A salt-activated inositol 1,3,4,5-tetrakisphosphate 3-phosphatase at the inner surface of the human erythrocyte membrane

The localization of the human erythrocyte membrane Ins(1,3,4,5)P₄ 3-phosphatase was investigated by saponin permeabilization of resealed 'isoionic' erythrocyte ghosts. This enzyme is active at the inner face of the plasma membrane, at the same site as a specific 5-phosphatase that degrades both Ins(1,4,5)P₃ and Ins(1,3,4,5)P₄. In the presence of EDTA, Ins(1,4,5)P₃ was the only product of Ins(1,3,4,5)P₄ metabolism. However, when Mg²⁺ was present both the 5-phosphatase and the 3-phosphatase attacked Ins(1,3,4,5)P₄, directly forming Ins(1,3,4)P₃ and Ins(1,4,5)P₃; some Ins(1,4)P₂ was also formed as a product of 5-phosphatase attack on the liberated Ins(1,4,5)P₃ The Ins(1,3,4,5)P₄ 3-phosphatase was potently activated by KCl, thus making the route of metabolism of Ins(1,3,4,5)P₄ by erythrocyte ghosts strikingly sensitive to variations in ionic strength: at 'cytosolic' K⁺ and Mg²⁺ levels, 3-phosphatase activity slightly predominated over 5-phosphatase. Ins(1,3,4,5)P₄ 3-phosphatase was potently inhibited by Ins(1,3,4,5,6)P₅ and InsP₆ at levels lower than those often observed within cells. This leaves open the question as to whether the cellular function of inositol polyphosphate 3-phosphatase is to participate in a physiological cycle that interconverts Ins(1,3,4,5)P₄ and Ins(1,4,5)P₃ or to metabolize other inositol polyphosphates in the cytosol compartment of cells.