Propriétés des formes allongées de l'acétylcholinestérase en solution

We present here several molecular properties of the asymmetric acetylcholinesterase forms, which we described in our previous publications [1–3]. From molecular sieve experiments, we determined the Stokes radius and compared its value with that calculated from micrograph measurements [3] for the A form which is a tetramer associated to an elongated element, called the “tail” (structure en grappe ). A good agreement is obtained with a prolate ellipsoid of identical volume and length; this indicates that the acetylcholinesterase is, in solution, a rigid structure. We also estimated the partial specific volume, v̄ , of acetylcholinesterase, by two independent methods: first, by the difference of sedimentation velocity in gradients of different overall density (H 2 O, 2 H 2 O); secondly by absolute density measurements in CsCl equilibrium gradients. Both methods are in reasonable agreement and yield a value of 0.74–0.75 for all forms of the enzyme. This enabled us to calculate, from sucrose gradient data, the sedimentation constant in water at 20 °C. Combining the values of the Stokes radius, sedimentation constant, and partial specific volume, we calculated the molecular mass of A, C, and D as 430000, 780000, and 11. These values justify our hypothesis that these forms possess one, two, and three tetramers, associated with the “tail”. The methods used in this work are all based on activity measurements, which is a major advantage since only minute quantities of protein are needed. However, these are mainly comparative determinations, and it is difficult to ascertain the precision of the published data for the reference proteins.