Effect of protein binding on 4-methylumbelliferyl sulfate desulfation kinetics in perfused rat liver

4-Methylumbelliferyl sulfate (4MUS), a polar metabolite of 4-methylumbelliferone (4MU), is known to undergo desulfation and participate in futile cycling with 4MU. Unusual parabolic or increasing profiles of the steady-state extraction ratio (Ess) of 4MUS with respect to concentration in rat livers perfused with a red cell (20%)-albumin (1%) medium have been reported (Ratna et al., 1993). In order to study this unusual phenomenon, we examined the desulfation of 4MUS in the single-pass rat liver in the absence of albumin. We further employed a tubular-flow model to describe the present observations and data previously obtained on 4MUS in order to predict the effects of protein binding and enzymatic constants for conjugation/deconjugation on the hepatic processing of 4MUS and its metabolites. The net hepatic extraction ratio from albumin-free perfusate decreased from 0.465 to 0.326 when the 4MUS input concentration was increased from 122 to 908 microM; moreover, the unusual profiles previously observed for ESS with increasing concentration in albumin-containing perfusate were not apparent. The hepatic clearances and desulfation rates of 4MUS were essentially identical to those observed in the presence of albumin, when the latter were expressed in terms of unbound concentrations (unbound input and logarithmic average unbound concentration of the input and output blood). Initial modeling indicated that first nonlinear protein binding (dissociation constant KD of 93 microM) and then saturable desulfation (Km of 382 microM) were responsible for the unusual increasing and then decreasing trend of ESS with concentration in the presence of albumin.(ABSTRACT TRUNCATED AT 250 WORDS)