ASCORBIC-ACID INHIBITS CHEMICALLY-INDUCED UROPORPHYRIA IN ASCORBATE-REQUIRING RATS

Ascorbate was previously shown to suppress accumulation of uroporphyri n (URO) in cultured chick embryo hepatocytes and to competitively inhi bit microsomal oxidation of uroporphyrinogen catalyzed by cytochrome P 4501A2. Here we used the Osteogenic Disorder Shionogi (ODS) mutant rat , which cannot synthesize ascorbic acid, to examine the in vivo effect of ascorbic acid on hepatic URO accumulation caused by treatment with 3-methylcholanthrene (MC) and 5-aminolevulinate (ALA). Female mutant rats maintained on three levels of dietary ascorbate (15, 200, and 800 ppm) were treated for a total of 24 days, On the 11th and 16th days, rats were administered 3-methylcholanthrene, and 5-aminolevulinate was present continuously in the drinking water from day 14. Hepatic URO a ccumulated at the two lowest ascorbate levels, but not at 800 ppm asco rbate. The latter dose produced normal hepatic ascorbate levels. Plasm a ascorbate levels were proportional to the hepatic values, Male rats also accumulated URO at the low dietary dose of ascorbic acid, The met hylcholanthrene-induced increase in microsomal levels of CYP1A1 and CY P1A2, total cytochrome P450, and activities of uroporphyrinogen oxidat ion and ethoxyresorufin deethylase were not affected by the dietary le vel of ascorbate. Neither male nor female Fischer 344 rats accumulated URO when treated with the MC/ALA regime, Hepatic ascorbate concentrat ions in these rats were five-fold to seven-fold higher than they were in mutant rats that developed uroporphyria on 150 ppm dietary ascorbat e. In ODS rats fed ascorbate at 90 but not 900 ppm in the diet, hexach lorobenzene caused hepatic URO accumulation, indicating that the effec t of ascorbic acid is not unique to the regimen using methylcholanthre ne. These results are consistent with a role for ascorbate in preventi ng chemically induced URO accumulation and suggest that ascorbate migh t have a role in preventing URO accumulation in humans.