Specificity of C-Reactive Protein for Choline Phosphate Residues of Pneumococcal C-Polysaccharide

C-reactive protein (CRP) is an acute phase protein appearing in the serum of man during various pathological conditions, and frequently employed as a clinical index of acute inflammation. It is precipitated from the serum by pneumococcal C-polysaccharide in the presence of Ca ions. Although this precipitation reaction, which led to the discovery of CRP, has been known for many years, there is little information about the active site(s) in the molecule of C-polysaccharide participating in the reaction with CRP. It has been recently reported that choline phosphate is a unit of the macro-molecular structure of C-polysaccharide (1, 2). In an attempt to determine the specificity of CRP for choline phosphate residues of C-polysaccharide, we conducted quantitative inhibition studies of CRP-C-polysaccharide precipitation. The substances tested for inhibitory effect included choline phosphate and a series of organic phosphate menoesters. The results of these experiments indicated that choline phosphate is the most active inhibitor of CRP-C-polysaccharide precipitation yet described, and suggested that this compound might provide the major reacting site of C-polysaccharide.A single lot of CRP-positive serum (no. 9 E.M.), obtained 2 days following surgery from a patient with an intertrochanteric fracture of the femur, was used for all the inhibition experiments described below. Tested by capillary precipitation against a commercial antiserum (Hyland Labs, Los Angeles, California), this serum gave a 4+ reaction for CRP. The results with this serum were representative of data obtained also with other CRP-positive sera. C-polysaccharide was prepared by the method of Anderson and McCarty (3) from a III R strain of pneumococcus. The following substances obtained from commerical sources were tested for inhibitory effect without further purification: Choline phosphate chloride, o-phosphoryl-ethanol-amine, DL-α-glycerophosphate, β-glycerophosphate, L-α-glycerophosphorylcholine, choline base, uridine-5′-monophosphate (UMP), adenosine-5′-monophosphate (AMP), and cytidine-5′-monophosphate (CMP).