Competition between two enzymes for substrate removal in liver: modulating effects due to substrate recruitment of hepatocyte activity.

Modulating effects of competing pathways, exemplified by sulfation (high affinity-low capacity) and glucuronidation (low affinity-high capacity), on drug disappearance and metabolite formation were investigated in a simulation study. The phenomenon of substrate recruitment of hepatocyte activity in drug removal and metabolite formation was shown with respect to inlet substrate concentration, and drug processing from inlet to outlet by enzyme systems localized differentially along the sinusoidal flow path in liver. Three enzymic distribution models: (A) sulfation and glucuronidation evenly distributed in liver, (b) sulfation occurring exclusively in the first half of the liver and glucuronidation in the second half, and (C) glucuronidation solely in the first half and sulfation in the second half, were described. The influence of Km and Vmax of the competing pathway, including enzyme induction (increase in Vmax), on any given pathway was also explored. Competing pathways exert their effects on other given pathways by modulating intrahepatic drug concentration from the inlet to outlet of the liver. When a competing pathway is similarly distributed or is at an anterior location to another pathway, the former pathway effectively reduces intrahepatic drug concentrations which reach downstream hepatocytes for recruitment of activity. For example, when glucuronidation activity is anterior to sulfation activity (defined with respect to flow direction), sulfation is without an effect on glucuronidation, but glucuronidation exerts a maximal influence over sulfation rates (Model C). When glucuronidation is in direct competition with sulfation (Model A) or is posteriorly distributed to sulfation (Model B), saturation of the high-affinity sulfation pathway leads to greater fluxes of substrate available downstream for glucuronidation. This results in an apparent compensatory increase in glucuronidation with reduced sulfation capacity, which occurs at input concentrations greater than the Km for sulfation but less than the Km for glucuronidation. This compensation pattern is more prominent for highly extracted compounds where both sulfation and glucuronidation are effective pathways in drug removal, and where large intrahepatic drug concentration gradients are expected. Since the physiologic description of intrahepatic drug concentration is often described by a concentration gradient from the inlet to outlet of the liver, the logarithmic average concentration has been used to estimate the mean liver concentration in the determination of kinetic constants for enzymic reactions.(ABSTRACT TRUNCATED AT 400 WORDS)