Lead intoxication: Effects on cytochrome P-450-mediated hepatic oxidations

Acute administration of lead to rats caused significant decreases in cytochrome P-450, ethylmorphine N-demethylase, and aniline hydroxylase activities and prolonged hexobarbital-induced sleeping times. However, chronic administration of lead to weanling rats caused no significant changes in hepatic cytochrome P-450 levels or in the microsomal oxidative enzymes over a 12-wk period. Eight patients exposed to lead in the process of burning through lead-painted steel structures for at least 3 mo showed marked effects of chronic lead intoxication on the erythropoietic system: inhibition of erythrocyte δ-aminolevulinic acid dehydratase, increased erythrocyte protoporphyrin levels, and increased urinary excretion of δ-aminolevulinic acid. Chelation therapy greatly alleviated the inhibitory effects on dehydratase activity and decreased urinary δ-aminolevulinic acid excretion. The plasma elimination rate of antipyrine, a drug primarily metabolized by hepatic microsomal enzymes, was determined in the 8 subjects prior to and following chelation therapy. In 7 of 8 subjects, chelation therapy shortened the antipyrine half-lives, but the effect was minimal. These studies show that chronic lead exposure results in significant hematopoietic inhibition of the heme biosynthetic pathway without causing significant changes in hepatic cytochrome P-450-associated enzymic activities.