ALAD form porphobilinogen（a precursor of heme， cytochromes and other hemoproteins) by catalyzing the compression of 2 molecules of delta-aminolevulinate. ALAD catalyzes the second step in the porphyrin and heme biosynthetic pathway； zinc is vital for enzymatic activity. ALAD has 8 identical subunits and its enzymatic activity is inhibited by lead. Mutations in the ALAD structural gene are the source for high sensitivity to lead poisoning and acute hepatic porphyria.
