Properties of 16S acetylcholinesterase from rat motor nerve and skeletal muscle

Rat obturator nerve 16S acetylcholinesterase (16S AChE) was separated by sucrose gradient velocity sedimentation and compared to the 16S form of AChE similarly derived from endplate regions of anterior gracilis muscles. The 16S AChE from both tissues could only be extracted in high ionic strength buffer; as it aggregated under low ionic strength conditions. Treatment of nerve and muscle 16S AChE with purified collagenase, in the presence of calcium, caused an identical shift in the enzyme's sedimentation coefficient to 17.5S. Other properties which were also equivalent for 16S AChE from both tissue sources included: an excess substrate inhibition above 2×10^–3 M acetylcholine andK_m of 1.6×10^–4 M, relative sensitivity to the specific inhibitors BW284C51 (I₅₀ of 5×10^–8 M) and Iso-OMPA (I₅₀ of 5×10^–4 M), and a half maximal thermal inactivation at 62.5°C. These and additional results indicate that the 16S forms of AChE in both tissues are analogous molecules, which have a highly asymmetric conformation probably containing a collagen-like domain. The present findings are also consistent with the view that motor neurons provide at least a fraction of the 16S AChE present at the neuromuscular junction.