Low-density lipoprotein receptor binding determinants switch from apolipoprotein E to apolipoprotein B during conversion of hypertriglyceridemic very-low-density lipoprotein to low-density lipoproteins

Using thrombin and trypsin as probes, we determined: first, that low-density lipoprotein (LDL) receptor binding determinants switch from apolipoprotein (apo) E to apo-B within the very-low-density lipoprotein (VLDL) S(f) 20-60 region of the metabolic cascade from VLDL₁ (S(f) 100-400) of hypertriglyceridemic (HTG) human subjects to LDL. Second, two different conformations of apo-E exist in HTG-VLDL S(f) > 60, one accessible (≥1 mol/mol of particle) and one inaccessible (1-2 mol/mol) to both thrombin and the LDL receptor; normal VLDL (S(f) > 60) have only the inaccessible conformation and therefore do not bind to the LDL receptor. Third, thrombin degrades apo-B into large fragments, three of which have electrophoretic mobilities similar to B-48, B-74, and B-26; this, however, has no effect on apo-B-mediated receptor binding. Fibroblast studies showed that thrombin could abolish receptor uptake of HTG-VLDL₁ and HTG-VLDL₂ (S(f) 60-100), had little or no effect on HTG-VLDL₃ (S(f) 20-60), and no effect on uptake of intermediate-density lipoprotein (IDL) or LDL. Trypsin abolished the binding of HTG-VLDL, and HTG-VLDL₂, reduced that of HTG-VLDL₃, but had little to no effect on IDL or LDL binding. Immunochemical techniques revealed that thrombin cleaved some apo-E into the E-22 and E-12 fragments; after trypsin treatment no apo-E was detected in any HTG-lipoprotein. Normal VLDL subclasses contained less apo-E than the corresponding HTG-VLDL subclasses and it was not cleaved by thrombin. Apo-B immunoreactivities of VLDL subclasses were not significantly changed after treatment with thrombin, although thrombin cleaved some of the B-100 of each VLDL subclass, and all apo-B in IDL and LDL, into 4-6 major large fragments. Trypsin converted all of the apo-B of each lipoprotein into smaller fragments (M(r) < 100,000). We conclude that apo-E of the thrombin-accessible conformation mediates uptake of HTG-VLDL₁ and HTG-VLDL₂ but that apo-B alone is sufficient to mediate receptor binding of IDL and LDL; the switch from apo-E to apo-B as the primary or sufficient binding determinant occurs within the VLDL₃ (S(f) 20-60) region of the metabolic cascased where receptor binding first appears in VLDL subclasses from normal subjects.