BILE-ACID MEASUREMENTS USING A CHOLESTYRAMINE-COATED TSM ACOUSTIC-WAVE SENSOR

A novel chemical sensor for bile acids is described. A 10-MHz piezoele ctric crystal operating in the thickness-shear mode (TSM) is coated on one side with cholestyramine resin and mounted in a batch-mode sensor block which exposes the coating to solution. After sample injection, the binding process is observed in real time as a drop in frequency as the bile salt binds to the coating, reaching >90% completion within 1 0 min with most of the binding occurring within the first minute. Line ar calibration curves are generated with sensitivity increasing in the order cholate approximate to glycocholate < taurocholate much less th an taurodeoxycholate. Detection limits in water are in the range 0.2-9 nmol and are better than those observed in phosphate buffer, A multis tep regeneration protocol allows the coating to be reused more than 40 0 times over a period of several months. Precision for replicate injec tions is similar to 10% RSD and depends on the reproducibility of the regeneration and injection steps. In terms of the binding process, hyd rophobic interactions are observed to be of importance in the ability of bile salts to displace other counterions, However anions with great er charge density also appear to compete effectively for binding sites on the resin, In particular, at equimolar concentrations of citrate a nd bile salt, the trivalent citrate anion reduces the amount of bile s alt binding by similar to 40%. This suggests that the efficiency of ch olestyramine-based bile salt sequestering drugs used in the reduction of hypercholesterolemia may be improved by eliminating citric acid as an excipient and avoiding the use of fruit juices during ingestion.