Mercaptide conjugation in the uptake and secretion of sulfobromophthalein

¹ Gastrointestinal Laboratory, San Francisco General Hospital, Departments of Pathology, Medicine and Biochemistry and Metabolic Unit for Research in Arthritis and Allied Diseases, University of California School of Medicine, San Francisco, California

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.