Mechanism of action of 5-aminolevulinic acid dehydratese: stepwise order of addition of the two molecules of 5-aminolevulinic acid in the enzymic synthesis of porphobilinogen

Incorporation of [5-14C]-5-aminolevulinic acid into porphobilinogen in single-turnover enzymic experiments with mammalian 5-aminoleuvlinic acid dehydratase has revealed that of the two molecules of 5-aminolevulinic acid which are utilised for the formation of porphobilinogen it is the one contributing to the propionic acid side [atoms 1, 2, 3, 6, 7, and 8 in (2)] which is initially bound to the enzyme.