Sulfation is rate limiting in the futile cycling between estrone and estrone sulfate in enriched periportal and perivenous rat hepatocytes.

The metabolic activities and tissue binding of estrone (E1) and estrone sulfate (E1S) on futile cycling were examined. Desulfation of E1S in the 9000g supernatant fraction (S9) of periportal (PP) and perivenous (PV) rat hepatocytes were of similar V (2.9 +/- 1.0 and 2.4 +/- 0.9 nmol/min/mg of S9 protein), K (30.4 +/- 8.3 and 34.8 +/- 6.6 microM), and desulfation intrinsic clearances (V/K of 77 and 55 microl/min/10(6) cells). The intrinsic clearance towards E1 sulfation (1 microM) in cytosolic preparations of PV hepatocytes was 4 times that of PP hepatocytes (V/K of 26.4 +/- 9.5 versus 6.1 +/- 2.2 microl/min/mg of cytosolic protein or 13 +/- 5 versus 3.1 +/- 1.1 microl/min/10(6) cells). The observation was consistent with the immunolocalization of estrogen sulfotransferase (PV/PP ratio of 3.4 +/- 1.1) but not hydroxysteroid sulfotransferase (PV/PP ratio of 0.29 +/- 0.21) nor phenol sulfotransferase (PV/PP ratio of 1.13 +/- 0.23). Upon incubation of E1S (1-125 microM) with hepatocytes (30 min), higher concentrations of E1S and E1 were observed within PP than in PV cells, and saturation was evident at the higher concentrations. Based on the in vitro metabolic and tissue binding parameters for E1S and E1 and the published zonal uptake clearances of E1S (116 microl/min/10(6) cells), fitting revealed that uptake of E1 (1484 and 1463 microl/min/10(6) cells) by PP and PV cells was rapid and similar, and E1 sulfation was the slowest step in futile cycling. The greater metabolism of E1 in PV region led to higher levels of E1 and E1S in PP hepatocytes, and the nonlinear uptake, binding, and vesicular accumulation of E1S resulted in different t(1/2) values for E1S and E1.