Dissociation of serum amyloid P from c4b-binding protein and other sites by lactic acid: Potential role of lactic acid in the regulation of pentraxin function

Serum amyloid P (SAP) and C-reactive protein (CRP) are two members of the pentraxin family of proteins. These proteins associate with a variety of other materials that are found in serum under normal or pathological circumstances. This study showed that carboxylated compounds, especially lactic acid, were capable of dissociating pentraxins from several macromolecular binding sites. When measured by sucrose density gradient ultracentrifugation, complete dissociation of the complex of hSAP (human SAP) with C4b-binding protein (C4BP) occurred at ≥5 mM lactate. Lactate dissociated the hSAPmembrane complex and prevented hSAP self-association. The only interaction that was not dissociated by 10 mM lactate was the hSAP-heparin complex. The relative efficacies of several dissociating agents were O-phosphorylethanolamine > lactate > succinate > carbonate > ε-amino-n-caproic acid. This suggested that the carboxyl group plus a hydrogen-bonding site on the hydrocarbon chain was important, but a charged amino group was not a contributor to function when the anion was provided by a carboxyl group. The concentration of lactic acid needed to dissociate hSAP from C4BP was dependent on protein concentration in a manner suggesting the cooperative binding of lactate (coefficient = 2) to hSAP. Pure proteins, at concentrations found in normal serum, required about 12 mM lactate for half-dissociation of the hSAP-C4BP complex. Other pentraxins also interacted with lactic acid, but with lower affinities. An important observation was that lactic acid was capable of dissociating rat CRP from lipoproteins in rat serum. Human CRP bound very weakly to lactate, so that lactate probably is not a significant regulator of this pentraxin. Overall, these results suggest that lactate may be a biological regulator of the functions of at least some pentraxins in normal and/or pathological situations.