Calcium-binding system in bile
Williamson, Barry W.A.
Gallstones are a common clinical problem associated with significant morbidity and mortality whether managed conservatively or by surgery. Since a sizeable proportion of the resources of the community is spent dealing with the clinical sequelae of these stones, much interest has been focused on their pathogenesis. This has been stimulated over the last ten years by improvements in methodology and, perhaps more importantly, by new concepts from physical chemistry. Gallstones, in the western hemisphere, consist of crystalline cholesterol or salts of calcium including the carbonate, phosphate, bilirubinate and palmitate, or a combination of these. A significant minority (20 - 40%) contain micro-organisms. The species involved have, however, changed over the years and, within the recent past, actinomycetes have been isolated. Early theories of gallstone pathogenesis are discussed in relation to the currently accepted ideas on the importance of the cholesterol-solubilising system in bile. The cascade of data on this aspect of stone formation is contrasted with the lack of information on either the solubilising system for, or solubility properties of, the calcium salts in bile. The work described here aims to fill part of this hiatus. The calcium-binding system in bile was analysed from two viewpoints. Initial studies showed that a substantial proportion (60 - 80%) of the calcium in hepatic and gallbladder bile would not pass through an ultrafiltration membrane with a molecular weight cut-off = 1,000. It seemed, therefore, that much of the calcium in bile was chemically bound to species retained by the filter. Subsequent studies dealt systematically with the potential contribution to the calcium-binding system of each major class of molecule in bile, namely, lipids, proteins, polysaccharides, and molecules with molecular weights less than 1,000. In hepatic bile, micelles of bile salt and lecithin could account for 78% of the calcium bound. This contrasted with gallbladder bile in which micelles accounted for only 50% of the detectable calcium binding. However, proteins and acidic polysaccharides were capable of binding substantial quantities of calcium in gallbladder bile. The low molecular weight fraction (less than 1,000 M.W.) accounted for significant quantities of bound calcium in both hepatic and gallbladder bile (12 - 18% of the total). These molecular species all have a role to play in calcium binding in bile under physiological or pathological conditions because it was demonstrated firstly, that calcium binding in bile was reversible and secondly that the affinity of each of these species for calcium was roughly similar. This was performed by quantitative binding studies which also showed that there was a large reserve of calcium-binding sites. Interestingly, several of the species which bound calcium in soluble form, under certain circumstances also formed insoluble deposits. The methods and results are discussed in relation to data in the field. The various weaknesses of the analytical techniques are highlighted in order to emphasise the difficulties which still exist in working with bile despite the recent advances in methodology. The manner in which the choice of techniques used to study calcium binding was dictated by the affinity of binding is described. The broader implications of calcium binding in bile in terms of movement of bile from hepatic duct to gall¬ bladder, of diurnal variations in the composition of bile, and of changes in the serum calcium concentration, are outlined. It is shown that those inorganic salts of calcium commonly found in gallstones, in all probability, exist in supersaturated conditions in bile. The calciumbinding system therefore seems to behave in this respect not unlike the cholesterol-solubilising system in bile. Finally, an hypothesis is offered to account for the distribution of calcium between the various binding species involved and the ionised form. The potential for precipitation of many of these components is indicated and the possible relevance of precipitation of calcium complexes to gallstone nucleation is discussed.