Inhibition of delta-aminolevulinic acid dehydratase in human red blood cells by lead and activation by zinc or cysteine.

Inhibition of blood delta-aminolevulinic acid dehydratase(ALA-D) activity by lead was studied in vivo and in vitro. In vivo, a negative linear correlation (r = -0.85) was found between the logarithmic values of ALA-D activity and blood lead levels. In vitro the inhibitory effect of lead on blood ALA-D activity increased both with contact time and contact temperature of lead with blood before ALA-D assay. Maximum enzyme inhibition occurred after 14 h of contact at 25 degrees C. Inhibition of ALA-D activity by lead, in vivo as well as in vitro, is suppressed by the addition of zinc or cysteine. The logarithmic values of the activity ratios increase linearly with blood lead concentrations. The increase in ALA-D activity brought about by the addition of zinc or cysteine can be used to identify cases of low enzyme activity with no lead intoxication involved. The same technique can also detect cases in which ALA-D inhibition may be concealed by a presumably high initial enzyme activity as observed in some patients.