Regulation by the “interfacial quality” of some biological activities

Abstract

A few examples are given of regulation by the “interfacial quality” of some biological activities. Experiments with monolayers have the unique advantage that the arrangement and packing of the molecules can be easily measured and controlled. The first part is devoted to proteins which do not degrade lipids. Soluble cardiotoxins are generally injected into the subphase below a preformed lipid monolayer and measurements are taken either at constant surface area or at constant surface pressure. These experiments can give information on the penetration capacity of the protein into the interface and its lipid specificity, with direct access to the area of the protein segment interacting with lipids. Most intrinsic membrane proteins are insoluble in water. In the absence of detergent they aggregate and display no affinity for lipid interfaces. These proteins can be spread from an organic solvent solution but with the risk of being denatured. In order to circumvent this difficulty a method for spreading an aqueous suspension of lipoproteins or natural membrane vesicles was developed. This spreading method allows the formation of lipoprotein films retaining biological activities and native membrane constituents. In the second part, the use of lipid monolayers as substrates for lipolytic enzymes is reviewed. The monolayer technique permits an accurate study to be made of the influence of surface pressure and protein cofactors on the hydrolysis velocity and lag time in lipolysis. Two examples are developed: first, the assistance provided by colipase during the penetration of phospholipid films by pancreatic lipase; second the activation by apolipoprotein C II of phospholipid-monolayer hydrolysis by lipoprotein lipase. The monolayer technique is ideally suited to the study of the mode of action of lipolytic enzymes on monolayers of controlled “interfacial quality”.