INTERACTION OF BOVINE GALLBLADDER MUCIN AND CALCIUM-BINDING PROTEIN -EFFECTS ON CALCIUM-PHOSPHATE PRECIPITATION

Background & Aims: Gallstones consist of calcium salts and cholesterol crystals, arrayed on a matrix of gallbladder mucin (GEM), and regulat ory proteins like calcium-binding protein (CBP). To determine if inter actions between CBP and GEM follow a biomineralization scheme, their m utual binding and effects on CaHPO4 precipitation were studied. Method s: Binding of CBP to GEM was assessed by inhibition of the fluorescenc e of the complex of GEM with bis-1,8-anilinonaphthalene sulfonic acid (bis-ANS). The effects of the proteins on precipitation of CaHPO4 were assessed by nephelometry and gravimetry. Precipitates were analyzed f or calcium, phosphate, and protein. Results: CBP and bis-ANS competiti vely displaced each other from 30 binding sites on mucin, with a 1:1 s toichiometry and similar affinity. The rate of precipitation of CaHPO4 was retarded by mucin and CBP. Precipitate mass was unaffected by GEM alone but decreased with the addition of CBP. Complexing CBP with GEM abolished or moderated this latter effect, altered precipitate morpho logy, and changed the stoichiometric ratios of Ca to PO4 in the precip itates from 1:1 to 3:2. Mucin and CBP were incorporated into the preci pitates. Conclusions: These studies suggest that the formation of calc ium-containing gallstones is a biomineralization process regulated by both GEM and CBP.