The effect of liver cytosol enzymes on the rate of formation of a glutathione (GSH) conjugate and on the rate of covalent binding of acetaminophen to microsomal proteins was studied in vitro. Mouse liver microsomes were incubated with [14C]acetaminophen, GSH, a NADPH-generating system, and Sephadex G-25-treated mouse liver supernatant. The addition of liver cytosol to the microsomal incubation increased the rate of conjugate production, as measured by high pressure liquid chromatography and decreased the rate of covalent binding of the reactive intermediate of acetaminophen to microsomal proteins at all concentrations of GSH and acetaminophen studied. The effect of the liver supernatant enzymes was most pronounced at low GSH concentrations. Although cysteine and N-acetyl-l-cysteine form conjugates with acetaminophen, the cytosol preparations did not facilitate conjugate formation with these nucleophiles. Cytosol enzymes may be important in the detoxification of acetaminophen particularly when the liver concentration of GSH is low and may contribute to the marked "threshold" effect seen with acetaminophen toxicity.
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