Modification of a phospholipid stabilized emulsion interface by bile salt: effect on pancreatic lipase activity

Martin Wickham,Martin Garrood,John Leney,Peter D.G Wilson,Annette Fillery-Travis

doi:10.1016/s0022-2275(20)33300-9

Martin Wickham, Martin Garrood + Show 3 more

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https://doi.org/10.1016/s0022-2275(20)33300-9

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Abstract

Lipase is activated by binding to an insoluble emulsified or aggregated substrate. The extent of binding is related to the physicochemical as well as the compositional structure of the interface, the quality of the interface. 'Quality’ is as yet undefined but thought to contain contributions from electrostatic interactions, orientation of substrate, and hydration forces. To investigate the electrostatic and compositional factors we have used olive oil-in-water emulsions prepared with phosphatidylcholine and four bile salts of varying hydrophobicities. By measurement of the droplet zeta potential we have monitored semi-quantitatively the incorporation of bile salts within the interface. No correlation was found between droplet surface charge as monitored by the zeta potential and lag phase. The duration of the observed lag phase was found to be inversely related to the degree of incorporation of the bile salts. Simultaneously there was evidence of lipase binding to monomeric bile salts, reducing its availability for adsorption. Calcium ions reduced the surface charge but there was no correlation with lag phase duration. The evidence presented here agrees with a more specific role for calcium ions, i.e., the formation of a new catalytically active enzyme complex, (enzyme)–(mixed micelle)–(calcium ion). —Wickham, M., M. Garrood, J. Leney, P. D. G. Wilson, and A. Fillery-Travis. Modification of a phospholipid stabilized emulsion interface by bile salt: effect on pancreatic lipase activity.

Highlights

Lipase is activated by binding to an insoluble emulsified or aggregated substrate
Our results indicate that this breakup of the phospholipid interface is partial for the bile salts of lower hydrophobicity, i.e., sodium taurocholate (NaTC) and sodium glycocholate (NaGC), but may be complete for the more highly hydrophobic bile salts, sodium taurodeoxycholate (NaTDC) and sodium taurochenodeoxycholate (NaTCDC)
Our results confirm that the addition of bile salts to a phospholipid stabilized emulsion destroys the wellpacked phospholipid interface resulting in a mixed phospholipid/bile salt interface

Summary

Introduction

Lipase is activated by binding to an insoluble emulsified or aggregated substrate. The extent of binding is related to the physicochemical as well as the compositional structure of the interface, the quality of the interface. The characteristic feature of pancreatic lipase is its specificity of action on insoluble emulsified substrates; the enzyme is activated when it encounters an oil/ water interface. The mechanism of this catalysis is still a subject of study but there is general agreement [1] that the initial stage involves a reversible penetration of the lipase into the interface. If the bound lipase has only limited access to substrate clusters above this critical size, the rate of hydrolysis will be limited Support for this view is provided by the work of Muderhwa and Brockman [5] who concentrated on the hydrolysis of 1,3-dioleoylglycerol with 1-palmitoyl-2-oleoylsn -glycero-3-phosphocholine as a relatively low-affinity lipid ‘spacer’ in mixed lipid monolayers.

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