Interfacial catalysis by phospholipase A2: dissociation constants for calcium, substrate, products, and competitive inhibitors.

Interpretation of the kinetics of interfacial catalysis in the scooting mode as developed in the first paper of this series [Berg et al. (1991) Biochemistry 30 (first paper of six in this issue)], was based on the binding equilibrium for a ligand to the catalytic site of phospholipase A2. In this paper, we describe direct methods to determine the value of the Michaelis-Menten constant (KMS) for the substrate, as well as the equilibrium dissociation constants for ligands (KL) such as inhibitors (KI), products (KP), calcium (KCa), and substrate analogues (KS) bound to the catalytic site of phospholipase A2 at the interface. The KL values were obtained by monitoring the susceptibility to alkylation of His-48 at the catalytic site of pig pancreatic PLA2 bound to micellar dispersions of the neutral diluent 2-hexadecyl-sn-glycero-3-phosphocholine. The binding of the enzyme to dispersions of this amphiphile alone had little effect on the inactivation rate. The half-time for inactivation of the enzyme bound to micelles of the neutral diluent depended not only on the nature of the alkylating agent but also on the structure and the mole fraction of other ligands at the interface. The KL values for ligands obtained from the protection studies were in excellent accord with those obtained by monitoring the activation or inhibition of hydrolysis of vesicles of 1,2-dimyristoyl-sn-glycerophosphomethanol. Since only calcium, competitive inhibitors, and substrate analogues protected phospholipase A2 from alkylation, this protocol offered an unequivocal method to discern active-site-directed inhibitors from nonspecific inhibitors of PLA2, such as local anesthetics, phenothiazines, mepacrine, peptides related to lipocortin, 7,7-dimethyleicosadienoic acid, quinacrine, and aristolochic acid, all of which did not have any effect on the kinetics of alkylation nor did they inhibit the catalysis in the scooting mode.