Pteridine modulation of lead inhibition of uroporphyrinogen synthesis in erythroid precursor cells.

The role of nutritional factors on heme synthesis and their influence on the development of anemia in the bone marrow during lead exposure is unclear. Previous reports suggested that pteridines could regulate the formation of uroporphyrinogen, a step midway along the heme synthetic pathway. Studies were undertaken to determine if pteridines could modulate lead inhibition of uroporphyrinogen synthesis in erythroid precursor cells. Pteroylpolyglutamates of various glutamate chain lengths were tested for the ability to protect against lead inhibition of uroporphyrinogen I (URO) synthase prepared from murine erythroleukemia cells (MELC). Pteroylpentaglutamate, the major endogenous polyglutamate form by chain length found to be present in MELC, afforded rapid and specific protection of URO synthase against lead inhibition. MELC are expected to be a useful in vitro model for studying the role of endogenous folates on uroporphyrinogen synthesis and heme formation in erythroid precursor cells following lead exposure.

Erythrocyte uroporphyrinogen synthase activity as a possible diagnostic aid in the diagnosis of lymphoproliferative diseases.

Patients with active lymphoproliferative diseases were shown to have high activity of erythrocyte uroporphyrinogen synthetase (URO-S), the enzyme which converts porphobilinogen to uroporphyrinogen. In a few patients examined the lymphocyte URO-S was markedly increased. No correlation was found between the high URO-S activity and the degree of anemia, reticulocytosis, or the presence of hemolysis. Patients with epithelial malignancies and with some common viral diseases had normal erythrocyte URO-S values. Three patients with nonalcoholic cirrhosis also had high erythrocyte URO-S activities. The determination of erythrocyte and lymphocyte URO-S activity may be of aid in the diagnosis of lymphoproliferative diseases. It may also indicate whether remission has been achieved and whether treatment should be continued or reinstituted. These preliminary observations justify the investigation of a larger patient and control material.

The role of iron in the pathogenesis of porphyria cutanea tarda. II. Inhibition of uroporphyrinogen decarboxylase.

Porphria cutanea tarda is characterized biochemically by excessive hepatic synthesis and urinary excretion of uroporphyrin I and 7-carboxylporphyrins. This pattern of excretion suggest an impaired ability to decarboxylate uroporphyrinogen to the paired ability to decarboxylate uroporphyringen to the 4-carboxyl porphyrinogen, coproporphyrinogen, a reaction catalyzed by the enzyme uroporphyringen decarboxylase. Because clinical evidence has implicated iron in the pathogenesis of porphyria cutanea tarda, these experiments were designed to study the effect of iron on uroporphyrinogen decarboxylase in procine crude liver extracts. Mitochondria-free crude liver extracts were preincubated with ferrous ion and aliquots were assayed for uroporphyrinogen decarboxylase activity. Uroporphyrinogens I and III, the substrates for the decarboxylase assay, were prepared enzymatically from (3H)porphobilinogen. The products of the decarboxylase reaction were identified and quantitated by three methods: (a) extraction into 1.5 N HCl and spectrophotometric quantitation; (b) adsorption onto talc, esterification, paper chromatographic identification, and quantitation by liquid scintillation counting; and (c) adsorption onto talc, esterification, thin-layer chromatographic identification on silica gel, and quantitation by liquid scintillation counting. The thin-layer scinllation method proved most sensitive as it was the only method which accurately identified and quantitated the 7-carboxyl porphyrin reaction product. Uroporphyrinogens I and III were decarboxylated at the same rate by porcine hepatic uroporphyrinogen decarboxylase, and the addition of iron induced marked inhibition of the decarboxylase activity. Ortholpehanthroline blocked the inhibitory effect of iron. The inhibition of uroporphyrinogen decarboxylase by ferrous ion, coupled with its previously reported inhibitory effect on uroporphyrinogen III cosynthetase, provides a possible biochemical explanation for the pattern of urinary porphyrin excretion observed in patients with porphyria cutanea tarda and the clinical association with disordered iron metabolism.