Folding and unfolding of δ-aminolevulinic acid dehydratase and porphobilinogen deaminase induced by uro- and protoporphyrin

In all the cutaneous porphyrias, alterations in the heme pathway lead to an excessive production and accumulation of porphyrins. Absorption of light energy by circulating porphyrins induces reactive oxygen species generation, which provoke enzyme inactivation and protein structure changes. Protein structure alterations induced by porphyrins with different physico-chemical properties on δ-aminolevulinic acid dehydratase (ALA-D) and porphobilinogen deaminase (PBG-D) were examined. The action of uroporphyrin (URO), a highly hydrophilic porphyrin, and protoporphyrin (PROTO), most hydrophobic, was tested. ALA-D and PBG-D were partially purified from bovine liver and exposed to URO or PROTO, both in the dark and under UV light. All experiments were performed in solution after removing the porphyrins. Treatment with 10 μM URO 1 or 10 μM PROTO IX reduced the activity of ALA-D and PBG-D. This effect increased with increasing time of exposure to porphyrins. Solubility of the enzymes in buffer containing 3 M KCl decreased with increasing time of porphyrin treatment; this may be because of exposure of hydrophobic residues that are normally shielded in the native protein structure. Tryptic digestion of ALA-D and PBG-D exposed to URO 1 or PROTO IX resulted in an increase of protein degradation products, indicating an enhanced susceptibility to proteolysis. Fluorescence emission of several enzymes aminoacids was greatly modified. The structural changes described were observed when the enzymes were exposed to porphyrins both in the dark or under UV light. However, they were more noticeable with UV light. These results suggest that porphyrins per se can act directly on protein structure and that this action may be enhanced by UV irradiation.