δ-ALA-D is a metalloenzyme that has 3 vicinal thiol/thiolate groups that coordinate with Zn(ii). The proximity between the sulfhydryl groups renders δ-ALA-D extremely sensitive to oxidation by soft electrophiles, such as Pb(ii), Hg(ii), As(iii) and organoseleno and organotellurium compounds. In fact, blood δ-ALA-D is a classical biomarker of lead exposure in humans. The inhibition of δ-ALA-D can increase the concentration of 5-aminolevulinate (δ-ALA), which is a pro-oxidant compound. δ-ALA can generate oxidative stress that can further increase δ-ALA-D inhibition. Recently, data have been obtained indicating that the δ-ALA-D could be a marker of oxidative stress in human pathologies. In summary, considering its high sensitivity to pro-oxidant situations, δ-ALA-D can be considered a universal marker of oxidative stress.