Acetylcholinesterase from Schistosoma mansoni: interaction of globular species with heparin

In the cercarial and schistosomal stages of the life cycle of the trematode Schistosoma mansoni, acetylcholinesterase occurs as two principal molecula r forms (both globular), present in approximately equal amounts, with sedim entation coefficients of 6.5 S and 8S. The 6.5S form is solubilized by bact erial phosphatidylinositol-specific phospholipase C from intact schistosomu la. It is thus located on the outer surface of the schistosomal tegument an d is most probably analogous to the glycosylphosphatidylinositol-anchored G (2) form of acetylcholinesterase found in the electric organ of Torpedo, on the surface of mammalian erythrocytes, and elsewhere. Both forms are fully solubilized by the non-ionic detergent Triton X-100. Upon passing such a d etergent extract over a heparin-Sepharose column, only the 8S form was reta ined on the column. The bound acetylcholinesterase could be progressively e luted by increasing the salt concentration, with approx. 0.5-0.6 M NaCl bei ng needed for complete elution. Selective inhibition experiments carried ou t on live parasites using the covalent acetylcholinesterase inhibitor echot hiophate (phospholine), which does not penetrate the tegument, selectively inhibited the 6.5 S form, but not the 8S form, suggesting an internal locat ion for the latter. Monoclonal antibodies raised against S. mansoni acetylc holinesterase also distinguished between the two forms. Thus monoclonal ant ibody SA7 bound the 6.5 S form selectively, whereas SA57 recognized the 8 S form. The selective binding of the 8 S form to heparin suggests that, with in the parasite, this form may be associated with the extracellular matrix of the musculature.