Low-density lipoproteins (hLDL) and beta-migrating-
very-low-density lipoproteins (
beta-VLDL) were isolated from the plasma of
cholesterol-fed White Carneau (WC) pigeons and
low-density lipoproteins (nLDL) were isolated from the plasma of grain-fed WC pigeons. The
lipoproteins were radiolabeled with 125I or 131I and injected into normocholesterolemic or hypercholesterolemic WC pigeons to determine their rate of clearance from the plasma. The fractional catabolic rate (FCR) of nLDL and hLDL in normocholesterolemic pigeons averaged 0.202 and 0.206 pools/h.respectively.
beta-VLDL was cleared at a significantly slower rate of 0.155 pools/h. The FCR of the same
lipoproteins injected into hypercholesterolemic pigeons was reduced by 17% for nLDL, 50% for hLDL and 57% for
beta-VLDL, indicating that the effect of
hypercholesterolemia on clearance in vivo was different for the three
lipoproteins. The FCR of reductively methylated pigeon
LDL (MeLDL), which gives a measure of receptor-independent clearance of
LDL, was shown previously to be 0.037 pools/h. These studies suggest therefore that
LDL and
beta-VLDL are cleared from the plasma of normocholesterolemic and hypercholesterolemic pigeons at a rate substantially greater than that predicted for non-specific processes. Despite the reduction in the clearance rate of hLDL and
beta-VLDL due to
cholesterol feeding, the absolute amount of
cholesterol that was cleared from the plasma by these
lipoproteins was increased from approx. 200 mg/kg
body weight per day in the normocholesterolemic pigeons to greater than 1000 mg/kg
body weight per day in the hypercholesterolemic pigeons. This is due principally to the enrichment in
cholesterol relative to
protein of the
lipoproteins isolated from
cholesterol-fed pigeons and the failure of
hypercholesterolemia to completely inhibit receptor-dependent clearance of
LDL and
beta-VLDL. The lower rate of clearance of
beta-VLDL relative to
LDL is in marked contrast to mammalian
beta-VLDL, which is cleared much faster than
LDL, but is consistent with the lack of
apo E on pigeon
lipoproteins.
Apo E is the
apoprotein that is thought to be responsible for the rapid clearance of
beta-VLDL in normocholesterolemic mammals. The low rate of
beta-VLDL clearance in pigeons also suggests that pigeons lack an
apolipoprotein that function like mammalian
apo E.