INHIBITION OF UROPORPHYRINOGEN DECARBOXYLASE ACTIVITY - THE ROLE OF CYTOCHROME-P-450-MEDIATED UROPORPHYRINOGEN OXIDATION

It was previously shown that uroporphyrinogen oxidation is catalysed by a form of cytochrome P-450 induced by 3-methylcholanthrene [Sinclair, Lambrecht and Sinclair (1987) Biochem. Biophys. Res. Commun. 146, 1324-1329]. We have now measured uroporphyrinogen oxidation and uroporphyrinogen decarboxylation simultaneously in 10,000 g supernatants from the livers of methylcholanthrene-treated mice and chick embryos incubated with an NADPH-generating system. We found that uroporphyrinogen oxidation is associated with inhibition of uroporphyrinogen decarboxylase activity. The decreased uroporphyrinogen decarboxylase activity was not due to depletion of substrate, since decarboxylase activity was not increased by a 2.6-fold increase in uroporphyrinogen. Uroporphyrinogen oxidation and the associated inhibition of decarboxylase activity were also observed with liver supernatant from methylcholanthrene-treated chick embryo; both actions required the addition of 3,3',4,4'-tetrachlorobiphenyl. Uroporphyrinogen oxidation catalysed by microsomes from a methylcholanthrene-treated mouse inhibited the uroporphyrinogen decarboxylase activity in the 100,000 g supernatant. Ketoconazole, an inhibitor of cytochrome P-450, prevented both uroporphyrinogen oxidation and the inhibition of uroporphyrinogen decarboxylation. The addition of ketoconazole to mouse supernatant actively oxidizing uroporphyrinogen inhibited the oxidation and restored decarboxylation. The latter finding suggested that a labile inhibitor was formed during the oxidation. These results suggest uroporphyrinogen oxidation may be important in the mechanism of chemically induced uroporphyria.