SEQUESTERED ENDOPLASMIC-RETICULUM SPACE FOR SEQUENTIAL METABOLISM OF SALICYLAMIDE - COUPLING OF HYDROXYLATION AND GLUCURONIDATION

The metabolic disposition of simultaneously delivered [C-14]salicylami de (SAM) (100 mu M) and a tracer concentration of its hydroxylated met abolite [H-3]gentisamide (GAM) was studied with single-pass followed b y recirculating rat liver perfusion (10 ml/min), The use of dual radio labeling of precursor-product pairs in single-pass and recirculating p erfusions allowed for characterization of the differential metabolism of preformed [H-3]GAM and formed [C-14]GAM, which arose in site in the liver with [C-14]SAM single-pass perfusion, and the behavior of circu lating [C-14]GAM, which behaved as a preformed species in recirculatio n. In both modes of perfusion, [C-14]SAM was mainly sequentially metab olized to [C-14]GAM-5-glucuronide, whereas [H-3]GAM predominantly form ed [H-3]GAM-5-sulfate. The steady-state and time-averaged clearances o f SAM were identical and approached the value of flow, yielding a high hepatic extraction ratio (E = 0.98), The apparent extraction ratio of formed GAM [E{mi} = 0.96] was greater than that of the preformed spec ies [E{pmi} approximate to 0.7], Because the coupling of (SAM) oxidati on and (GAM) glucuronidation was a plausible explanation for the obser vation, a novel physiological pharmacokinetic model was developed to i nterpret the data. In this model, the liver was divided into three zon al units, within which acinar distribution of enzymatic activities was considered, namely periportal sulfation, evenly distributed glucuroni dation, and perivenous hydroxylation. Each zonal region was subdivided into extracellular, cytosolic, and endoplasmic reticulum compartments , with cytosolic (sulfotransferases) and microsomal (cytochromes P-450 and UDP-glucuronosyltransferase) enzymes being segregated intracellul arly into the cytosolic compartment and endoplasmic reticulum compartm ent, respectively, The simulations provided a good prediction of the p resent experimental data as well as previously obtained data with incr easing SAM concentration and retrograde flow and supported the content ion that SAM oxidation and GAM glucuronidation are coupled.