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 vari ety of other materials that are found in serum under normal or patholo gical circumstances. This study showed that carboxylated compounds, es pecially lactic acid, were capable of dissociating pentraxins from sev eral macromolecular binding sites. When measured by sucrose density gr adient ultracentrifugation, complete dissociation of the complex of hS AP (human SAP) with C4b-binding protein (C4BP) occurred at greater tha n or equal to 5 mM lactate. Lactate dissociated the hSAP-membrane comp lex and prevented hSAP self-association. The only interaction that was not dissociated by 10 mM lactate was the hSAP-heparin complex. The re lative efficacies of several dissociating agents were O-phosphoryletha nolamine > lactate > succinate > carbonate > epsilon-amino-n-caproic a cid. This suggested that the carboxyl group plus a hydrogen-bonding si te on the hydrocarbon chain was important, but a charged amino group w as not a contributor to function when the anion was provided by a carb oxyl group. The concentration of lactic acid needed to dissociate hSAP from C4BP was dependent on protein concentration in a manner suggesti ng the cooperative binding of lactate (coefficient=2)to hSAP. Pure pro teins, at concentrations found in normal serum, required about 12 mM l actate for half-dissociation of the hSAP-C4BP complex. Other pentraxin s also interacted with lactic acid, but with lower affinities. An impo rtant observation was that lactic acid was capable of dissociating rat CRP from lipoproteins in rat serum. Human CRP bound very weakly to la ctate, so that lactate probably is not a significant regulator of this pentraxin. Overall, these results suggest that lactate may be a biolo gical regulator of the functions of at least some pentraxins in normal and/or pathological situations.