Binding of sulfobromophthalein (BSP) sodium by plasma albumin. Its role in hepatic BSP extraction.

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Abstract
Binding of sulfobromophthalein (BSP) sodium and its major glutathione conjugate by human and bovine plasma albumin has been studied by equilibrium dialysis over a 200 fold concentration range. Radioactive sulfur labelled BSP was used in the study of binding at molar concentration ratios (BSP/albumin) approaching those encounter-ed in vivo. Analysis by the Scatchard method was used to obtain association constants for BSP (n1=l, k1= 1. 7 x 107; n2= 2, k2= 1. 6 x 106; and n3= 14, k3 = 6 x 103 and for the BSP conjugate n1 = 1, kl = 1 x 105, n2 = 2, k2= 3 x 104). These values indicate that less than 0. 1% of its conjugate are unbound as free fraction at physiologic plasma concentrations. Calculation shows that hepatic extraction of BSP in dog and man is too large to be accounted for by removal of the free fraction alone. It follows that the protein-bound dye must move across the sinusoidal endothelium to the hepatocyte surface. The fenestrations in the endothelium permitting such a movement need to occupy no more than 0. 002 of the surface.