Binding of apolipoprotein A-I and A-II after recombination with phospolipid vesicles to the high density lipoprotein receptor of luteinized rat ovary

To determine the apolipoprotein specificity of high density lipoprotein (HDL) receptor, apolipoprotein A-I (apo-AI) and apolipoprotein A-II (apo-AII) purified from high density lipoprotein₃ (HDL₃) were reconstituted into dimyristoyl phosphatidylcholine vesicles (DMPC) and their ability to bind to luteinized rat ovarian membranes was examined. Both ¹²⁵I-apo-A-I·DMPC and ¹²⁵I-apo-A-II·DMPC were shown to bind to ovarian membranes with K(d) = 2.87 and 5.70 μg of protein/ml, respectively. The binding of both ¹²⁵I-apo-A-I·DMPC and ¹²⁵I-apo-A-II·DMPC was inhibited by unlabeled HDL₃, apo-A-I·DMPC, apo-A-II·DMPC, apo-C-I·DMPC, apo-C-II·DMPC, apo-C-III₁·DMPC, and apo-C-III₂·DMPC, but not by DMPC vesicles, bovine serum albumin·DMPC or low density lipoprotein. Since the binding labeled apo-A-I·DMPC and apo-A-II·DMPC was inhibited by the DMPC complexes of apo-C groups, the direct binding of ¹²⁵I-apo-C-III₁·DMPC was also demonstrated with K(d) = 9.6 μg of protein/ml. In addition, unlabeled apo-A-I·DMPC, and apo-A-II·DMPC, as well as apo-C·DMPC, inhibited ¹²⁵I-HDL₃ binding. ¹²⁵I-apo-A-I, ¹²⁵I-apo-A-II, and ¹²⁵I-apo-C-III₁ in the absence of DMPC also bind to the membranes. These results suggest that HDL receptor recognizes apolipoprotein AI, AII, and the C group and that the binding specificity of the reconstituted lipoproteins is conferred by their apolipoprotein moiety rather than the lipid environment. In vivo pretreatment of rats with human chorionic gonadotropin resulted in an increase of ¹²⁵I-apo-A-I·DMPC, ¹²⁵I-apo-A-II·DMPC, and ¹²⁵I-apo-C-III₁·DMPC binding activities. However, no induction of binding activity was observed when the apolipoprotein was not included in DMPC vesicles. An examination of the equilibrium dissociation constant and binding capacity for ¹²⁵I-I-apo-A-I·DMPC and ¹²⁵I-apo-A-II·DMPC after human chorionic gonadotropin treatment revealed that the increase in binding activity was due to an increase in the number of binding sites rather than a change in the binding affinity. These results further support our contention that apo-A-I, apo-A-II, and the apo-C group bind to HDL receptor. In conclusion, the HDL receptor of luteinized rat ovary recognizes apolipoproteins A-I, A-II, and the C group but not low density lipoprotein, and the binding is induced by human chorionic gonadotropin in vivo.