Ultracentrifugal subclasses of low and intermediate density lipoproteins.

Low density lipoprotein (LDL) and intermediate density lipoprotein (IDL) classes have been shown to be composed of discrete metabolic entities or subclasses. Present ultracentrifugal methods are unable to precisely determine these subclasses. A new analytical micro-ultracentrifugal method was developed that facilitates the determination of IDL and LDL subclasses and their F1.21 flotation coefficient from ultracentrifugal scans. The method is based on the modification of a published equation (Fujita, H. 1956. J. Chem. Phys. 24: 1084-1090) adapted to calculate concentration gradient boundary curves for IDL and LDL that are approximately Gaussian in form. Using an extension of this modified equation, theoretical distributions of the gradient curves were calculated. By applying the theoretical distributions, IDL and LDL subclasses were resolved from absorbance scans as Gaussian concentration gradient boundary curves. Both theoretically calculated and experimentally determined boundary curves for IDL and LDL lipoproteins were plotted and found to be in excellent agreement. Three subclasses of LDL and four subclasses of IDL were determined. The mean flotation rates of the LDL subclasses were: LDL1 = 37.2 +/- 0.6, LDL2 = 31.1 +/- 0.9, and LDL3 = 26.7 +/- 0.7. The mean flotation rates of the IDL subclasses were: IDL1 = 61.6 +/- 0.9, IDL2 = 53.9 +/- 1.0, IDL3 = 50.1 +/- 0.6, and IDL4 = 45.6 +/- 1.1.