Soluble and membrane-bound polyphosphoinositide phosphohydrolases in mammalian erythrocytes

Abstract

Phosphatases and phosphodiesterases that hydrolyse polyphosphoinositides are described in both membrane and cytosol fractions of human, pig, rat, rabbit, and sheep erythrocytes using exogenous substrates. With suitably optimized assay conditions, Ca2+-dependent phosphatidylinositol bisphosphate (PIP2) phosphodiesterase activity was found in the hemoglobin-free cytosol fraction, as well as the membrane. Membrane activity is completely dependent upon Triton X-100 and salt and inhibited by cetyltrimethylammonium bromide (CTAB), while the soluble activity requires CTAB and is inhibited by Triton. A low Ca2+-dependent PIP2 phosphatase activity, not present in other tissues, was also detected. The cation-independent phosphatidylinositol phosphate (PIP) phosphatase is localized in the membrane in most species, while the diesterase and the PIP2 phosphatases (both Mg2+ and Ca2+ dependent) are localized in the cytosol. Rat and rabbit erythrocytes are atypical in having a substantial proportion of their Mg2+ -dependent PIP2 phosphatase activities in the membrane. All activities are lowest in sheep erythrocytes, except the PIP phosphatase, most of which is soluble in this species. Ca2+-dependent PIP2 phosphatase activity is not correlated with the activity or subcellular distribution of any of the other hydrolases and seems to be a separate enzyme. All the phosphoinositide hydrolase activities, particularly the diesterase, are orders of magnitude lower in erythrocytes than in other tissues. Both soluble and membrane diesterase activities are lost as erythrocytes age. Soluble polyphosphoinositide diesterase does not seem to be active with membrane-bound substrate, since pig and sheep erythrocytes that have negligible membrane activity do not respond to Ca2+ loading, yet have substantial diesterase activity in the cytosol. This supports the view that the diesterase is not physiologically functional in normal erythrocytes.