Human
porphyria cutanea tarda (PCT) is an unusual consequence of common hepatic disorders such as
alcoholic liver disease and
iron overload, where hepatic
iron plays a key role in the expression of the metabolic lesion, i.e., defective hepatic decarboxylation of
porphyrinogens. In this investigation, kinetic studies on a partially purified rat liver
uroporphyrinogen decarboxylase have been conducted under controlled conditions to determine how
iron perturbs porphyrinogen decarboxylation in vitro. The
enzyme, assayed strictly under anaerobic conditions in the dark, was inhibited progressively by ferrous
iron. Approximately 0.45 mM
ferrous ammonium sulfate was required to observe about 50% inhibition of
enzyme activity measured with uroporphyrinogen I as substrate. We showed that (a) all the steps of enzymatic decarboxylation (octa-, hepta-, hexa-, and
pentacarboxylic porphyrinogen of isomer I series) were inhibited by ferrous
iron. The inhibition was competitive with respect to uroporphyrinogen I and III substrates; (b) the
cations, e.g., Fe3+ and Mg2+, had no effect, whereas sulfhydryl group specific
cations and compounds such as Hg2+, Zn2+,
p-mercuribenzoate, and 5,5'-dithiobis(2-nitrobenzoate) all inhibited the
enzyme; (c) the
enzyme could be protected from inhibition by Fe2+ and
p-mercuribenzoate by preincubation with
pentacarboxylic porphyrinogen, a natural substrate and competitive inhibitor. These data suggest for the first time a direct interaction of ferrous
iron with cysteinyl residue(s) located at the active site(s) of the
enzyme.