Porphyrias are inherited or acquired disorders of heme biosynthesis characterized by heme deficiency and accumulation of toxic intermediates. In δ-aminolevulinic acid dehydratase deficiency porphyria (ALADP), patients consistently present elevated urinary δ-aminolevulinic acid (δ-ALA) and coproporphyrin III (COPRO III), yet the mechanistic basis of COPRO III accumulation remains unclear. This metabolic disturbance is also observed in the porphyria-like associated crises in hereditary tyrosinemia type I (HT1). Here, we investigated the effects of δ-ALA, COPRO III, and lead (Pb2⁺) on human coproporphyrinogen oxidase (CPOX), a key mitochondrial enzyme in heme biosynthesis. Using purified recombinant CPOX, we show that COPRO III binds with high affinity (KD ≈ 2.1 µM) and acts as a competitive inhibitor, while δ-ALA inhibits CPOX at millimolar concentrations through a non-competitive, likely covalent, mechanism. In vivo, δ-ALA accumulation in an HT1 mouse model led to hepatic COPRO III buildup, consistent with our in vitro findings and supporting a synergistic inhibition model in which δ-ALA promotes secondary COPRO III accumulation that further impairs CPOX. Additionally, Pb2⁺ was found to inactivate CPOX, likely through oxidative damage, providing a molecular explanation for enzyme dysfunction in porphyrin abnormalities in response to lead intoxication. Together, these results identify multiple metabolite- and toxin-dependent mechanisms that converge on CPOX inhibition, offering new insights into the pathophysiology of ALADP among other porphyrias, lead intoxication and HT1.
Schedlbauer et al. (Wed,) studied this question.