Molecular forms of acetylcholinesterase in two sublines of human erythroleukemia K562 cells. Sensitivity or resistance to phosphatidylinositol-specific phospholipase C and biosynthesis.

Acetylcholinesterase (AChE) in K562 cells exists in two molecular forms. The major form, an amphiphilic dimer (G2a) which sediments at 5.3 S, and the minor form, an amphiphilic monomer (G1a) which sediments at 3.5 S. Extraction in the presence of the sulfhydryl alkylating agent N-ethylmaleimide was required to preserve the G2a form. In Triton X-100 extracts of the subline K562-243, phosphatidylinositol-specific phospholipase C (PtdIns-PLC) from Bacillus thuringiensis converted most of the G2a AChE into a hydrophilic dimer (G2h), indicating that the G2a form possessed a hydrophobic glycoinositol phospholipid that mediated its attachment to the membrane. Treatment of intact K562-243 cells with PtdIns-PLC released approximately 60% of the total AChE activity and provided an estimate of the externally exposed AChE. The direct conversion from an amphiphilic to a hydrophilic dimeric form by PtdIns-PLC was not obtained in extracts or intact cells of the subline K562-48. Instead, pretreatment with alkaline hydroxylamine was necessary to render the amphiphilic G2 form of this subline susceptible to digestion by the phospholipase. In this respect, the amphiphilic dimer of K562-48 AChE resembles the G2a form of human erythrocyte AChE, which is resistant to PtdIns-PLC because of the direct palmitoylation of an inositol hydroxyl group in the anchor [Roberts et al. (1988) J. Biol. Chem. 263, 18766-18775]. Release of this acyl chain by hydroxylamine renders the enzyme susceptible to PtdIns-PLC [Toutant et al. (1989) Eur. J. Biochem. 180, 503-508]. In both K562 sublines, sialidase decreased the migration of the G2a form but not of the G1a form of AChE. G1a forms thus appear to represent an intracellular pool of newly synthesized molecules residing in a compartment proximal to the trans-Golgi apparatus. The sialidase-resistant G1a molecules were also resistant to PtdIns-PLC digestion; possible explanations for this resistance are presented.