INCREASE IN THE APPARENT AChE ACTIVITY IN THE MOUSE DIAPHRAGM INDUCED BY PROTEOLYTIC TREATMENT

— Isolated endplate regions from the mouse diaphragm were treated with different agents before or after homogenization in order to solubilize junctional AChE and study the effect of solubilization on its apparent activity. Total AChE activity (solubilized + nonsolubilized) of samples treated with collagenase or papain before homogenization was nearly twice as high as in control samples. If collagenase was added after homogenization no increase in apparent activity was observed although in both cases about 70–80% of AChE activity was solubilized. The access of ACh to the membrane‐bound enzyme is probably not a limiting factor in the AChE assay as is the case in the electric organ homogenates. Both 1 m‐NaCl and Triton X‐100 were quite ineffective as solubilizers when applied before homogenization and had an insignificant effect on the apparent AChE activity. The increase in apparent AChE activity cannot be explained either by a de novo synthesis or by the change in kinetic properties of different species of AChE, or by the release of AChE possibly sequestrated in the membrane vesicles. The possibility is discussed that a part of junctional AChE is inactivated at the beginning of homogenization while it can be preserved by previous solubilization, or that proteolytic treatment may activate some ‘silent’ AChE sites in motor endplates. However, the mere fact that the difference does exist suggests that all AChE activity present in intact motor endplates may not be measurable after homogenization.