1. Analysis of urine by (1)H-nuclear magnetic resonance (NMR) spectroscopy is used to detect biochemical disturbances predictive of toxicological changes. Recent studies, using (1)H-NMR spectroscopy have suggested that Alderley Park rats can be classified as hippuric acid (HA) or m-(hydroxyphenyl)propionic acid (m-HPPA) excretors. Evidence exists for the role of intestinal microflora in the excretion of aromatic phenolic compounds including HA and m-HPPA. 2. We sought to investigate whether intestinal microflora contribute to the difference in excretion. Urinary HA and m-HPPA levels were monitored to characterize excretion over time. The effect of intestinal microflora on the (1)H-NMR spectrum was also investigated using antibiotics to sterilize the intestine. Finally, the levels of m-HPPA and phenylpropionic acid (a precursor for HA) were analysed in the caecum and colon (entire tissue, including contents). 3. Characterization confirmed the presence of HA and m-HPPA excretors; enquiries revealed that the rats were obtained from two floors within a barriered breeding unit. Housing the rats from the two floors together for 21 days resulted in comparable levels of HA and m-HPPA excretion demonstrating that the profiles are not stable. 4. Following antibiotic treatment, HA and m-HPPA excretion decreased, indicating that intestinal microflora contribute to the excretion of these compounds. Finally, m-HPPA levels were higher in the colon of rats that excreted m-HPPA whilst PPA was increased in the caecum and colon of rats that excreted HA. 5. These results demonstrate that the observed difference in HA/m-HPPA excretion is due to differences in the intestinal microflora.

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Aims NTBC (2-(2-nitro-4-¯uoromethylbenzoyl)-1,3-cyclohexanedione) and mesotrione (2-(4-methylsulphonyl-2-nitrobenzoyl)-1,3-cyclohexanedione) are inhibitors of 4-hydroxyphenyl pyruvate dioxygenase (HPPD). NTBC has been successfully used as a treatment for hereditary tyrosinaemia type 1 (HT-1), while mesotrione has been developed as an herbicide. The pharmacokinetics of the two compounds were investigated in healthy male volunteers following single oral administration. The aim of the NTBC study was to assess the bioequivalence of two different formulations and to determine the extent of the induced tyrosinaemia. The mesotrione study was performed to determine the magnitude and duration of the effect on tyrosine catabolism. Additionally, the urinary excretion of unchanged mesotrione was measured to assess the importance of this route of clearance and to help develop a strategy for monitoring occupational exposure. Methods A total of 28 volunteers participated in two separate studies with the compounds. In the ®rst study, the relative bioavailability of NTBC from liquid and capsule formulations was compared and the effect on plasma tyrosine concentrations measured. In the second study the pharmacokinetics of mesotrione were determined at three doses. Plasma tyrosine concentrations were monitored and the urinary excretion of mesotrione and tyrosine metabolites was measured. Results Both compounds were well tolerated at the dose levels studied. Peak plasma concentrations of NTBC were rapidly attained following a single oral dose of 1 mg kg x1 body weight of either formulation and the half-life in plasma was approximately 54 h. There were no statistical differences in mean (t s.d.) AUC(0,?) (capsule 602t154 vs solution 602t146 mg ml x1 h) or t 1/2 (capsule 55t13 vs solution 54t8 h) and these parameters supported the bioequivalence of the two formulations. Mesotrione was also rapidly absorbed, with a signi®cant proportion of the dose eliminated unchanged in urine. The plasma half-life was approximately 1 h and was independent of dose and AUC(0,?) and C max increased linearly with dose. Following administration of 1 mg NTBC kg x1 in either formulation, the concentrations of tyrosine in plasma increased to approximately 1100 nmol ml x1 . Concentrations were still approximately 8 times those of background at 14 days after dosing, but had returned to background levels within 2 months of the second dose. Administration of mesotrione resulted in an increase in tyrosine concentrations which reached a maximum of approximately 300 nmol ml x1 following a dose of 4 mg kg x1 body weight. Concentrations returned to those of background within 2 days of dosing. Urinary excretion of tyrosine metabolites was increased during the 24 h immediately following a dose of 4 mg mesotrione kg x1 , but returned to background levels during the following 24 h period. Conclusions NTBC and mesotrione are both inhibitors of HPPD, although the magnitude and duration of their effect on tyrosine concentrations are very different. When normalized for dose...

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The benzoylcyclohexane-1,3-diones, the triketones, are potent bleaching herbicides whose structure-activity relationships and physical properties are substantially different from classical bleaching herbicides, which affect phytoene desaturase. The first clue to their unique mechanism of action was the discovery that rats treated with a triketone were found to be tyrosinemic. Additionally, examination of the rat urine revealed the accumulation of p-hydroxyphenylpyruvate (HPP) and p-hydroxyphenyllactate. These results suggested that this chemically induced tyrosinemia was the result of the inhibition of p-hydroxyphenylpyruvate dioxygenase (HPPD, EC 1.13.11.27), and this suggestion was confirmed when a triketone was shown to be a potent inhibitor of rat liver HPPD. In plants, HPPD is a component of the biosynthetic pathway to plastoquinone (PQ), which in turn is a key cofactor of phytoene desaturase. The expectation that triketone-treated plants should accumulate tyrosine while having reduced PQ levels was dramatically demonstrated in the meristematic tissue of ivyleaf morningglory. Plant HPPD, like the mammalian enzyme, was inhibited in vitro by triketones. These biochemical effects provide evidence that the triketone herbicidal mechanism of action is HPPD inhibition leading to a deficiency of PQ, a key cofactor for carotenoid biosynthesis. Other chemical classes of bleaching herbicides were also examined for their ability to elevate tyrosine and deplete PQ as a definitive means of establishing their mode of action and for delineating the structural and physical chemical requirements for an HPPD herbicide. Evidence is provided to support the claim that a 2-benzoylethen-1-ol substructure is the minimum substructure required for a potent HPPD inhibitor.