Capillary Endothelium in the Clearance of Chylomicrons A Model for Lipid Transport from Blood by Lateral Diffusion in Cell Membranes

TRIGLYCERIDE, phospholipid, and cholesterol absorbed from the intestines are transported in the blood stream in the form of chylomicrons and very low density lipoproteins (VLDL)." There is considerable evidence that triglyceride of chylomicrons and VLDL is hydrolyzed during uptake from blood and that this process is catalyzed by lipoprotein l ipase. * Lipoprotein lipase activity is found in tissues that utilize triglyceride, and the level of activity in individual tissues (adipose tissue, heart, muscle, and mammary gland) varies with nutritional and physiological states that affect triglyceride uptake, such as fasting,*"" diabetes," pregnancy,'» and lactation.*" Lipoprotein lipase activity is released readily to the blood stream by heparin injection, suggesting that the enzyme is present in or near the vascular bed.'• "• lf Studies in several laboratories have shown that chylomicron triglyceride is not utilized directly by liver."" Instead, the triglyceride is hydrolyzed to free fatty acid (FFA) by lipoprotein lipase in peripheral tissues, and some of the FFA formed is released to the blood to be taken up by liver."" Although cholesteryl ester in chylomicrons is taken up primarily by liver,*'" this process occurs only after most of the triglyceride in chylomicrons has been removed by the action of lipoprotein lipase in peripheral tissues." The fate of phospholipids and unesterified cholesterol in chylomicrons has not been determined. It is possible that lecithimcholesterol acyltransferase activity (LCAT) in