Erythrocyte delta-aminolevulinic acid dehydratase in birds. I. The effects of lead and other metals in vitro

The effects of buffering ions, pH, temperature, and various metal cations on the activity of avian red blood cell aminolevulinic acid dehydratase (RBC-ALA-d) in blood hemolysates was studied. The pH optimum of the enzyme was approximately 6.6 in both citrate and morpholinethanesulfonic acid (MES) buffers, and maximal activity was comparable in both buffer systems. Enzyme activity was increased by about 20% at 42 degrees C (avian body temp.) relative to that at 37 degrees C. Pb2+ was 10-100-fold more potent than Cu2+, Cd2+, Hg2+ or CH3Hg+ as an inhibitor of RBC-ALA-d activity. The IC50 for Pb2+ was 0.03-0.04 mumol/ml blood. Zn2+ added to Pb-pretreated hemolysates was able to induce up to 65% recovery of enzyme activity. Pb2+ was a more effective inhibitor of delta-aminolevulinic acid dehydratase (ALA-d) activity in MES buffer than in citrate buffer, possibly because of chelation of Pb2+ by citrate.