Choline acetyltransferase in skeletal muscle from patients with myasthenia gravis.

Acetylcholine synthesis in homogenates of human intercostal muscle was measured by a radiochemical method. Choline acetyltransferase activity in control muscle was about 20 nmol . g-1 . h-1. The enzyme was found only in the endplate area of the muscle. At high substrate concentrations its activity was overshadowed by the acetylcholine synthesizing activity of a different enzyme not saturated by 10 mM-choline. The nonspecific enzyme was present at and away from the endplate area. Choline acetyltransferase in parasternal samples of intercostal muscle from myasthenia gravis patients was about 2.5 times higher than in samples, taken from a more lateral location, of control patients, but the Km for choline was not altered (0.24 mM). It is suggested that in myasthenia gravis the shortage of acetylcholine receptors is partially compensated for by increased synthesis, storage, and release of the transmitter.