PT  - JOURNAL ARTICLE
AU  - WALTER N. PIPER
AU  - ROBERT B. L. VAN LIER
TI  - Pteridine Regulation of Inhibition of Hepatic Uroporphyrinogen I Synthetase Activity by Lead Chloride
DP  - 1977 Nov 01
TA  - Molecular Pharmacology
PG  - 1126--1135
VI  - 13
IP  - 6
4099  - http://molpharm.aspetjournals.org/content/13/6/1126.short
4100  - http://molpharm.aspetjournals.org/content/13/6/1126.full
SO  - Mol Pharmacol1977 Nov 01; 13
AB  - A dialyzable factor has been purified from rat hepatic cytosol that protects against PbCl2-mediated inhibition of uroporphyrinogen I synthetase (EC 4.3.1.8), an enzyme of the heme biosynthetic pathway. This factor also stimulated enzymatic activity which was antagonized by folic acid. The factor also produced growth of the folate-requiring microorganisms Lactobacillus casei (ATCC 7469) and Streptococcus faecalis (ATCC 8043), and antagonized binding of [3H]folic acid to a specific folate-binding protein. These results suggested that the factor was a pteridine derivative. The protective effect against inhibition of uroporphyrinogen I synthetase activity by PbCl2 was abolished by prior treatment of factor with γ-glutamyl carboxypeptidase (EC 3.4.12.10), indicating that the factor is a polyglutamated pteridine derivative. The estimated concentrations of factor (as pteroylglutamate) protecting against inhibition of enzyme activity by PbC12 (10 µM) were 3.3 pM (competitive protein binding radioassay) and 16 pM (microbiological assays), which indicates that the glutamate portion of the pteroylpoly-glutamate factor does not merely chelate lead. Additional evidence ruling out chelation as a mechanism is the finding that concentrations of the factor in excess of those protecting against inhibition of uroporphyrinogen I synthetase activity by PbCl2 failed to protect against inhibition of another lead-sensitive enzyme, δ-aminolevulinic acid dehydratase (EC 4.2.1.24). Pteroylhexaglutamate was found to protect against inhibition of uroporphyrinogen I synthetase activity by PbCl2 at 1-10 µM concentrations. Pteroyltriglutamate and pteroylmonoglutamate were unable to protect against inhibition of enzymatic activity by PbCl2. It is suggested that pteroylpolyglutamate derivatives may be regulators of uroporphyrinogen I synthetase activity, and may serve as coenzymes for this enzyme. ACKNOWLEDGMENTS The authors express their gratitude for the technical assistance of Ms. V. Pascucci.