Subcellular distribution of acetylcholinesterase forms in chromaffin cells

Abstract

The presence of acetylcholinesterase (AChE) in chromaffin granules has been controversial for a long time. We therefore undertook a study of AChE molecular forms in chromaffin cells and of their distribution during subcellular fractionation. We characterized four main AChE forms, three amphiphilic forms (G1a, G2a and G4a), and one non-amphiphilic form (G4na). Each form shows that same molecular characteristics (sedimentation, electrophoretic migration, lectin interactions) in the different subcellular fractions. All forms are glycosylated and seem to possess both N-linked and O-linked carbohydrate chains. There are differences in the structure of the glycans carried by the different forms, as indicated by their interaction with some lectins. Glycophosphatidylinositol-specific phospholipases C converted the G2a form, but not the other amphiphilic forms, into non-amphiphilic derivatives. The distinct patterns of AChE molecular forms observed in various subcellular compartments indicate the existence of an active sorting process. G4na was concentrated in fractions of high density, containing chromaffin granules. We obtained evidence for the existence of a lighter fraction also containing chromogranin A, tetrabenazine-binding sites and G4naAChE, which may correspond to immature, incompletely loaded granules or to partially emptied granules. The distribution of G4na during subcellular fractionation suggested that this form is largely, but not exclusively, contained in chromaffin granules, the membranes of which may contain low levels of the three amphiphilic forms.