The measurement of C-reactive protein in human sera; comparison of the clinical tests on the basis of a quantitative method

The career of Maclyn McCarty

On January 2, 2005, the scientific community lost a valued colleague and friend. Maclyn McCarty, or "Mac," as he was better known, was perhaps most recognized for his part in the discovery of DNA as the carrier of genetic information. But McCarty's scientific career was long and fruitful, and his contributions to science were vast. This retrospective offers a look at some of Mac's other notable scientific achievements.

Conversion of native oligomeric to a modified monomeric form of human C-reactive protein

C-reactive protein (CRP) is a pentameric oligoprotein composed of identical 23 kD subunits which can be modified by urea-chelation treatment to a form resembling the free subunit termed modified CRP (mCRP). mCRP has distinct physicochemical, antigenic, and biologic activities compared to CRP. The conditions under which CRP is converted to mCRP, and the molecular forms in the transition, are important to better understand the distinct properties of mCRP and to determine if the subunit form can convert back to the pentameric native CRP form. This study characterized the antigenic and conformational changes associated with the interconversion of CRP and mCRP. The rate of dissociation of CRP protomers into individual subunits by treatment in 8 M urea-10 mM EDTA solution was rapid and complete in 2 min as assayed by an enzyme-linked immunofiltration assay using monoclonal antibodies specific to the mCRP. Attempts to reconstitute pentameric CRP from mCRP under renaturation conditions were unsuccessful, resulting in a protein retaining exclusively mCRP characteristics. Using two-dimensional urea gradient gel electrophoresis, partial rapid unfolding of the pentamer occurred above 3 M urea, a subunit dissociation at 6 M urea, and further subunit unfolding at 6-8 M urea concentrations. The urea gradient electrophoresis results suggest that there are only two predominant conformational states occurring at each urea transition concentration. Using the same urea gradient electrophoresis conditions mCRP migrated as a single molecular form at all urea concentrations showing no evidence for reassociation to pentameric CRP or other aggregate form. The results of this study show a molecular conversion for an oligomeric protein (CRP) to monomeric subunits (mCRP) having rapid forward transition kinetics in 8 M urea plus chelator with negligible reversibility.

A capillary electrophoresis-based assay for the binding of Ca2+ and phosphorylcholine to human C-reactive protein

Affinity capillary electrophoresis was performed to quantitate the binding of Ca2+ and phosphorylcholine to human C-reactive protein (CRP). The assay requires no modifications of any of the molecules involved, uses minuscule amounts of protein (8.5 x 10(-15) mol per analysis, i.e., less than 1 pmol for 15 triplicate data points), and the binding could be examined under conditions of physiological ionic strength and pH. The values for the dissociation constants obtained here (KD = 59 microM for Ca(2+)-CRP and 18 microM for the phosphorylcholine-CRP interaction) were in close agreement with previous studies using gel filtration and equilibrium dialysis. As long as one of the reactants can be detected and recovered quantitatively in the capillary electrophoresis system, the method is generally useful to study interactions where complexed molecules display an electrophoretic mobility that is different from that of unbound molecules and where the rates of association and dissociation are sufficiently fast.

An appraisal of polystyrene-(ELISA) and nitrocellulose-based (ELIFA) enzyme immunoassay systems using monoclonal antibodies reactive toward antigenically distinct forms of human C-reactive protein

The purpose of this study was to compare and contrast two enzyme immunoassay systems: the enzyme-linked immunosorbent assay (ELISA), which utilizes polystyrene microtiter plates as the adsorptive surface and the enzyme-linked immunoflow assay (ELIFA), which utilizes nitrocellulose membranes. The principal parameter under scrutiny was the denaturing or unfolding effects caused by the interaction of the protein with the adsorptive surfaces in each assay system. These effects were monitored by utilizing two conformationally distinct forms of human C-reactive protein (CRP), the native form of CRP and a denatured form (M-CRP), with a corresponding panel of monoclonal antibodies (MAbs) specific to either CRP or M-CRP. The results show that the ELIFA system was less sensitive than the ELISA system but that the ELIFA assay can be completed in less time than the ELISA. Also, adsorption of native CRP to the polystyrene surface in the ELISA system resulted in conformational changes of the adsorbed native CRP protein such that M-CRP reactive determinants were available for binding with anti-M-CRP MAbs, whereas native CRP adsorbed to the nitrocellulose membrane in the ELIFA system resulted in very limited conversion of CRP to M-CRP reactive epitopes. These results have important implications for development of immunoassays and screening of MAbs for proteins whose conformations may be affected by adsorption to various surfaces.

Protein volumes and hydration effects. The calculations of partial specific volumes, neutron scattering matchpoints and 280-nm absorption coefficients for proteins and glycoproteins from amino acid sequences

Amino acid sequences, carbohydrate compositions and residue volumes are used to compare critically calculations of partial specific volumes v, neutron scattering matchpoints and 280-nm absorption coefficients with experimental v values for proteins and glycoproteins. The v values that are obtained from amino acid densitometry underestimate experimental v values by 0.01-0.02 ml/g while the v values from crystallographic volumes overestimate the experimental v values by 0.04-0.05 ml/g. An intermediate consensus volume set of amino-acid-residue volumes is proposed in order to predict experimental v values using sequence information. The method is extended to carbohydrates and glycoproteins. Neutron scattering matchpoints can be calculated from crystallographic residue volumes on the basis of the non-exchange of 10% of the main-chain NH protons. Crystallographic results on protein-bound water are used to account for the experimental values of v and matchpoints. Finally, 280-nm absorption coefficients, A1%, 1 cm 280, of 5-27 are found to be well predicted by the Wetlaufer procedure based on the totals of Trp, Tyr and Cys residues. Average errors are +/- 0.7, and the experimental A(1%,1cm)280 values can be larger than the predicted values by 3%.

Binding of human C4 to C-reactive protein-pneumococcal C-polysaccharide complexes during activation of the classical complement pathway

Sequential interaction of CRP-PnC aggregates, made at slight CRP excess, with purified human C1, C4 and C2oxy resulted in formation of an effective C3-convertase, indicating the binding of C1, C4 and C2 on the aggregates. Immunoprecipitation experiments demonstrated that, following cleavage of 125I-C4 by CRP-PnC-C1 complexes, approximately 3% of the 125I-C4 was bound to CRP while a lower percentage was bound to PnC, CRP-C4 complexes could also be demonstrated by substituting 125I-CRP for 125I-C4. The nature of the CRP-C4 bond was examined by electrophoretic analysis. Complexes of 125I-C4-CRP prepared as earlier were incubated at 100 degrees C for 2 min in buffer containing 2% SDS and 5% beta-mercaptoethanol and subjected to electrophoresis in SDS-containing polyacrylamide gradient slab gels. Autoradiography of the dried gels revealed the presence of high mol. wt bands containing the alpha'-chain of C4b. CRP could also be demonstrated in these high mol. wt bands which apparently represented covalent complexes between the alpha'-chain of C4b and CRP monomers. Since CRP contains no detectable carbohydrate, it seems likely that an amide bond is formed between the two proteins.

Platelet inhibitory effects of CRP preparations are due to a co-isolating low molecular weight factor

We previously reported that C-reactive protein (CRP), an acute phase reactant, inhibits platelet activation through an effect upon a factor(s) critical to ADP-mediated secondary wave platelet aggregation but independent of a direct effect upon platelet contractile elements. However, a role for an accessory factor in this inhibitory effect became of concern because of an inconsistency in the effects of CRP preparations upon the platelet: inhibition was lost upon storage and CRP preparations differed, on a weight basis, in inhibitory capacity and sensitivity to the presence of the CRP ligand C-polysaccharide (CPS(. The studies presented herein were thus intended to assess whether an accessory factor was involved in the inhibition of platelet activation observed with CRP. We report that the activity of the inhibitory CRP preparations resulted from association with a low molecular weight factor (LMF) with an apparent nominal molecular weight of 8300-12,500 and an A280:A260 ratio of approximately 0.4. Purified CRP did not inhibit platelet responsiveness but CRP with associated LMF (CRP-LMF) did. Moreover, the inhibitory capacity of CRP-LMF but not LMF was substantially reversed in the presence of CPS. These studies indicate that the platelet inhibitory properties of CRP preparations result from and are contingent upon the presence of a co-isolating low molecular weight factor.

Cross-reactivity between C-reactive protein and idiotypic determinants on A phosphocholine-binding murine myeloma protein

Binding of human 125I-C-reactive protein (CRP) to sheep erythrocytes sensitized with pneumococcal C polysaccharide (E-PnC) was found to be Ca++ dependent and inhibitable by phosphocholine, CRP, and HOPC 8. Binding of 125I-HOPC 8 to EPnC was Ca++ -independent but could also be inhibited by phosphocholine, CRP, and HOPC 8. Thus, CRP and HOPC 8, despite a differential Ca++ requirement, share a common binding specificity for phosphocholine. A monoclonal anti-idiotypic antibody (MAB), GB4-10, prepared in A/J mice immunized with BALB/c HOPC 8 inhibited the binding of both 125I-CRP and 125I-HOPC 8 to E-PnC. In addition, both proteins bound to GB4-10 immobilized on polysterene tubes. Interestingly, binding of 125I-CRP to GB4-10 required Ca++. Similar results were also obtained with another MAB (AB1-2) prepared similarly to GB4-10, whereas neither protein bound to a control MAB (EB3-7) against an alpha1 leads to 3 dextran-binding myeloma protein, J558. Binding of 125I-HOPC 8 to GB4-10 could be inhibited by HOPC 8, keyhole limpet hemocyanin-phosphocholine but not phosphocholine but not phosphocholine, and in the presence of Ca++ by CRP. These data indicate that CRP bears antigenic determinants cross-reacting with certain idiotypic determinants on HOPC 8. They also suggest that Ca++ acts as an allosteric effector, perhaps stabilizing the phosphocholine-binding site of CRP.

Significance of serum C-reactive protein elevation in patients with systemic lupus erythematosus

Serum C-reactive protein (CRP) concentration was determined by a radial immunodiffusion method in serum samples collected over a mean period of 19 months from a group of 17 patients with systemic lupus erythematosus (SLE). Thirty-two episodes of CRP elevation were detected. Twenty of these were associated with active SLE, and 9 with proven or possible infection or bone fracture. In 3 cases no explanation for CRP elevation was readily apparent. There were 8 instances of onset or exacerbation of lupus activity without accompanying CRP elevation. These data indicate that CRP elevation in the course of SLE is frequently associated with activation of lupus, and that detection of such elevation does not differentiate between lupus activity and infection.

A fluorescent immunoassay for the quantification of C-reactive protein

A sensitive fluorescent binding immunoassay (FBIA) was developed for measuring C-reactive protein (CRP), at a level of 20 ng/ml, in human and mouse sera. The advantages of the FBIA over radioimmunoassays are the long shelf-life of the FITC-labelled antibody, the re-use of the antibody bound microbeads, the short assay time and the use of inexpensive instruments.

C-reactive protein: a molecule composed of subunits

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C-reactive protein in virus infection

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An attempt has been made to correct cerebral hypertension induced by direct compression of the brain in a group of six dogs. The animals, which had been previously fitted with an inflatable subdural rubber balloon, were either kept eupneic (isolated mechanical hypertension) or deliberately hypoventilated (mixed mechanical and acidotic hypertension). In the first instance, administration of urea brought the intracranial pressure back to control values while, in the second case, injection of an amine buffer controlled only the acidotic component of intracranial hypertension.

The occurrence during acute infections of a protein not normally present in the blood. V. Physical-chemical properties of the C-reactive protein crystallized by a modified technique

A method is described for obtaining crystalline C-reactive protein from serous fluids in which the protein is associated with lipid. Most pathological fluids currently available as a source of this protein appear to fall in this category. Crystalline C-reactive protein has its isoelectric point at pH 4.82 as determined by free electrophoresis in McIlvaine's buffer. Its mobility in the electrophoresis cell, both alone and after addition to normal serum, coincides with that of the beta-globulin fraction of the serum. In contrast to this finding, by the method of zone electrophoresis on a starch supporting medium the protein migrates with the gamma(1)-globulin. The significance of this discrepancy is discussed. Studies in the ultracentrifuge indicate an s(20,w) of 7.5.

The relationship between the acute phase response and antibody production in the rabbit. I. Correlation between the early appearance of Cx-reaction protein and subsequent antibody production

It has been found that some rabbits respond to the administration of either human C-reactive protein or human gamma globulin with the production of an acute phase substance, Cx-reactive protein. A correlation was found to exist between the amount of Cx-protein produced and the subsequent production of significant titers of precipitating antibody to the two antigens employed.

The relationship between the acute phase response and antibody production in the rabbit. II. The stimulation of Cx-reactive protein response by certain adjuvants and the relation of this response to the enhancement of antibody formation

The ability of an adjuvant and its individual constituents to induce the production of Cx-reactive protein in rabbits has been studied. It was found that the adjuvant stimulated rabbits to produce large amounts of the acute phase protein for 3 to 6 days. Melted aquaphor blended with saline stimulated the production of Cx-reactive protein for 3 or 4 days. Mineral oil was less effective in stimulating the production of the protein than either adjuvant or aquaphor. Heat-killed Jamaica strain tubercle bacilli suspended in mineral oil did not induce the Cx-protein response. The ability of subcutaneously administered adjuvant without antigen incorporated in the saline phase to potentiate the antibody response of rabbits to the intravenously administered antigens, C-reactive protein and human gamma globulin, was investigated. It was found that the adjuvant-treated animals produced more precipitating antibody to the two intravenously administered antigens than did the control animals given intravenous antigen alone.