High-density lipoprotein (HDL) does not represent one structure but rather a series of lipoproteins that are sequentially produced. HDL dynamically switches between different conformations. Fractionated HDL particles can be classified based on physiochemical properties, cholesterol component, apolipoprotein composition, electrophoretic mobility, density, and biological function.^[1][2] HDL concentration in the serum can be classified into either low and high when the serum HDL concentration ≥ 60mg/dL and < 40 mg/dL, respectively.^[3]

High HDL level is defined by the National Cholesterol Education Program (NCEP) as serum HDL concentration ≥ 60mg/dL.^[3]

Low HDL level is defined by the National Cholesterol Education Program (NCEP) as serum HDL concentration < 40 mg/dL.^[3]

HDLs are highly heterogenous in their physiochemical characteristics due to the differences in relative composition of intercalated apolipoproteins thus may be separated based upon buoyant density, size, charge, or apolipoprotein composition. Moreover, the amphiphatic helical structure of apoA-I possesses a hinge domain that allows it to switch between different conformations corresponding to HDLs of variable size.^[4] With density gradient ultracentrifugation, HDLs can be separated into HDL₂, HDL₃, and very-high-density lipoprotein (VHDL). Nondenaturing polyacrylamide gradient gel electrophoresis may also be used to fractionate HDLs into five distinct populations of descending size: HDL_2b, HDL_2a, HDL_3a, HDL_3b, and HDL_3c.^[5][6] Isopycnic density gradient ultracentrifugation has been used to analyze the fasting plasma to yield equivalent delineations as well.^[7][8] Yet another method of classifying HDL lipoprotein particles by size is nuclear magnetic resonance (NMR).^[9] Two-dimensional gel electrophoresis technique can be used to resolve HDL particles in the plasma into lipid-poor pre-β-HDLs and α-HDL that predominantly contains mature spherical cholesteryl esters.^[10]

HDLs may be ultracentrifugally fractionated into two major populations.^[11][12][13]

HDL may be isolated on the basis of size by non-denaturing polyacrylamide gradient gel electrophoresis or by isopycnic density gradient ultracentrifugation.^[11]

In the order of decreasing size:^[13]

• HDL_2b
• HDL_2a
• HDL_3a
• HDL_3b
• HDL_3c

Another classification is:

• Very large HDL particles (VL-HDL)
• Large HDL particles (L-HDL)
• Medium HDL particles (M-HDL)
• Small HDL particles (S-HDL)
• Very-small HDL particles (VS-HDL)
• Pre-β-1 HDL (role in macrophage cholesterol efflux)^[1]

High density lipoproteins can be immunoseparated on the basis of apolipoprotein composition into particles containing different apolipoproteins. The two major apolipoproteins found within HDL particles are the apoA-I and apoA-II.^[14][15] Several other minor apolipoproteins associated with HDL include apoA-IV, apoA-V, apoC-I, apoC-II, apoC-III, and apoE.^[16] However, LpA-I and LpA-I:LpA-II constitute the major portions of ciculating HDLs.

• LpA-I (contains only apoA-I)
• LpA-I:LpA-II (contains both apoA-I and apoA-II)
• LpA-IV
• LpE^[11][13]

HDL can be separated according to charge by agarose gel electrophoresis.^[11]

Note that pre-beta < pre-alpha < alpha.

• Pre-beta (positive)
• Pre-alpha
• Alpha (negative)

Template:WikiDoc Sources