Cellular binding and degradation of lipoprotein (a)

GM Fless - Journal of Atherosclerosis and Thrombosis, 1995 - jstage.jst.go.jp
GM Fless
Journal of Atherosclerosis and Thrombosis, 1995jstage.jst.go.jp
S2 Fless nM lipoprotein was 0.324•} 0.126 pmol/mg cell protein (n= 5) and 0.388•} 0.106
pmol/mg cell protein(n= 5) for LDL (Table 1). Specific binding was derived by subtracting the
nonspecfic binding curve (binding obtained in the presence of a 50-fold molar excess
unlabeled LDL) from the total binding curve. Specific binding of Lp (a) at 100 nM lipoprotein
as measured in three experiments was 79% that of LDL, indicating substantial binding of Lp
(a) to LDL receptors. LDL specific binding curves by nonlinear least square regression …
S2 Fless nM lipoprotein was 0.324•} 0.126 pmol/mg cell protein (n= 5) and 0.388•} 0.106 pmol/mg cell protein(n= 5) for LDL (Table 1). Specific binding was derived by subtracting the nonspecfic binding curve (binding obtained in the presence of a 50-fold molar excess unlabeled LDL) from the total binding curve. Specific binding of Lp (a) at 100 nM lipoprotein as measured in three experiments was 79% that of LDL, indicating substantial binding of Lp (a) to LDL receptors. LDL specific binding curves by nonlinear least square regression indicated that the binding of both Lp (a) and LDL to HMDM was of low affinity having a Kd of 0.80 ƒÊM for Lp (a) and 0.23 ƒÊM for LDL (Table 2). Maximal lipoprotein binding for Lp (a) was 2.23•} 0.44 pmol/mg cell protein and for LDL 1.05•} 0.07 pmol/mg cell protein. These results show that the affinity for the LDL receptor on HMDM is at least two orders of magnitude lower than in fibroblasts indicating that the LDL receptor in these two cells is probably different. This conclusion is in keeping with the finding of Knight and Soutar(14), and Van Lenten and Fogelman(15) who demonstrated the HMDM have receptors with low affinity for LDL. Van Lenten and Fogelman proposed the existence of two classes of LDL receptor sites on HMDM, one class with high affinity that may correspond to the classical LDL receptor and a second, also specific for LDL, but of lower affinity. The latter was hypothesized to channel ligands into a different pathway and to operate at high ligand concentrations such as those used in the present study.
37• Ž Degradation Studies In contrast to good cell surface binding at 4• Ž, Lp (a) was degraded much less efficiently than LDL both in terms of total degradation and LDL specific degradation (see Table 3). Results averaged from five experiments indicate that at 100 nM lipoprotein, the total degradation of Lp (a) was only 31.8% that of LDL. LDL specific degradation was obtained by subtracting the nonspecific degradation (degradation obtained in the presence of a 50-fold molar excess unlabeled LDL from the total degradation of either lipoprotein. The LDL specific degradation of Lp (a) was only 17.3% that of LDL. For Lp (a), the specific degradation was 0.738•} 0.062 pmol/mg cell protein(n= 3) and 4.26•} 0.18 pmol/mg cell protein(n= 3) for LDL. Both the total and specific degradation curves for 125I-Lp (a) in HMDM were linear and non-saturable, whereas those of
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