Conversion of human erythrocyte acetylcholinesterase from an amphiphilic to a hydrophilic form by phosphatidylinositol‐specific phospholipase C and serum phospholipase D

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Abstract
Each catalytic subunit in the amphiphilic dimer of human erythrocyte acetylcholinesterase (AChE) is anchored in the plasma membrane exclusively by a glycoinositol phospholipid. In contrast to erythrocyte AChEs in other mammalian species, the human enzyme is resistant to direct cleavage by phosphatidylinositol‐specific phospholipase C (PtdIns‐specific PLC). The resistance is due to the existence of an additional fatty acyl chain on the inositol ring which blocks the action of PtdIns‐specific PLC [Roberts et al. (1988) J. Biol. Chem. 263, 18766–18775]. In this report, nondenaturing polyacrylamide gel electrophoresis was applied to permit rapid and unambiguous distinction between amphiphilic AChE, in which each catalytic subunit binds one nonionic detergent micelle, and hydrophilic AChE, which does not interact with detergent. Deacylation of human erythrocyte AChE by an alkaline treatment with hydroxylamine rendered the amphiphilic AChE susceptible to PtdIns‐specific PLC with the consequent release of hydrophilic AChE. Although serum anchor‐specific phospholipase D (PLD) cleaves the intact human erythrocyte AChE anchor, this treatment, as judged by nondenaturing electrophoresis, did not release hydrophilic AChE. Hydroxylamine treatment before or after PLD digestion was necessary to achieve the conversion. These observations indicate that binding of a single detergent micelle was maintained when any of the three fatty acyl or alkyl groups in the human erythrocyte AChE anchor phospholipid were retained. For proteins that can be identified following nondenaturing gel electrophoresis, these procedures provide methods both for detecting glycoinositol phospholipid anchors resistant to PtdIns‐specific PLC and for indicating fatty acyl and/or alkyl chains in these anchors.