Bilirubin conjugates sulfate

Bilirubin is nonpolar and would persist in cells (eg, bound to lipids) if not rendered water-soluble. Hepatocytes convert bilirubin to a polar form, which is readily excreted in the bile, by adding glucuronic acid molecules to it. This process is called conjugation and can employ polar molecules other than glucuronic acid (eg, sulfate). Many steroid hormones and drugs are also... 

Another sinusoidal transporter catalyzes Na+-independent uptake of organic anions and is instrumental for biliary clearance of glucuronidated and sulfated steroids, the diagnostic chemical bromosulfophthalein (BSP) and possibly bilirubin. Canalicular transport of glucuronidate and GSH conjugates is coupled to ATP... 

Figure 28.3. Transport of bile acids and other constituents across the hepatocyte. The Na+ dependent bile salt (taurocholate) transporter (BA-) is shown on the sinusoidal membrane that utihzes the Na+ gradient maintained by the NAK pump, shown here on the lateral aspect of the plasmalemma. Bile salt transcellular transport involves microtubules, which then dehver substrate to the canahcular bile salt transporter (1). Bilary excretion of GSH, gluc-uronate (GluA), and sulfate conjugates of compounds such as 17P-estradiol (E2), bilirubin, and bromosulfothalein (BSP) is catalyzed by the multispecific organic anion transporter (MOAT 2). Both 1 and 2 are members of the ABC family of ATP-dependent transporters that also includes P-glycoprotein (3), another canalicular transporter catalyzing excretion of hpophihc compounds such as the chemotherapeutic drug, daunorubicin.

Finally it must be stressed that a sulfate of bilirubin does not occur naturally, despite the fact that such a compound has been repeatedly inferred to represent the alkali-stable fraction. Experiments performed with a synthetic sulfate compound (Kuenzle, 1970b,c) have shown that diis or a similar derivative would not have escaped detection with the method employed to isolate the alkali-labile conjugates. 

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