Mercaptide conjugation in the uptake and secretion of sulfobromophthalein

Abstract

Sulfobromophthalein (BSP) was injected into rats, and the livers were excised at timed intervals. The ratio of free to mercaptide-conjugated BSP in the liver was determined after extraction and chromatographic separation. After 2 minutes, 12.5% of the total dose was concentrated in the liver; of this, 65% was free BSP. At 5 minutes, 33% of the total dose appeared in the liver, of which approximately 50% was still in the free form. Increasing concentrations of BSP were infused or administered as a single injection, and free and conjugated pigment appearing in the bile was measured. The maximum secretion rate for free BSP was 3–7 µg/ min/gm liver. Maximum capacity for secretion of conjugates was not reached, even when 60 mg BSP/kg body weight was administered. When purified metabolites were injected, they were rapidly secreted into the bile. It was concluded that uptake of BSP from the blood is a dynamic process which precedes and is relatively independent of the conjugation step. In contrast, conjugation may be the rate-limiting step in determining the secretion rate of the pigment from the hepatic cell.