Potassium-Activated Phosphatase

Abstract

The existence of a phosphatase activated by K in piasma membranes was first demonstrated by Judah et al. (1962). These authors showed that human red blood cell (RBC) membranes, incubated at neutral pH and in the presence of Mg, are capable of accelerating the hydrolysis of P -nitrophenylphosphate (P-NPP), the rate of hydrolysis in Mg-containing media being almost doubled by K. The activating effect of K required Mg since the activity in the absence of the divalent cation was shown to be very low and insensitive to K. A distinctive property of the enzyme was that 10⁻⁴ M ouabain abolished the activating effect of K, leaving unaltered the activity in the absence of the monovalent cation. The report of Judah et al. (1962) was followed by many others describing activities with characteristics similar to that from human RBCs in membrane preparations from tissues as diverse as brain (Ahmed and Judah, 1964; Fujita et al., 1965; Israel and Titus, 1966; Yoshida et al., 1966; Nagai et al., 1966), kidney (Ahmed and Judah, 1964; Bader and Sen, 1966; Nagai et al., 1966), gastric mucosa (Forte et al., 1967), intestinal epithelia (Boyd et al., 1968), liver (Ahmed and Judah, 1964; Nagai et al., 1966), and smooth muscle (Ahmed and Judah, 1964). The distribution and some kinetic parameters of K-activated phosphatase in different tissues are summarized in Table 1.