ALAD is a cytoplasmic enzyme that catalyzes the second step of the heme biosynthesis pathway, that is, the condensation of two molecules of delta-aminolevulinic acid into porphobilinogen. ALAD is a zinc-dependent enzyme; thiol groups are essential for its activity; and in vitro experiments show that ALAD can be activated or inhibited by several metal ions including A;3+, Pb2+, Cd2+, Hg2+, Ag2+, and Cu2+. To explain these effects, it has been postulated that metals bind to thiol groups of allosteric sites and, according to their structure, provoke allosteric transitions to the active or inactive form of the enzyme. Under current environmental and occupational exposure levels, lead is practically the only metal that can affect ALAD activity. Erythrocyte ALAD is the most sensitive indicator of lead exposure: effects of exposure are detectable even when blood lead levels are within the "normal" range. Zinc protects ALAD in vitro and in vivo from the inhibitory effect of lead. There is also some suggestion that aluminum could be responsible for the decreased erythrocyte ALAD activity observed in patients on chronic hemodialysis.