title: Enthalpy of acetylcholine hydrolysis by acetylcholinesterase.
abstract: Direct microcalorimetric measurements were made of the reaction between acetylcholine chloride and acetylcholinesterase (EC 3.1.1.7) that was extracted from electric eel (Electrophorus electricus) and purified by affinity chromatography. Tris-HCl, sodium phosphate and potassium phosphate were used as buffers and sources of ions for the reaction. At pH 7.2 and in 0.1-0.2 M phosphate buffer, the delta H for acetylcholine hydrolysis was found to be -0.107 kcal/mol (under buffered conditions) and -0.931 kcal/mol under unbuffered conditions (water). At pH 8.0 in 0.1 M Tris-HCl buffer, values greater than -2.5 kcal/mol were obtained, with the highest value of -9.2 kcal/mol being seen with bovine erythrocyte acetylcholinesterase. Tris-HCl buffer at 4 X 10(-2) M enhanced the reaction velocity by 51.2% over that of 4 X 10(-3) M buffer. Enzyme purity, pH and ionic milieu of reaction mixture, and substrate concentration affected the measured delta H value.
mesh_terms: D000109:Acetylcholine / Q000378:metabolism*; D000110:Acetylcholinesterase / Q000378:metabolism*; D000818:Animals; D002151:Calorimetry; D004557:Electric Organ / Q000201:enzymology; D004593:Electrophorus; D006868:Hydrolysis; D007700:Kinetics; D013816:Thermodynamics
chemical_list: D000110:Acetylcholinesterase; D000109:Acetylcholine