Ascorbic acid inhibition of cytochrome P450-catalyzed uroporphyrin accumulation

Previous studies on the mechanism of the uroporphyria caused by polyhalogenated aromatic hydrocarbons have indicated a key role of cytochrome P450 of the 1A subfamily in catalyzing uroporphyrinogen (UROgen) oxidation. Here we report that ascorbic acid (ASC) inhibits UROgen oxidation in primary cultures of chick embryo hepatocytes and hepatic microsomes from chickens and mice. In hepatocyte cultures, 0.15 mM ASC totally prevented the accumulation of uroporphyrin (URO) induced by treatment of cells with the combination of 3,4,3',4'-tetrachlorobiphenyl (TCB) and 2-propyl-2-isopropylacetamide (PIA), but had no effect on the induction of protoporphyrin accumulation by PIA and desferrioxamine. However, addition of 5-aminolevulinic acid (ALA) to cultures treated with PIA plus TCB decreased the ability of ASC to prevent URO accumulation, suggesting that the effectiveness of ASC was dependent on the intracellular concentration of ALA or its metabolites. Similarly, when chick hepatocyte cultures were treated with TCB plus exogenous ALA to produce URO accumulation, the effectiveness of ASC was also less than when ALA was produced endogenously. Under this condition, addition of piperonyl butoxide, a P450 inhibitor, increased ASC inhibition of URO accumulation. ASC competitively inhibited the oxidation of UROgen by hepatic microsomes from chicks or mice treated with 3-methylcholanthrene (MC) with Ki for ASC being about 0.1 mM. ASC prevented formation of a 500-nm absorbing compound, probably tetrahydrouroporphyrin, the first intermediate in UROgen oxidation. These results are consistent with ASC preventing URO accumulation in hepatocytes by competitive inhibition of the first step of UROgen oxidation and suggest a new physiological role of ASC, that of maintaining UROgen in the reduced state.