Modulation of ascorbic acid metabolism by cytochrome P450 induction revealed by metabonomics and transcriptional...

In the present study, NMR-based urinary metabonomic profiles resulting from dosing with widely recognized microsomal enzyme inducers were evaluated in male rats. Wistar or Sprague-Dawley rats were dosed daily by oral gavage with phenobarbital (PB; 100 mg/kg), diallyl sulfide (DAS; 500 mg/kg), the investigational compound DMP-904 (150 mg/kg), or beta-naphthoflavone (BNF; 100 mg/kg) for 4 days, and urine was collected daily for analysis. Compounds known to increase cytochrome P450 2B enzymes, including PB, DAS and DMP-904, increased the urinary excretion of gulonic and ascorbic acid in a time-dependent manner, reaching a maximum following 3-4 days of dosing. In contrast, BNF, an agent that induces primarily Cyp1A enzymes, did not increase gulonic or ascorbic acid excretion, despite inducing Cyp1A1 more than 200-fold. Given the metabonomic results, hepatic transcriptional changes in the regulation of ascorbic acid biosynthesis were determined by RT-PCR. All Cyp2B inducers increased hepatic mRNA levels of aldo-keto reductase 1A1, an enzyme that catalyzes the formation of gulonic acid from glucuronate with concurrent decreased expression of both regucalcin (Rgn), the enzyme responsible for conversion of gulonic acid to gulono-1, 4-lactone and gulonolactone oxidase (Gulo), the rate-limiting enzyme in ascorbate biosynthesis. These effects would be expected to increase levels of gulonic acid. In addition, Cyp2B inducers also increased hepatic expression of enzymes regulating ascorbic acid reutilization including glutaredoxin reductase (Glrx2) and thioredoxin reductase (Txnrd1). In contrast, BNF did not effect hepatic expression of any enzyme regulating gulonic or ascorbic acid biosynthesis. Thus, some microsomal enzyme inducers alter transcriptional regulation of ascorbic acid biosynthesis, and these changes are detected by noninvasive metabonomic profiling. However, not all microsomal enzyme inducers appear to alter ascorbic acid metabolism. Finally, the work illustrates how metabonomic results can direct additional studies to determine the biochemical mechanisms underlying changes in urinary metabolite excretion.