Antagonistic effect in vivo of zinc on inhibition of delta-aminolevulinic acid dehydratase by lead.

Delta-aminolevulinic acid dehydratase (ALAD) is an enzyme involved in the biosynthesis of heme, in which it catalyzes the condensation of two molecules of delta-aminolevulinic acid to one molecule of porphobilinogen. It is a sulfhydryl enzyme, which means, among other things, that its activity is inhibited by many heavy metals. In the present investigation rabbits were given either zinc or lead or both. Zinc had a strong activating effect on ALAD in vivo, and the inhibitory effect of lead was almost completely eliminated. A close positive correlation was found between ALAD in the red blood cells and zinc in the plasma, but there was no correlation between ALAD and zinc in the red blood cells. These observations are of particular interest in the light of recent findings, suggesting that zinc is an essential metal for ALAD.