AbstractThe effects of lead (Pb2+) and various pesticides on δ‐aminolevulinic acid dehydratase (ALA‐d) in avian blood, liver and kidney were studied. In blood, virtually all of the ALA‐d activity was localized in the cellular fraction. Complete inhibition of RBC ALA‐d occurred at an in vitro Pb2+concentration of ∼10 μmol/g protein, with an IC50 of ∼0.9 μmol/g protein. Pesticides (carbaryl, carbofuran, chlorpyrifos, diazinon, dieldrin, fenitrothion, hexachlorobenzene) added to blood samples up to 10 mM in vitro had no inhibitory effect on ALA‐d, nor did the presence of pesticides greatly modify the response of ALA‐d to inhibition by increasing concentrations of Pb2+. Recovery of Pb‐inactivated RBC ALA‐d activity was accomplished by treatment of blood hemolysates with Zn2+and an SH‐reducing agent such as dithiothreitol (DTT). A combination of Zn2+(4 mM) and DTT (120 mM) was required to achieve complete recovery of Pb‐inhibited enzyme activity. Normal avian ALA‐d activity was greatest in blood, followed by liver, then kidney. Unlike blood hemolysates, liver and kidney homogenates contained a fraction of ALA‐d activity which was very resistant to inhibition by in vitro Pb2+. In vivo, hepatic ALA‐d was unaffected by Pb exposure (2.5 μg Pb2+/g body weight injected intraperitoneally), but renal ALA‐d was decreased as a result of greater deposition of Pb in kidney than in liver. Inhibition of ALA‐d in avian blood, particularly when expressed as a ratio of fully restored:nonrestored activity (activity ratio), is a highly specific, sensitive indicator of Pb exposure, deposition and corresponding enzyme inhib
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