A new look at the lipid composition of the plasma membrane of human blood platelets relative to the GPIIb/IIIa (integrin cxIIβ3) content.
The total lipids of the human platelet plasma membrane (HPPM) from 50 ml of blood of healthy subjects were extracted, quantified and related to the mass content of the major intrinsic membrane protein, the glycoprotein IIb/IIIa (GPIIb/IIIa). The HPPM total lipid/GPIIb/IIIa weight ratio determined was 5.40 ± 0.20, independently of the membrane washing procedure used, with the cholesterol/GPIIb/IIIa and phospholipid/GPIIb/IIIa molar ratios of 800 ± 50 and 1200 ± 40, respectively. If the distribution of lipids around each intrinsic protein were proportional to its mass, the lipids around a molecule of GPIIb/IIIa will occupy about 120 nm(2) of the membrane plane, which is about one and a half times the cross-sectional area of the extracellular head of GPIIb/IIIa, as estimated by electron microscopy. The lipid extracts were further subjected to thin-layer chromatography to separate and quantify the different phospholipid fractions, the free fatty acids and the neutral lipid fraction and the distribution of fatty acids in each fraction was determined by gas chromatography after methanolysis. The phospholipid molar distribution was SPM(22.3 ± 0.9%), PC(36.2 ± 1.0%), PE(24.9 ± 0.9%), PS(12.1 ± 0.6%) and PI(4.5 ± 0.4%) and the free fatty acid fraction represented 2.9 ± 0.4% of the total fatty acids in HPPM. The fatty acid chain length ranged from 14 to 24 carbons, comprising unsaturated fatty acids (47.3% molar per cent of the total) of which 40.7 ± 2.0% were monosaturated and 40.7 ± 0.9% tetraunsaturated. Palmitic, stearic, oleic and arachidonic acids represent 66% of the total fatty acids of HPPM, being: 68.9 ± 5.3% of palmitic acid and 63.3 ± 6.9% of oleic acid in PC; 50.9 ± 3.8% of arachidonic acid in PE; and 30.5 ± 2.4% of stearic acid in PS. We discuss the methodological modifications and the new data in relation with the major differences in HPPM lipid composition found in the literature. The data obtained provides a comprehensive and accurate description of the lipid composition of HPPM on which to rely as a reference for basic and medical studies.
Departamento de Biofísica, Instituto de Química Física, C.S.I.C., Serrano 119, E-28006, Madrid, Spain.
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R
García-Guerra
J A
JA
García-Domínguez
J
J
González-Rodríguez