Polymer networks in pigment and cholesterol gallstones assessed by equilibrium swelling and infrared spectroscopy

Sixty-five gallstones were assessed for content of network polymer by equilibrium swelling and infrared spectroscopy. All types of pigment gallstones exhibited swelling in aqueous buffers, indicating network polymer content, but swelling was greatest in black pigment gallstones. Gallstones composed principally of cholesterol did not swell. Swelling increased with increasing pH values from 6 to 10.5, suggesting that titratable carboxyl side chains of the pigment were intact. Swelling increased in the presence of 10 mM ethylenediaminetetraacetic acid, indicating that calcium ions provided secondary crosslinks in the network. In all types of pigment gallstones, the equilibrium swelling ratio at the maximum (pH = 10.5) was correlated inversely with the proportion of intact vinyl groups in the pigment assessed by infrared spectroscopy. This suggests that pigment vinyl groups are consumed in forming the covalent crosslinks of the polymer network. A weaker direct correlation between equilibrium swelling ratio and the percentage of unmeasured residue in the gallstones suggested that this residue may also have contributed to the swelling. The high content of cross-linked network polymer in black pigment gallstones renders their therapeutic dissolution improbable by physicochemical means.