3-Hydroxymethylene-2-thioxopyrrolidine (HMTP), which is generated from the pungent principle of radish root (Raphanus sativus L.) in an aqueous medium, was studied for its mutagenicity and antimutagenicity on Salmonella typhimurium TA 98 and TA 100 by a mutation frequency assay. When incubated without a rat microsomal fraction (S9), HMTP showed weak mutagenic activity on both tester strains at a 260-520 mug (2-4 mu mol) dose, but the activity was largely decreased or lost by simultaneous treatment of the compound with the S9 fraction at any dose level examined. Against the heterocyclic amines 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) and 3-amino-1,4-dimethyl-5H-pyrido[4,3-b]indole (Trp-P-1), HMTP largely inhibited their mutagenic activities at a dose range of 390-520 mug (3-4 mu mol). HPLC analysis of change in amounts of HMTP, IQ and Trp-P-1 during their incubations with the S9 fraction showed that HMTP could be metabolized to form an S9-inhibitor with the S9 fraction, by which metabolic activation of the heterocyclic amines was inhibited. Lineweaver-Burk plot of the inhibition suggested that HMTP can act as an uncompetitive inhibitor of cytochrome P-450s in the S9 fraction. (C) 2000 Academic Press.