Accumulation of uroporphyrin does not provoke further inhibition of liver uroporphyrinogen decarboxylase activity in hexachlorobenzene-induced porphyria.

The inhibition of uroporphyrinogen decarboxylase (Uro-D) is the basic pathogenetic mechanism in porphyria caused by hexachlorobenzene (HCB). This study aimed to establish whether hepatic accumulation of uroporphyrin in this porphyria could provoke a further decrease of Uro-D activity. Male C57Bl/6 mice were treated for 8 weeks with a diet containing 0.02% HCB. In some of them the deposition of liver porphyrins was additionally increased by intraperitoneal application of delta-aminolaevulinic acid (ALA). Uro-D activity was determined by measuring unconverted substrate uroporphyrinogen after its oxidation to uroporphyrin by reversed-phase high performance liquid chromatography. The value of endogenously formed uroporphyrin was also obtained from the sample by subtraction, using a blank assay. HCB treatment resulted in reduced activity of hepatic Uro-D, but this activity was not significantly less in animals loaded with ALA than in non-loaded mice. Uroporphyrin deposition tended to decrease 6 weeks after withdrawal of HCB, but the activity of Uro-D was still markedly inhibited. There was no evidence that the accumulation of uroporphyrin promoted a supplementary decrease of Uro-D activity in HCB porphyria.