Aminolevulinate inhibition of human coproporphyrinogen oxidase clarifies coproporphyrin III accumulation in porphyrias

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.