Phospholipase Kinetics in Mixed Micelles

California State University Northridge, Northridge CA

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Abstract

Phospholipid hydrolysis by digestive enzymes is an interface phenomenon that occurs at the lipid/bile-salt[unreadable] aggregate and solvent interface in the human digestive tract. Unequivocal evidence exists, showing that[unreadable] interface properties (lipid surface concentration, size, curvature, interface hydration and charge and lipid[unreadable] conformation) modulate enzyme activity. However there are no functional relations developed because[unreadable] enzymology studies to date have been performed on poorly defined substrate aggregates. Factors that control[unreadable] phospholipase kinetics at micellar interfaces are yet to be established. The proposed studies are designed to[unreadable] remove this shortcoming. Complementary studies of physico-chemical characterization and phospholipase[unreadable] kinetics studies on the same substrate aggregate systems will be conducted, thus bridging the gap between the[unreadable] two types of studies. This will impact the approach to biochemical assays that presently treat micelles as a[unreadable] black box. A basic question in enzymology is what rate law is obeyed by an enzyme. No hypothesis or model[unreadable] can be tested without well-characterized substrate aggregates. Results of the past year on phospholipase C[unreadable] enzyme kinetic studies on bile salt/lipid aggregates, characterized by time-resolved fluorescence quenching[unreadable] showed a clear quantitative correlation between the physicochemical aggregate properties and enzyme activity.[unreadable] These results form the foundation of and motivate the present proposed studies, the specific aims of which are:[unreadable] (i) Develop reliable assays for investigating phospholipase kinetics and test the predictions of the putative[unreadable] Michaelis-Menten model applied to interface enzyme kinetics, (ii) Determine the quantitative correlation[unreadable] between phospholipase activity and the physicochemical properties of the micellar substrate and the kinetic[unreadable] pathway and thus show how the interface modulates enzyme activity. Included in the studies are other[unreadable] detergent/phospholipid model substrate systems, that form stable, globular mixed micelles with well-defined[unreadable] properties and geometries, serving thereby as controls of the interface and leading to a minimum of ambiguity[unreadable] in the interpretation of results. Phospholipase kinetics will be measured by the pH-Stat method. Micelle[unreadable] characterization is performed by a complementary set of techniques: time-resolved fluorescence quenching,[unreadable] electron spin resonance, solution viscosity, and small-angle neutron scattering. The properties determined are[unreadable] the mixed micelle aggregation numbers, micelle shape and size, surface concentration of lipids, surface charge,[unreadable] interface hydration, interface microviscosity. The tunability of these properties over a considerable range[unreadable] depending on mixture compositions and concentrations will be exploited in developing the interface[unreadable] microstructure-activity correlation scale. The long-term goal is to understand the mechanism of enzymatic[unreadable] catalysis at interfaces. Knowledge of the kinetic pathway is fundamental to understanding the mechanism,[unreadable] because proposed mechanisms make kinetic predictions which can be tested with reliable assays. Establishing[unreadable] the kinetic pathway is intrinsically valuable in providing knowledge that can be used in improving the efficiency[unreadable] of metabolism. Improving and refining the knowledge of what controls digestion of fats is important to human[unreadable] health.