allosteric modifier to increase the apparent affinity toward K⁺ but to decrease it toward Na⁺. These findings further imply that the phosphatase represents an aspect of the over-all ATPase reaction. Finally, the data raise the possibility that the inhibition of sugar transport by phorizin and phloretin may be mediated in part through an effect on the Na⁺-dependent aspects of that transport system.