The apolipoprotein containing C-terminal alanine (apoLP-Ala) from very low density lipoprotein and phosphatidylcholine were used as a prototype to study lipidprotein interactions in human plasma lipoproteins. ApoLP-Ala strongly inhibited the reactivation of delipidated mitochondrial /3-hydroxybutyrate dehydrogenase, an enzyme which requires phosphatidylcholine for biological activity. When apoLP-Ala was sonicated with a 100-fold molar excess of phosphatidylcholine, a lipid-protein complex resulted which could be isolated free of excess lipid by ultracentrifugal flotation in potassium bromide solution at a density of 1.063-1.21 g/ml. The complex contained an average of 38 ± 5 phosphatidylcholine molecules for every 1 apoLP-Ala molecule. Further experiments showed that ultracentrifugation on a sucrose density gradient afforded a heterogeneous population of complexes whose average stoichiometry was 46 phosphatidylcholine molecules per apoLP-Ala molecule. When a saline gradient was used, the average stoichiometry was reduced to 30:1. When the titration of apoLP-Ala with a sonicated dispersion of phosphatidylcholine was observed by circular dichroism, the calculated -helical content of T Xhe very low density lipoproteins (VLDL)* 1 of human plasma represent the major vehicle for the transport of endogenously synthesized triglycerides in blood. By weight, the VLDL particles have an approximate composition of 51 % triglyceride, 20% cholesterol, 19% phospholipid, and 8% protein (Oncley and Harvie, 1969). The protein constituents of VLDL are heterogeneous (

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1. Short-term incubation of the everted intestinal sacs of rats in media containing cholesterol oleate or cholesterol plus oleic acid resulted in rapid hydrolysis, but no synthesis, of the sterol ester. 2. On separation of the brush border from the rest of the mucosal cell, almost all of the hydrolytic activity and appreciable amounts of the synthetic activity of the whole cell were found to be present in the brush-border fraction. 3. The isolated brush-border fraction contained considerable amounts of cholesterol, which was always present in the unesterified state; the rest of the cell contained about an equal amount of unesterified cholesterol, but, in addition, small but definite amounts of the esterified sterol were also found in this fraction. 4. On feeding rats with [4-(14)C]cholesterol, which was diluted with 3mg. of cholesterol, it was found that the brush border very rapidly took up the fed sterol without changing its net content of cholesterol. No traces of radioactive cholesterol ester could ever be detected in the isolated brush border after feeding with (14)C-labelled esterified or unesterified cholesterol. 5. The appearance of the labelled sterol was quite rapid in the rest of the cell also, where small proportions were found in the esterified state. 6. Therefore the sequence of events in the absorption of cholesterol appears to be: the dietary cholesterol esters are hydrolysed by the cholesterol ester hydrolase of pancreas or of the mucosal brush border or both, after which the brush border rapidly absorbs the de-esterified sterol and transfers it into the mucosal cell, by a mechanism of displacement, where it is slowly re-esterified for transport through the lymph.

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