Transport of lipid across capillary endothelium.

Fatty acids, monoacylglycerol, cholesterol, and phospholipids are taken up from blood by many different tissues. These substances, which are strongly amphipathic at physiological pH, are poorly soluble in water and neutral lipids (tyriacylglycerol nd cholesteryl ester). They are transported in blood as components of lipoprotein particles or, in the case of fatty acids, s monomers bound to albumin. Fatty acids derived from the diet are carried as triacylglycerol in chylomicrons and very low density lipoprpoteins (VLDL). Phospholipids and cholesterol absorbed from intestines are also transported in chylomicrons and VLDL. We propose that transport of fatty acids and monoacylglycerol from chylomicrons Tand VLDL) across capillary endothelium in extrahepatic tissues requires 1) conversion of chylomicron triacylglycerol to amphipathic lipids (fatty acids and monoacylglycerol) by lipoprotein lipase at the capillary surface, 2) location and lateral movement of ipolytic products in a continuous interface composed of the chylomicron surface film and the external leaflet of plasma and intracellular membranes of endothelial and parenchymal cells, and 3) removal of lipolytic products from the interface in endoplasmic reticulum where they are reesterified to traicylglycerol and accumulate between leaflets of endoplasmic reticulum. We suggest that cholesterol and phospholipids from chylomicrons, and fatty acids from plasma albumin, also cross capillary endothelium by lateral movement in cell membranes.