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When sulfobromophthalein (Bromsulphalein, BSP) sodium was introduced some 40 years ago as a tool for the measurement of liver function, Rosenthal and White (1) reported that solution in serum completely prevented ultrafiltration of the dye through a collodion membrane. They suggested that binding of protein might make BSP and other substances with similar properties, such as bilirubin and rose bengal, more readily available for hepatic removal by preventing their loss in the urine. Although their observation has been repeatedly confirmed (2-7), precise quantitative data referable to BSP concentrations employed in clinical and physiological studies are still lacking. This is so, unfortunately, because the plasma proteins must be diluted 50to 100-fold during examination of the dye-protein interaction by equilibrium dialysis or by ultrafiltration in order to minimize interference by oncotic pressure effects, changing protein concentrations, Gibbs-Donnan equilibria, and various membrane phenomena. To obtain measurable concentrations of BSP, one must work at molar ratios of dye-to-protein far in excess of those ever encountered in zivo. Since saturation of binding sites and alteration in binding capacity may occur at such high ratios, the resulting figures are of somewhat dubious physiological significance.
Baker et al. (Tue,) studied this question.