The Ratio of Erythrocyte Zinc-Protoporphyrin to Protoporphyrin IX in Disease and its Significance in the Mechanism of Lead Toxicity on Haem Synthesis

Protoporphyrin and zinc-protoporphyrin were measured in the erythrocytes of normal subjects, workers exposed to lead and patients with iron deficiency and erythropoietic protoporphyria (EPP). Results showed significantly higher levels of zinc-protoporphyrin in the lead-exposed workers (P < 0.0001), patients with iron deficiency (P < 0.0001) and EPP patients (P < 0.001) compared with normal subjects. The lead-exposed workers showed the highest levels of zinc-protoporphyrin, which were significantly greater than both the iron-deficient and EPP patients (P < 0.0001). They also showed a higher ratio of zinc-protoporphyrin to free protoporphyrin compared with normal subjects (P < 0.0001) but no significant difference in this ratio was found when compared with iron-deficient patients (P = 0.1). These results are discussed in light of the controversy concerning the mechanism of formation of zinc-protoporphyrin in lead exposure.