Crystallization of C-Reactive Protein Following Removal of Associated Lipid-Containing Material by Antiserum to Normal Human Beta Lipoprotein **

C-reactive protein (CRP) as it occurs in serum and in all but a very few pathological fluids is associated with a phosphorus-containing substance or substances of lipid character.' It can be obtained from the fraction of serum precipitable by 0.5 to 0.75 saturation with ammonium sulfate, following dialysis against dilute calcium chloride. The complex obtained by this procedure is insoluble in distilled water and in 0.85 per cent sodium chloride, but is completely soluble in a solution 0.1M in respect to both sodium chloride and sodium citrate. C-reactive protein rendered free of lipid-containing material is not precipitable by calcium ion alone, but does precipitate as a complex with the somatic C carbohydrate of pneumococcus in the presence of ionized calcium. The first crystallization of this protein was described by McCarty, who obtained it in crystalline form, containing no phosphorus, from two unusual serous fluids in which the protein existed unbound to lipid.' In McCarty's procedure, washed CRP-C carbohydrate complex, dissolved in a solution O.1M with respect to both sodium citrate and sodium chloride, was brought to 0.75 saturation with Na2SO4 and kept at 37°C. An amorphous precipitate formed and over a period of days gave rise to crystalline material. Crystalline CRP typically appears as flat rhomboid plates. Since both serum-associated lipid material and pneumococcal C carbohydrate contain phosphorus, it was evident from the elementary analysis of McCarty's two preparations that crystalline Creactive protein was free of both lipid and carbohydrate. Wood, Slater and McCarty subsequently described a simple modification of method by which lipid-free, crystallizable CRP could be obtained.' Essentially this consisted of gently stirring concentrated CRP complex over chloroform in the cold several times. The defatted protein was then further purified and crystallized according to McCarty's original method.