Futile cycling between 4‐methylumbelliferone and its conjugates in perfused rat liver

Futile cycling between 4‐methylumbelliferone and its sulfate and glucuronide conjugates was examined in the single‐pass perfused rat liver preparation. The steady‐state hepatic extraction ratio of 4‐methylumbelliferone was found to be high (0.97) at a low input concentration of 0.005 μmol/L (tracer), with a net 4‐methylumbelliferyl sulfate/4‐methylumbelliferyl glucuronide ratio of about 5:1; at 63 μmol/L the steady‐state extraction ratio had remained constant despite a shift from net sulfation to net glucuronidation. At higher input 4‐methylumbelliferone concentrations, saturation was evidenced by a decreased steady‐state extraction ratio and reduced net sulfation and net glucuronidation. Because 4‐methylumbelliferyl sulfate and 4‐methylumbelliferyl glucuronide deconjugation would result in an intracellular accumulation of 4‐methylumbelliferone, the phenomenon was monitored with a shift in tracer [3H]4‐methylumbelliferone metabolism from sulfation to glucuronidation with increased intracellular 4‐methylumbelliferone concentration. When 4‐methylumbelliferyl sulfate (0 to 890 μmol/L) or 4‐methylumbelliferyl glucuronide (0 to 460 μmol/L) was delivered simultaneously with tracer [3H]4‐methylumbelliferone to the rat liver, notable desulfation of 4‐methylumbelliferyl sulfate (18% to 38% rate in) but little deglucuronidation of 4‐methylumbelliferyl glucuronide (1.2% to 2.1% rate in) was observed. With 4‐methylumbelliferyl sulfate, 4‐methylumbelliferone and 4‐methylumbelliferyl glucuronide were readily found as metabolites, whereas with 4‐methylumbelliferyl glucuronide, levels of the metabolites, 4‐methylumbelliferone and 4‐methylumbelliferyl sulfate, were much reduced. 4‐Methylumbelliferyl sulfate and not 4‐methylumbelliferyl glucuronide shifted tracer [3H]4‐methylumbelliferone metabolism from [3H]4‐methylumbelliferyl sulfate to [3H]4‐methylumbelliferyl glucuronide formation in a concentration‐dependent fashion. The steady‐state ex traction ratio for 4‐methylumbelliferyl sulfate (0.1 to 0.3) was comparatively higher than that for 4‐methylumbelliferyl glucuronide (0.05), and it was found to increase with concentration, an observation explained by the nonlinear protein binding of 4‐methylumbelliferyl sulfate. Biliary excretion rates for 4‐methylumbelliferone and 4‐methylumbelliferyl sulfate were proportional to their input or net formation rates, regardless of whether 4‐methylumbelliferyl erone, 4‐methylumbelliferyl glucuronide or 4‐methylumbelliferyl sulfate was a