Journal of Lipid Research, Vol. 9, 562-569, September 1968
Copyright © 1968 by Lipid Research, Inc.

Lipid monolayers: interactions with the apoprotein of high density plasma lipoprotein

German Camejo , Giuseppe Colacicco , and Maurice M. Rapport

Department of Biochemistry, Albert Einstein College of Medicine, Yeshiva University, Bronx, New York 10461

Monolayer techniques were used to study the interactions of various lipids (cholesterol, lysophosphatidyl choline, phosphatidal ethanolamine, phosphatidyl choline, sphingomyelin, stearic acid, and lipids extracted from plasma high density lipoproteins and very low density lipoprotein) with the lipid-free protein subunit of rat plasma high density lipoprotein and with rat plasma albumin. The proteins were injected under the lipid monolayer at fixed area, and the increase in surface pressure (decrease in surface tension) was measured as a function of time.

With all lipids, both the rate and magnitude of this increase were greater with the apolipoprotein than with albumin. The degree of film penetration of pure lipid films (at an initial film pressure of 15 dynes/cm) by the two proteins followed the same order: cholesterol > phosphatidal ethanolamine > phosphatidyl choline > stearic acid > sphingomyelin > lysophosphatidyl choline. Other variables studied were protein concentration, initial film pressure, and pH.

Two distinctive properties of the apolipoprotein were the penetration of lipid films at pressures above the collapse pressure of the protein, and the formation of a film even at low salt concentration. High surface activity and strong interaction of HDL-protein with lipid monolayers may be associated with the flexibility of the protein molecule due to absence of disulfide bridges. The unusual surface activity of HDL-protein may be intimately related to the mechanism of formation of the lipoprotein.

Supplementary key words lipid-protein interactions • high density lipoprotein • apolipoprotein • albumin • monolayers • stearic acid • cholesterol phosphatidyl choline • phosphatidal ethanolamine • sphingomyelin • lysophosphatidyl choline

Submitted on November 15, 1967
Accepted on March 15, 1968

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