LIPID METABOLISM 



■77 



with respect to molecular weight, end group analyses 

 for specific peptide chains, amount and type of N — 

 and C — terminal amino acids, etc. Such information 

 remains preliminary in nature. A schematic concep- 

 tion of the various human plasma lipoproteins is 

 shown in figure 4. 



The lipid moieties of the various lipoproteins (other 

 than chylomicrons) may comprise from 40 to (per- 

 haps) 90 per cent of the molecule. These include a 

 small amount of free fatty acids, and virtually all the 

 esterified fatty acids as esters of glycerol or more 

 complex alcohols, and cholesterol. Free cholesterol 

 also is present. Normal human postabsorptive plasma 

 contains approximately 400 mg per 100 ml esterified 

 fatty acids. Approximately 70 per cent of such fatty 

 acids exist as triglycerides and phospholipids, and 

 the remainder as cholesterol esters (141). Small 

 amounts of esterified fatty acids may be found as 

 diglycerides and monoglycerides, cerebrosides and 

 acetals (71). 



Despite the high proportion of lipid in the lipo- 

 proteins, they have the chemical and physical char- 

 acteristics of protein molecules. Such behavior sug- 

 gests that the protein moiety is on the surface of the 

 molecule. For instance, it has been estimated (35) 

 that a chylomicron of 0.5 ju may be covered com- 

 pletely by protein, assuming the protein to be all at 

 the surface and constituting 1.5 per cent of chylo- 



micron. However, it has been pointed out that in the 

 larger ^-lipoprotein there is only enough protein to 

 cover about half the surface, assuming a thickness of 

 one peptide chain. On the basis of titration data, 

 Oncley el at. (154) have postulated a kind of mosaic 

 surface comprising both peptide and phospholipids, 

 the latter being oriented with their charged groups 

 at the surface. 



Under ordinary circumstances, the alpha lipopro- 

 teins, or high-density lipoproteins, probably do not 

 transport triglyceride for oxidative purposes. Recent 

 studies (176) with labeled amino acids have suggested 

 that the plasma does not contribute a major portion of 

 the protein found in either the chylomicrons or the 

 high-density lipoproteins in thoracic lymph. Since the 

 cells of the intestinal mucosa incorporate amino acids 

 into proteins having the same electrophoretic mobility 

 as chylomicron protein, it was theorized that the in- 

 testine may be the source of the protein of both the 

 high-density (alpha) lipoproteins and the chylo- 

 microns. 



The alpha lipoproteins contain approximately 40 

 per cent lipid; are not remarkably influenced by diet 

 or fasting and do not increase with age. In terms of 

 blood level, they are relatively stable. 



In contrast, the low-density (beta) lipoproteins, 

 which contain 75 per cent or more lipid, are labile; 

 they are affected by diet, fasting, age and gonadal 



DENSITY 



40,000 



LIPOPROTEIN SPECIES 



PRINCIPAL 



N-TERMINAL AMINO 



ACID RESIDUE 



fig. 4. Schematic conception of 

 human lipoproteins. Cross-hatching or 

 stippling represents polypeptide portions 

 of the molecules. [From Frederickson 

 & Gordon (71).] 



