Kinetic analysis of differences in brain acetylcholinesterase from fish or mammalian sources

A kinetic analysis of the interactions of gold fish and rat brain AChE (acetylcholine acetylhydrolase, EC 3.1.1.7) with acetylthiocholine substrate and sulfonyl fluoride inhibitors revealed several differences between these enzymes. The fish brain AChE had a K m of 250 μM while the mammalian brain AChE had a K m of 55 μM under the same assay conditions. In addition, the mammalian brain enzyme reacted with methanesulfonyl fluoride at more than three times the rate at which the fish enzyme reacted, and fish AChE did not react with phenylmethanesulfonyl fluoride at a measurable rate while mammalian AChE was found to react rapidly. Leptocurares were found to accelerate methanesulfonylation of both enzymes; however, they inhibited phenylmethanesulfonylation. These results suggest that fish and mammalian AChE may have topographic differences in the vicinity of the anionic portion of the active site.