The chemical step is not rate-limiting during the hydrolysis by phospholipase A2 of mixed micelles of phospholipid and detergent

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Abstract
The effect of detergents on the overall catalytic turnover by secreted phospholipase A2 (PLA2) on codispersions of the substrate phospholipid is characterized. The overall rate of interfacial catalytic turnover depends on the effective substrate "concentration" (mole fraction) that the bound enzyme "sees" at the interface. Therefore, besides the intrinsic catalytic turnover rate determined by the Michaelis-Menten cycle in the interface [Berg et al. (1991) Biochemistry 30, 7283], two other interfacial processes significantly alter the overall effective rate of hydrolysis: first, the fraction of the total enzyme at the interface; second, the rate of replenishment of the substrate. At low mole fractions (< 0.3), bile salts promote the binding of pig pancreatic PLA2 to zwitterionic vesicles, and the rate of hydrolysis increases with the fraction of the enzyme in the interface. At higher (> 0.3) mole fractions of the detergent, the bilayer is disrupted, and the rate of hydrolysis decreases by more than a factor of 10. The detergent-dependent decrease in the rate of hydrolysis of the sn-2-oxyphospholipids is much larger than that of sn-2-thiophospholipid, and therefore the element effect (O/S ratio) decreases from about 10 in bilayers to less than 2 in mixed micelles. This loss of the element effect in mixed micelles shows that the chemical step is no longer rate-limiting during the hydrolysis of mixed micelles formed by the disruption of vesicles by the detergent. Such effects were observed with phospholipase A2 from several sources acting on substrates dispersed in a variety of detergents including bile salts, 2-deoxylysophosphatidylcholine, and Triton X-100.(ABSTRACT TRUNCATED AT 250 WORDS)