Structural properties and lipid binding of human apolipoprotein A-IV

Differences in the hydrophobic properties of discrete alpha-helical domains of rat and human apolipoprotein A-IV

The primary structure of human apolipoprotein A-IV has been analyzed to identify amphipathic alpha-helical regions and compare their hydrophobic properties with analogous segments of rat apolipoprotein A-IV. Significant differences were found in the mean hydrophobicity and/or mean helical hydrophobic moment of several discrete alpha-helical domains of rat and human apolipoprotein A-IV, which may determine the differences in their biophysical and biological behavior.

Free apolipoproteins A-I and A-IV present in human plasma displace high-density lipoprotein on cultured bovine aortic endothelial cells

Adult bovine aortic endothelial (ABAE) cells, exposed to serum-free medium, specifically bind 125I-labeled human high-density lipoprotein (125I-HDL). Addition of human lipoprotein-deficient serum (LPDS) reduces the specific binding of 125I-HDL in a concentration-dependent manner, such that LPDS at a concentration of 6 mg protein/ml almost completely inhibits the specific binding of 125I-HDL. ABAE cultures exposed to 125I-labeled LPDS (125I-LPDS) specifically bind two peptides, which appear as minor iodinated components in 125I-LPDS. The binding of these two components is abolished in the presence of excess amounts of unlabeled LPDS or HDL. Preincubation of ABAE cells with 25-hydroxycholesterol (25-HC) results in an increase in the binding of the two 125I-LPDS components, similar to the increase observed in 125I-HDL binding in the presence of 25-HC. These two LPDS components comigrate on sodium dodecyl sulfate/polyacrylamide gel electrophoresis (SDS-PAGE) with apolipoproteins A-I and A-IV of molecular masses 28 kDa and 43 kDa respectively. Furthermore, these two proteins were transferred from the SDS gel to nitrocellulose paper and interacted specifically with anti-(A-I) and anti-(A-IV) sera respectively. When ABAE cultures, pretreated with 25-HC in the presence of LPDS, are subjected to cell-surface iodination, the A-IV appears as one of the major proteins on the cell surface accessible to iodination. The interaction of A-IV with the cell surface of 25-HC-treated cells is not specific to ABAE cells and appears also in human skin fibroblasts. Analysis of the relative amounts of various apolipoproteins in the 125I-HDL bound to ABAE cells demonstrates a decrease in the relative amount of iodinated A-II concomitant with increase in the relative amounts of the other iodinated apolipoproteins, when compared to the composition of the native 125I-HDL. These changes are similar whether the binding is done in the presence or absence of LPDS. It indicates that the decrease in 125I-HDL binding in the presence of LPDS is not due to displacement of the iodinated apolipoproteins A-I and A-IV in the 125I-HDL by unlabeled A-I and A-IV present in LPDS. The results indicate that free apolipoproteins A-I and A-IV, present in LPDS, can displace HDL on the cell surface of ABAE cells. Thus, free A-I and A-IV, present in plasma, control the binding of HDL to endothelial cells and may regulate the process of cholesterol removal from the cells performed by HDL.

Lipoprotein affinity of human apolipoprotein A-IV during cholesterol esterification

We have studied the lipoprotein distribution of human apo A-IV during cholesterol esterification by the action of endogenous lecithin-cholesterol acyltransferase. Using immunologic and radiotracer techniques at 4 degrees C, apo A-IV was found in two discrete monomeric and dimeric populations, unassociated with plasma lipoproteins. With incubation at 37 degrees C, apo A-IV initially associated with the high density lipoprotein-3 fraction, but thereafter dissociated from its surface, and reappeared as unbound protein and in association with a complex in the low density lipoprotein size range. Inclusion of LCAT inhibitors in the incubations abolished these changes. We conclude that the changes in lipoprotein distribution of human apo A-IV closely parallel the formation and exchange of plasma cholesteryl esters.