Acetylcholinesterase activity of Xenopus laevis oocytes

The cholinesterase activity of Xenopus laevis oocytes was assessed using [3H]acetylcholine in a simple radiometric procedure. The cholinesterase activity of mature (stage V-Vl) oocytes was very sensitive to inhibition by the specific acetylcholinesterase inhibitor, BW284-C5l, and relatively insensitive to an inhibitor of non-specific, or butyrylcholinesterase. The Km and Vmax of the acetylcholinesterase measured in homogenates of oocytes were 312 microM and 4.6 nmol-oocyte 1-h 1, respectively. Triton X-100 increased the enzyme activity of homogenates four- to five-fold while collagenase treatment displaced into the medium none of the acetylcholinesterase activity from either homogenates or intact oocytes. Cations were found generally to diminish the acetylcholinesterase activity of oocyte homogenates, and lanthanum ions inhibited acetylcholine hydrolysis with an IC50 of 0.63 mM. Subcellular fractionation of oocytes revealed that the bulk of enzyme activity was associated with particulate fractions. Acetylcholinesterase activity was also detected on the surface, and in homogenates, of immature oocytes. Peak enzyme activity resided in stage IV oocytes. Eggs obtained from females induced to spawn were found to have acetylcholinesterase activity in homogenates but little or no hydrolytic activity was detected on the egg surface. These results provide a point of departure for further investigations of the functional significance of this enzyme in Xenopus oocytes.