DELTA-AMINOLEVULINIC-ACID DEHYDRATASE INACTIVATION BY UROPORPHYRIN-I IN LIGHT AND DARKNESS

1. The action of uroporphyrin I on erythrocytic ALA-D activity under d ark and light conditions was examined. 2. Photo and non-photoinactivat ion of ALA-D induced by uroporphyrin I were observed. 3. Both effects were dependent on uroporphyrin concentration, temperature and time of exposure of the protein to the porphyrin. 4. Light-dependent effect of uroporphyrin I is related with the phototoxicity of porphyrins and co uld be produced by primary amino acid photooxidation followed by secon dary cross-linking of the protein. 5. Light-dependent effect of uropor phyrin I could be ascribed to a direct enzyme inhibition due to bindin g of the porphyrin to the protein inducing structural changes at or ne ar its active site.