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  • Title: A potential biochemical explanation for the genesis of porphyria cutanea tarda. Studies on the inherent biochemical defect in highly purified human erythrocyte uroporphyrinogen decarboxylase and its amplification by iron.
    Author: Mukerji SK, Pimstone NR, Tan KT.
    Journal: FEBS Lett; 1985 Sep 23; 189(2):217-20. PubMed ID: 4043380.
    Abstract:
    Familial porphyria cutanea tarda (PCT) is a photocutaneous disease in which subnormal activity of uroporphyrinogen decarboxylase is observed both in the liver and red cells. Hepatic iron plays a key role in the genesis of overt biochemical and clinical PCT. In this report, we have studied the properties of 10 000-fold purified erythrocyte uroporphyrinogen decarboxylase preparations from two familial PCT patients and a non-porphyric control subject. The apparent Michaelis constants (Km), determined by using uroporphyrinogen III substrate, were approx. 3.2-times higher for the enzyme from the diseased subjects (Km = approximately 1.0 microM) as compared to the normal (Km = 0.3 microM). Though both abnormal and normal enzymes were inhibited progressively with increasing concentrations of iron, the enzymes from diseased subjects exhibited greater susceptibility e.g. 0.1 mM Fe2+ inhibited the former about 50% and the latter about 20%. These observations suggest that the inherent biochemical defect in PCT is the reduced enzyme-substrate affinity and the intrinsic abnormal conformation renders the PCT enzyme particularly susceptible to inhibition by iron.
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