Interaction of bile salts with calcium hydroxyapatite: Inhibitors of apatite formation exhibit high–affinity premicellar binding

Abstract

Of the major human bile salts, only the glycine-conjugated dihydroxy species prevent the transformation of amorphous calcium phosphate to calcium hydroxyapatite, a component of gallstones; we have proposed that this inhibition occurs by competition between the bile salt and HPO4(2-) anions for binding site on the apatite crystal embryo. Now we show that the binding affinity of bile salts to fully mature hydroxyapatite has the following order: glycine-conjugated dihydroxy salts > taurine-conjugated dihydroxy salts > glycocholate approximately taurocholate. Glycine-conjugated dihydroxy bile salts bound with high affinity as "premicellar" aggregates, but the remaining species appeared to bind as a wider range of aggregate sizes. Glycochenodeoxycholate binding was decreased as the pH increased from 6.6 to 9.8 and the apatite surface charge reversed from net positive to net negative. Binding was competitively inhibited by HPO4(2-), but not by H2PO4-. Ca2+ promoted the binding of glycochenodeoxycholate, taurochenodeoxycholate and glycocholate, and for the latter two bile salts the increase was associated with enhanced "premicellar" binding. The binding of taurocholate was not influenced by Ca2+. When either glycocholate or taurocholate was mixed with glycochenodeoxycholate, mixed aggregates were formed that had a lower affinity for apatite than had pure glycochenodeoxycholate aggregates. Because only glycine-conjugated dihydroxy bile salts inhibit apatite formation, these results suggest that inhibition depends on high-affinity "premicellar" bile salt-apatite binding.