The effects of bile salts on the radioimmunoassay of hormonal peptides.

The effect of bile salts on RIAs of secretin, glucagon, insulin, and gastrin have been studied. Increasing concentrations of the sodium salts of taurocholic, glycochenodeoxycholic, taurochenodeoxycholic, glycocholic, and taurodeoxycholic acids progressively inhibit the binding of 125I-secretin to specific antibody, resulting in significant lowering of the B/F ratio at concentrations as low as 0.04 mM and almost complete inhibition at concentrations above 2.4 mM. The nonspecific inhibition by taurodeoxycholate results in a B/F vs. concentration curve resembling a secretion standard curve. The binding of 125I-secretin to charcoal is also inhibited by increasing concentrations of bile salts, although this effect is less marked than their effects on the immune reaction. The binding of 125I-glucagon, 125I-insulin, and 125I-gastrin to specific antisera is also inhibited by sodium taurocholate. Insulin binding is least affected. However, gastrin binding is inhibited by sodium taurocholate at a concentration as low as 0.2. The binding of 125I-insulin and 125I-gastrin to charcoal is also inhibited by sodium taurocholate. Thus bile salts interfere in the RIA of hormonal peptides by inhibiting both the immune reaction and the binding of labeled antigen to charcoal. These nonspecific effects must therefore be considered in RIA of body fluids containing high concentrations of bile salts. Treatment of plasma samples with anionic-binding resins can eliminate interference caused by high bile salt concentrations. However, these resins will also remove anionic hormonal peptides such as gastrin.