C-reactive protein (CRP) can be structurally modified by heat, acid, or urea-chelation to express a neoantigen designated by us as neo-CRP. This antigen is also expressed on the in vitro primary protein translation products of both human and rabbit CRP. Unmodified CRP and CRP complexed with pneumococcal C-polysaccharide (CPS) do not express neo-CRP. Forms of CRP expressing neo-CRP but not native CRP antigenicity (even in the presence of CPS) consistently and in a dose-dependent manner potentiated the respiratory burst response of human polymorphonuclear leukocytes and peripheral blood monocytes to heat-modified IgG. Forms of CRP expressing neo-CRP antigenicity also induced reactions of aggregation and secretion from isolated platelets and potentiated platelet activation stimulated by ADP in platelet-rich-plasma, while native CRP alone or complexed with CPS again did not. Unlike CRP-CPS complexes, forms of CRP expressing neo-CRP were not able to activate the complement system. These data emphasize the biologic potential inherent in this humoral acute-phase reactant, particularly in the activation of the formed elements of the blood important in the inflammatory response. Since these cell-activating properties are preferentially observed when CRP is structurally modified to express the neo-CRP antigen, such a molecular conversion may be central to the structure-function relationships of CRP at local sites of inflammation and tissue injury.

Serum amyloid P component (SAP) is a decamer of 10 identical 25.5-kDa subunits. Limited proteolysis of SAP with a-chymotrypsin cleaves the subunit into two fragments of 18 and 7.5 kDa, although the fragments stay together in the decamer under nondenaturing conditions. Proteolysis does not occur in the presence of CaZ+ (10 mM). Cleavage with a-chymotrypsin prevents the Ca2+-dependent binding of SAP to zymosan extract, nucleosomes, and DNA. The a-chymotrypsin cleavage site identified is in a region of SAP that is highly conserved in members of the human C-reactive protein (CRP) family of proteins (pentraxins) to which SAP belongs and is similar to the Ca2+-binding site in calmodulin and related Caz+-binding proteins (Nguyen, N.Y., Suzuki, A., Boykins, R.A., & Liu, T.-Y., 1986, J. Biol. Chem. 261,[10456][10457][10458][10459][10460][10461][10462][10463][10464][10465]. Treatment of SAP with other proteases (trypsin, Pronase, and Nagarse protease) yields fragmentation patterns upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) that are similar to those obtained with a-chymotrypsin. Two other members of the pentraxin family of proteins, hamster female protein and rabbit CRP, also exhibit similar fragmentation patterns on SDS-PAGE when treated with the various proteases. Recently, it has been shown that the homologous protein, human CRP, is cleaved in the same homologous position as cleavage of SAP by a-chymotrypsin, resulting in the loss of Ca2+ binding (as shown by equilibrium dialysis) and Caz+-dependent binding reactivities (Kinoshita, C.M., Ying, S.-C., Hugli, T.E., Siegel, J.N., Potempa, L.A., Jiang, H.J., Houghten, R.A., & Gewurz, H., 1989, Biochemistry 28,9840-9848). These results indicate that the protease sensitivity of this proposed CaZ+-binding region has been conserved and may play an important regulatory role, perhaps via the control of Ca2+-dependent properties of these proteins.

No abstract

Various testicular metal-binding proteins having apparent mol wt in the range of 10-30 kD have been demonstrated by gel filtration of 109Cd- or 65Zn-labeled cytosol, but in no case has a purified metalloprotein been isolated that contains stoichiometric amounts of the metal. The purpose of this work was to purify from rat testes a testes-specific 30 kD Cd-binding protein (Cd-testin) following in vitro addition of 109Cd to testis cytosol. Conventional purification methods similar to those used for purification of metallothionein could not be used because Cd was not retained in stoichiometric amounts by the 30 kD species when these methods were employed. However, using ammonium sulfate fractionation, hydrophobic interaction and gel filtration chromatography, a 30 kD protein containing 2.6 mol of Cd/mol of protein was isolated. Two-dimensional sodium dodecyl sulfate polyacrylamide gel electrophoresis demonstrated that the isolated protein contained one major polypeptide with a mol mass of 22 kD and a pI of 4.6 (22 kD/pI 4.6) and two minor polypeptides (16 kD/pI 4.6 and 10 +/- 4 kD/pI 6.3). Two-dimensional gel electrophoresis demonstrated that the 22 kD species is a major low mol mass (less than 60 kD) protein in rat testis cytosol. The 22 kD protein was not detectable in cytosol of rooster testis, a tissue that is insensitive to Cd-induced damage and devoid of the 30 kD Cd-binding protein. Gel filtration and hydrophobic interaction chromatography of 109Cd- and 65Zn-labeled cytosol demonstrated that 109Cd and 65Zn cochromatograph with the 30 kD protein. The function of this novel 30 kD testicular metal-binding protein is not known, but our work and other studies suggest that its occurrence in testes is linked to the production of a unique 22 kD polypeptide.