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A B S T R A C T Rat livers were perfused for 6 h without added plasma proteins using washed erythrocytes and buffer in a recirculating system. An inhibitor to the enzyme lecithin-cholesterol acyltransferase (5,5'-dithionitrobenzoic acid) was added in some experiments to prevent modification of substrate-lipids contained in secreted lipoproteins. The inhibitor did not detectably alter hepatic ultrastructure or gas exchange, but it inhibited the secreted lecithin-cholesterol acyltransferase by more than 85%. Very low density lipoproteins in perfusate were unaltered but the high density lipoproteins obtained from livers perfused with the inhibitor appeared disk-shaped in negative stain by electron microscopy with a mean edge thickness of 46±5 A and a mean diameter of 190±25 A. The high density lipoproteins were composed predominantly of polar lipids and protein with only small amounts of cholesteryl esters and triglycerides. The major apoprotein of these discoidal fractions had the same electrophoretic mobility as the arginine-rich apoprotein, whereas plasma high density lipoproteins contained mainly the A-I apoprotein. In all these respects the discoidal perfusate high density lipoproteins closely resemble those found in human plasma which is deficient in lecithin-cholesterol acyltransferase. Perfusate high density lipoproteins obtained in the absence of the enzyme inhibitor more closely resembled plasma high density lipoproteins in Portions of this work have appeared in an abstract and in a review (1, 2).Dr. Hamilton is the recipient of a Research Career Development Award. Dr. Williams was the recipient of a Postdoctoral Fellowship from the National Heart and Lung Institute. Dr. Fielding was an Established Investigator of the American Heart Association.Received for publication 30 January 1976 and in revised form 12 April 1976. chemical composition (content of cholesteryl esters and apoproteins) and in electron microscopic appearance. Purified lecithin-cholesterol acyltransferase synthesized cholesteryl esters at a substantially faster rate from substrate lipids of perfusate high density lipoproteins than those from plasma. The discoidal high density lipoproteins were the best substrate for this reaction. Thin sections of plasma high density lipoproteins indicated a spherical particle whereas discoidal high density lipoproteins stained with the characteristic trilaminar image of membranes. These observations suggest that the liver secretes disk-shaped lipid bilayer particles which represent both the nascent form of high density lipoproteins and preferred substrate for lecithin-cholesterol acyltransferase. INTRODUCTIONThe plasma lipoproteins are usually separated into four major physically defined groups although it is recognized that heterogeneity exists within each. This heterogeneity is further complicated by movements of both lipids and apoproteins between particles of the different groups. One approach to obtain a clearer understanding of the physiological significance of the different plasma lipoproteins h...

A minor fraction of plasma high-density lipoprotein (pre beta-1 HDL) has been shown to promote cholesterol efflux from peripheral cell membranes [Castro, G. R., & Fielding, C. J. (1988) Biochemistry 27, 25-29]. When isolated native plasma is incubated at 37 degrees C, this fraction is specifically decreased. On the other hand, the level of plasma pre beta-1 HDL is fully protected in the presence of even very low levels of fibroblasts, vascular smooth muscle cells, or macrophages. Blood cells were completely inactive in maintaining plasma pre beta-1 HDL levels in the absence of peripheral cells, even at the relatively high levels present in whole blood. The loss of pre beta-1 observed in isolated plasma was dependent upon lecithin-cholesterol acyltransferase (LCAT) activity. These data suggest that reverse cholesterol transport catalyzed by pre beta-1 HDL, and subsequent LCAT-mediated cholesterol esterification, is directly dependent upon the interaction between this HDL species and competent peripheral cells.