Apoprotein (E--A-II) complex of human plasma lipoproteins. II. Receptor binding activity of a high density lipoprotein subfraction modulated by the apo (E--A-II) complex

Innerarity, T.L.; Mahley, R.W.; Weisgraber, K.H.; Bersot, T.P.

Journal of Biological Chemistry 253(17): 6289-6295


ISSN/ISBN: 0021-9258
PMID: 210175
Accession: 068524197

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Normal human high density lipoproteins (HDL) of the d = 1.063-1.125 ultracentrifugal fraction can be separated by Geon-Pevikon block electrophoresis into 2 subclasses, HDL-I and HDL-II. HDL-I, characterized by the presence of the E apoprotein and the apo(E-A-II) complex along with the A-I and A-II apoproteins, accounted for most, if not all, of the high affinity binding of the human HDL (d = 1.063-1.21) to the low density lipoprotein (LDL) receptors on normal human fibroblasts. By contrast, the HDL-II, the major subclass of the d = 1.063-1.125 fraction, lacked the E apoprotein and the apo(E-A-II) complex and exhibited no significant binding activity. Likewise, the HDL3 (d = 1.125-1.21) did not bind to the LDL receptor. In competitive binding studies using iodinated LDL, the high affinity binding of HDL-I correlated directly with the content of the E apoprotein. Treatment of the HDL-I with disulfide reducing agents resulted in a marked enhancement of the binding activity of the HDL-I, a response which was correlated with the appearance of increased amounts of the E apoprotein. The amount of E. apoprotein in the HDL-I was increased by reduction and alkylation (mercaptoethanol/iodoacetamide) of the apo(E-A-II) complex, which resulted in conversion of this complex to its E and A-II apoprotein subunits. Moreover, stimulation of intracellular cholesterol esterification paraleled the enhanced binding activity of the reduced and alkylated HDL-I. The HDL-I, biologically active with normal human fibroblasts, had no effect on the cellular metabolism of fibroblasts from a receptor-negative homozygous type II hypercholesterolemic patient. Even after reduction and alkylation, neither HDL3 (d = 1.125 to 1.21) nor the HDL-II subclass of the d = 1.063-1.125 fraction exhibited receptor binding activity. Both of these lipoproteins lack the apo(E-A-II) complex. Furthermore, reduction and alkylation did not change the binding activity of LDL. Previously, 1,2-cyclohexanedione was shown to modify the arginyl residues of the B and E apoproteins, thereby selectively blocking the high affinity receptor binding activity of lipoproteins which contained either of these apoproteins (LDL and HDLc, respectively). Likewise, treatment of HDL-I with this reagent abolished the biological activity of these apo-E-containing lipoproteins. These studies show that the HDL-I subclass accounts for the binding activity of human HDL and that the activity of this subclass can be enhanced by conversion of the inactive apo(E-A-II) complex of HDL-I to the biologically active E apoprotein by reduction of the mixed disulfide. Whether or not this interconversion occurs in vivo and operates as a modulator of HDL binding to the LDL receptors remains to be determined.