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 (E-1) and estrone su lfate (E1S) on futile cycling were examined. Desulfation of E1S in the 9000 g supernatant fraction (S9) of periportal (PP) and perivenous (PV) rat hepa tocytes were of similar V [GRAPHICS] (2.9 +/- 1.0 and 2.4 +/- 0.9 nmol/ min/ mg of S9 protein), K [GRAPHICS] (30.4 +/- 8.3 and 34.8 +/- 6.6 muM), and desulfation intrinsic clearances [GRAPHICS] of 77 and 55 mul/min/10(6) cells). The intrinsic clearance towards E-1 sulf ation (1 muM) in cytosolic preparations of PV hepatocytes was 4 times that of PP hepatocytes [GRAPHICS] of 26.4 +/- 9.5 versus 6.1 +/- 2.2 mul/min/mg of cytosolic protein or 13 +/ - 5 versus 3.1 +/- 1.1 mul/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 i ncubation of E1S (1-125 muM) with hepatocytes (30 min), higher concentratio ns of E1S and E-1 were observed within PP than in PV cells, and saturation was evident at the higher concentrations. Based on the in vitro metabolic a nd tissue binding parameters for E1S and E-1 and the published zonal uptake clearances of E1S (116 mul/min/10(6) cells), fitting revealed that uptake of E-1 (1484 and 1463 mul/min/10(6) cells) by PP and PV cells was rapid and similar, and E-1 sulfation was the slowest step in futile cycling. The gre ater metabolism of E-1 in PV region led to higher levels of E-1 and E1S in PP hepatocytes, and the nonlinear uptake, binding, and vesicular accumulati on of E1S resulted in different t(1/2) values for E1S and E-1.