EFFECT OF PROTEIN-BINDING ON 4-METHYLUMBELLIFERYL SULFATE DESULFATIONKINETICS IN PERFUSED-RAT-LIVER

4-Methylumbelliferyl sulfate (4MUS), a polar metabolite of 4-methylumb elliferone (4MU), is known to undergo desulfation and participate in f utile cycling with 4MU. Unusual parabolic or increasing profiles of th e steady-state extraction ratio (E(SS)) of 4MUS with respect to concen tration in rat livers perfused with a red cell (20%)-albumin (1%) medi um have been reported (Ratna et al., 1993). In order to study this unu sual phenomenon, we examined the desulfation of 4MUS in the single-pas s rat liver in the absence of albumin. We further employed a tubular-f low model to describe the present observations and data previously obt ained on 4MUS in order to predict the effects of protein binding and e nzymatic constants for conjugation/deconjugation on the hepatic proces sing of 4MUS and its metabolites. The net hepatic extraction ratio fro m albumin-free perfusate decreased from 0.465 to 0.326 when the 4MUS i nput concentration was increased from 122 to 908 muM; moreover, the un usual profiles previously observed for E(SS) with increasing concentra tion in albumin-containing perfusate were not apparent. The hepatic cl earances and desulfation rates of 4MUS were essentially identical to t hose observed in the presence of albumin, when the latter were express ed in terms of unbound concentrations (unbound input and logarithmic a verage unbound concentration of the input and output blood). Initial m odeling indicated that first nonlinear protein binding (dissociation c onstant K(D) of 93 muM) and then saturable desulfation (K(m) of 382 mu M) were responsible for the unusual increasing and then decreasing tre nd of E(SS) with concentration in the presence of albumin. When estima tes for net conjugation of 4MU and net deconjugation of 4MU conjugates were utilized to predict data with a tubular-flow model, good agreeme nt was found between predictions and observations. We conclude that pr otein binding is responsible for the reduced elimination of 4MUS at lo w concentrations, whereas enzyme saturation is involved at higher conc entrations.