Studies on the interaction of rat serum phosphorylcholine binding protein (PCBP) with plasma lipoproteins

Saxena, Uday (1986) Studies on the interaction of rat serum phosphorylcholine binding protein (PCBP) with plasma lipoproteins. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Rat serum phosphorylcholine binding protein (PCBP) was previously shown to inhibit heparin-lipoprotein-precipitation reaction. Comparison of the effect of PCBP on heparin-lipoprotein precipitation reaction with similar proteins from other species revealed a striking difference between the glycosylated rat PCBP and female hamster FP and the non-glycosylated varieties (human and rabbit CRP’s). Whereas FP shared the inhibitory effect with rat PCBP, human and rabbit CRP failed to inhibit the precipitation reaction suggesting a role of the sialic acid residues on PCBP and FP in the mechanism of inhibition of heparin-lipoprotein precipitation reaction. -- The binding of PCBP to multilamellar liposomes prepared with egg phosphatidylcholine (PC) and lysophosphatidylcholine (LPC) was studied and the binding was found to be Ca²⁺ dependent and required the incorporation of 25% LPC into the liposomes. The binding was inhibited by phosphorylcholine (P-choline). Substitution of phosphorylcholine head groups by phosphorylethanolamine and phosphorylserine on the PC of the liposomes reduced the binding considerably indicated involvement of the phosphorylcholine head groups in the bind of PCBP to liposomes. -- Studies on the binding of human plasma lipoproteins to PCBP immobilized on Sepharose showed that very low density lipoproteins (VLDL) were partially bound and the bound fraction contained higher amounts of apoprotein (apo) B and E. All the low density lipoproteins (LDL) were bound to the column. In case of high density lipoproteins (HDL) only a small fraction was retained on the column but that bound fraction contained all the apo E and Lp(a) applied. The binding of lipoproteins was Ca²⁺ dependent and the bound lipoproteins were eluted by a P-choline gradient. Prior equilibration of Sepharose-PCBP column with P-choline prevented the binding of LDL but the removal of sialic acid from PCBP had no effect on the binding of LDL to immobilized desialylated PCBP. Chemical modification of arginyl residues on apo B in LDL resulted in marked reduction of binding whereas modification of lysine residue had no effect. The results suggest a preferential binding of apo B and E containing lipoproteins with PCBP. -- Investigations to examine the effect of PCBP on the binding of human LDL to LDL receptors on liver membranes from estradiol treated rats showed that PCBP inhibited the binding of LDL to receptors. Preincubation of liver membranes with PCBP did not affect the binding of ¹²⁵I-LDL to the membranes. Gel filtration analysis of the incubation products from the LDL-receptor assays showed a concentration dependent binding of ¹²⁵I-PCBP to LDL. These results suggest that the inhibitory effect of PCBP is probably due to fluid phase interactions between LDL and PCBP and not due to the binding of PCBP to the LDL receptor site. -- The effectiveness of Sepharose-PCBP columns to bind plasma VLDL and LDL from control and hypercholestrolemic rabbits when used in a extracorporeal plasmapheretic system was tested. Results showed that Sepharose-PCBP columns bound some circulating plasma lipoproteins and most (>90%) of the bound lipoprotein fraction contained VLDL + LDL. The results obtained in this study support the possibility of a role of rat PCBP and similar circulation phosphorylcholine binding proteins of other species in lipoprote in metabolism.

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