Basis for the anomalous effect of competitive inhibitors on the kinetics of hydrolysis of short-chain phosphatidylcholines by phospholipase A2.
The effect of four specific competitive inhibitors on the kinetics of hydrolysis of short-chain diacyl-sn-glycero-3-phosphocholines below their critical micelle concentrations was examined. The kinetics of hydrolysis of short-chain substrates dispersed as solitary monomers were generally consistent with the classical Michaelis-Menten formalism; i.e., hydrolysis began without any latency period, the steady-state rate was observed at higher substrate concentrations, the steady-state initial rate showed a linear dependence on the enzyme concentration, and the hyperbolic dependence of the initial rate on the substrate concentration could be described in terms of KM and Vmax parameters. The competitive nature of the inhibitors used in this study has been established by a variety of techniques, and the equilibrium dissociation constants for the inhibitors bound to the enzyme were measured by the protection method [Jain et al. (1991) Biochemistry 30, 7306-7317]. The kinetics of hydrolysis in the presence of competitive inhibitors could be described by a single dissociation constant. However, the value of the dissociation constant obtained under the kinetic conditions was comparable to that obtained by the protection method for the inhibitor-enzyme complex bound to a neutral diluent, rather than to the value of the dissociation constant obtained with solitary monomeric inhibitors and the enzyme in the aqueous phase. Spectroscopic methods showed that the effectively lower dissociation constant of an inhibitor bound to PLA2 at the interface is due to the stabilization of the enzyme-inhibitor complex by interaction with other amphiphiles present in the reaction mixture.(ABSTRACT TRUNCATED AT 250 WORDS)
Rogers
J
J
Department of Chemistry and Biochemistry, University of Delaware, Newark 19716.
Yu
B Z
BZ
Jain
M K
MK
D001665:Binding Sites; D001667:Binding, Competitive; D002384:Catalysis; D006639:Histidine / Q000378:metabolism; D006860:Hydrogen Bonding; D006863:Hydrogen-Ion Concentration; D006868:Hydrolysis; D007700:Kinetics; D008823:Micelles; D015394:Molecular Structure; D010713:Phosphatidylcholines / Q000378:metabolism*; D010741:Phospholipases A / Q000037:antagonists & inhibitors* / Q000737:chemistry / Q000378:metabolism; D054467:Phospholipases A2; D013050:Spectrometry, Fluorescence; D013056:Spectrophotometry, Ultraviolet
D008823:Micelles; D010713:Phosphatidylcholines; D006639:Histidine; D010741:Phospholipases A; D054467:Phospholipases A2