A novel phosphorylcholine-binding protein from rat serum and its effect on heparin-lipoprotein complex formation in the presence of calcium.

An adsorbent was synthesized by attaching 4-aminophenylphosphorylcholine to cyanogen bromide-activated Sepharose. A phosphorylcholine (P-choline)-binding protein from rat serum was adsorbed on this affinity column which was eluted by 4 mM P-choline. The protein separated into two bands of Mr = 47,000 and 24,000 on sodium dodecyl sulfate-polyacrylamide gradient gels and contained 18% carbohydrate. A serum protein factor, precipitable by 30-50% (NH4)2SO4, was previously shown to inhibit Ca2+-heparin-rat serum very low density lipoprotein (VLDL) precipitation reaction, whereas P-choline counteracted the action of this protein (Mookerjea, S. (1978) Can. J. Biochem. 56, 746-752). It is now demonstrated that purified P-choline-binding protein prevents Ca2+-heparin-chylomicron or VLDL complex formation and P-choline reverses the effect of this protein. Antibody to P-choline-binding protein raised in rabbits produced a single precipitin line against the pure antigen. The antiserum, however, did not react against rat serum chylomicron, VLDL, low density lipoproteins, or high density lipoprotein. Human serum appears to lack P-choline-binding protein, since (a) the affinity column did not adsorb any such protein, (b) P-choline had no effect on the Ca2+-heparin-serum lipoprotein precipitation reaction, and (c) an immunodiffusion test against the antiserum was negative. However, when P-choline-binding protein was added to human serum, the lipoprotein precipitation was inhibited, and P-choline counteracted the effect of this protein. Preliminary experiments suggested a stoichiometric interaction between P-choline-binding protein and VLDL. Hydrophilic P-choline groups exposed on the surface of VLDL may possibly interact with the P-choline-binding protein and thereby affect the precipitation of lipoproteins by heparin and Ca2+.