CHOLESTEROL ESTERASE CATALYZED-HYDROLYSIS OF MIXED MICELLAR THIOPHOSPHATIDYLCHOLINES - A POSSIBLE CHARGE-RELAY MECHANISM

Mechanistic features of cholesterol esterase catalyzed hydrolysis of two thiophospholipids, rac-l-(hexanoylthio)-2-hexanoyl-3-glycerophosphorylcholine (6TPC) and rac-l-(decanoylthio)-2-decanoyl-3-glycerophosphorylcholine (10TPC), have been characterized. The hydrolysis of 10TPC that is contained in mixed micelles with Triton X-100 occurs strictly at the micellar interface, since the reaction rate is independent of the micelle concentration but depends hyperbolically on the mole fraction of the substrate in the micelles. This latter observation allows one to calculate the interfacial kinetic parameters V(max) and K(m)*. The hydrolyses of 10TPC and p-nitrophenyl butyrate are similarly inhibited by the transition state analogue inhibitor phenyl-n-butylborinic acid, and therefore, physiological and nonphysiological substrates are processed at the same active site. The similarity of k(cat)* values for the acyl-similar substrates 10TPC and p-nitrophenyl decanoate indicates that the phospholipase A1 activity of cholesterol esterase is partially rate limited by turnover of a decanoyl-enzyme intermediate. Solvent isotope effects on V(max)* and V(max)*/K(m)* (which monitors acylation only) are approximately 2-3 and are consistent with transition states that are stabilized by general acid-base proton transfers. Proton inventories of V(max)*/K(m)* indicate that simultaneous proton transfers stabilize the acylation transition state, which requires a multifunctional acid-base machinery (perhaps a charge-relay system) in the cholesterol esterase active site. Similar results are obtained for the 6TPC reaction, both in the presence and absence of Triton X-100 micelles.