Harpers

206 / CHAPTER 25Table 25–1. Composition of the lipoproteins in plasma of humans.CompositionDiameter Density Protein Lipid Main LipidLipoprotein Source (nm) (g/mL) (%) (%) Components ApolipoproteinsChylomicrons Intestine 90–1000 < 0.95 1–2 98–99 Triacylglycerol A-I, A-II, A-IV, 1 B-48, C-I, C-II, C-III,EChylomicron Chylomicrons 45–150 < 1.006 6–8 92–94 Triacylglycerol, B-48, Eremnantsphospholipids,cholesterolVLDL Liver (intestine) 30–90 0.95–1.006 7–10 90–93 Triacylglycerol B-100, C-I, C-II, C-IIIIDL VLDL 25–35 1.006–1.019 11 89 Triacylglycerol, B-100, EcholesterolLDL VLDL 20–25 1.019–1.063 21 79 Cholesterol B-100HDL Liver, intestine, Phospholipids, A-I, A-II, A-IV, C-I, C-II, C-III, D, 2 EHDL 1 VLDL, chylo- 20–25 1.019–1.063 32 68 cholesterolHDL 2microns10–20 1.063–1.125 33 67HDL 3 5–10 1.125–1.210 57 43Preβ-HDL 3 < 5 > 1.210 A-IAlbumin/free Adipose > 1.281 99 1 Free fatty acidsfatty acids tissueAbbreviations: HDL, high-density lipoproteins; IDL, intermediate-density lipoproteins; LDL, low-density lipoproteins; VLDL, very lowdensity lipoproteins.1 Secreted with chylomicrons but transfers to HDL.2 Associated with HDL 2 and HDL 3 subfractions.3 Part of a minor fraction known as very high density lipoproteins (VHDL).25–1). The main apolipoprotein of LDL (β-lipoprotein)is apolipoprotein B (B-100) and is found also inVLDL. Chylomicrons contain a truncated form of apoB (B-48) that is synthesized in the intestine, whileB-100 is synthesized in the liver. Apo B-100 is one ofthe longest single polypeptide chains known, having4536 amino acids and a molecular mass of 550,000 Da.Apo B-48 (48% of B-100) is formed from the samemRNA as apo B-100 after the introduction of a stop signalby an RNA editing enzyme. Apo C-I, C-II, andC-III are smaller polypeptides (molecular mass 7000–9000 Da) freely transferable between several differentlipoproteins. Apo E is found in VLDL, HDL, chylomicrons,and chylomicron remnants; it accounts for 5–10% of total VLDL apolipoproteins in normal subjects.Apolipoproteins carry out several roles: (1) they canform part of the structure of the lipoprotein, eg, apo B;(2) they are enzyme cofactors, eg, C-II for lipoproteinlipase, A-I for lecithin:cholesterol acyltransferase, or enzymeinhibitors, eg, apo A-II and apo C-III for lipoproteinlipase, apo C-I for cholesteryl ester transfer protein;and (3) they act as ligands for interaction with lipoproteinreceptors in tissues, eg, apo B-100 and apo E forthe LDL receptor, apo E for the LDL receptor-relatedprotein (LRP), which has been identified as the remnantreceptor, and apo A-I for the HDL receptor. Thefunctions of apo A-IV and apo D, however, are not yetclearly defined.FREE FATTY ACIDS ARERAPIDLY METABOLIZEDThe free fatty acids (FFA, nonesterified fatty acids, unesterifiedfatty acids) arise in the plasma from lipolysisof triacylglycerol in adipose tissue or as a result of theaction of lipoprotein lipase during uptake of plasma triacylglycerolsinto tissues. They are found in combinationwith albumin, a very effective solubilizer, in concentrationsvarying between 0.1 and 2.0 µeq/mL ofplasma. Levels are low in the fully fed condition andrise to 0.7–0.8 µeq/mL in the starved state. In uncontrolleddiabetes mellitus, the level may rise to as muchas 2 µeq/mL.