Predominant phosphatase in the ocular lens regulated by physiological concentrations of magnesium and calcium

A magnesium-dependent phosphatase with a molecular weight of about 55 kDa was found in the lens of chicken embryo, mouse, rabbit and bovine. It appears to be unique to the lens and, when activated by magnesium, accounts for the majority of the phosphatase activity in the lens.Phosphatases in the lens were separated by using high-resolution gel permeation column chromatography, and phosphatase activity was determined with p-nitrophenyl phosphate (pNPP) as a substrate. The 55-kDa phosphatase showed very low basal activity, but the activity was increased concentration-dependently by magnesium ion (Mg(2+)), and at 1 mM Mg(2+), this enzyme accounted for over 50% of the total phosphatase activity in the lens. Calcium potently inhibited the magnesium-activated phosphatase activity in a dose-dependent (IC(50); about 50 microM), uncompetitive manner. The phosphatase activity was high in the acidic pH range, with an optimum pH value of 5.5. The characteristics of the lens Mg(2+)-dependent phosphatase, such as substrate specificity and sensitivity to various phosphatase inhibitors, showed little similarity to those of any reported phosphatase. The Mg(2+) and Ca(2+) concentrations that were found to be effective in this study are similar to physiological concentrations in the lens; therefore it is likely that these ions physiologically regulate the 55-kDa phosphatase activity in the lens.