Effect of occupational lead exposure on lymphocyte enzymes involved in heme biosynthesis

Lead exposure is a well-known cause of increases in urinary coproporphyrin and erythrocyte zinc-protoporphyrin, so these compounds are often used to monitor occupational lead exposure. The increased concentrations are usually assumed to result from lead inhibition of two of the mitochondrial enzymes of heme biosynthesis, coproporphyrinogen oxidase (EC 1.3.3.3) and ferrochelatase (EC 4.99.1.1). We studied 88 subjects in whom the degree of occupational lead exposure was established by measuring erythrocyte lead and protoporphyrin. Assay of lymphocyte coproporphyrinogen oxidase and ferrochelatase activities showed that these enzymes were unaffected by lead exposure, as was a related enzyme, lymphocyte NADH-ferricyanide reductase. We propose alternative explanations for the increased concentrations of coproporphyrin and zinc-protoporphyrin seen in lead exposure.