Lead concentration in endothermic vertebrates and ALAD polymorphism
Lead – the heavy metal affects human and animals health. Low levels of exposure of lead can iduced cognitive and neurobehavioral changes. High levels can be responsible for encephalopathy, even it can cause death. Previously even considered Pb as one of the causes contributing to the extinction of the Roman Empire. The main reason of destroying health by lead exposure is because of his potential inhibition of δ-aminolevulinic acid dehydratase (ALAD), coproporphyrinogen oxidase, and ferrochelatase, enzymes that catalyze the second, sixth, and final steps, respectively, in the biosynthesis of heme (Warren et al. 1998). In this review it will be discussed ALAD gene that encoded δ-aminolevulinic acid dehydratase. ALA dehydratase (E.C. 18.104.22.168) also known as porphobilinogen synthase is an octameric metaloenzyme. This enzyme catalyzes the second step in heme biosynthesis, condensation of two molecules of δ-aminolevulinic acid (ALA) into one molecule of monopyrrole porphobilinogen (PBG), which is precursor of heme. For full activity ALAD recquires zinc ions as a cofactor. Normally, this ions bind enzyme´s SH group. But in the presenece of lead in the organism, SH group binds the lead because of the higher affinity for this metal and so activity of enzyme is inhibited. This situation can lead to porphyria.
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