Abstract
Two distinct classes of acetylcholinesterase (AChE) from the fruit fly Drosophila melanogaster are reported: a soluble species that shows heterogeneity of forms and a particulate species. The subunit composition of the particulate enzyme was studied using the active site label [3H]diisopropylfluorophosphate. Comparison of the electrophoretic patterns on nondenaturing gels using the activity stain and the active site label shows that the label is specific to AChE. The smallest active site-containing subunit of the enzyme is a monomer of approximately 60,000 daltons MW. Two such units are linked by disulphide bonds to produce a dimer of about 110,000 daltons. Another monomeric form of MW approximately 64,000 daltons, although present, does not participate in the dimerisation. The particulate enzyme when solubilised exists as a 9-10S species as determined by sucrose gradient centrifugation. This species has a MW greater than 200,000, as shown by its behaviour on a coarse-bead Sephadex-G200 column. Electrophoretic analysis suggests a MW of nearly 250,000 daltons for this form. Thus, this species is likely to be a tetramer. One possibility is that this tetramer is made up of two units of 64,000 daltons each and a dimer of 110,000 daltons. Preliminary data on mutant enzymes that support such a possibility are also presented.
