title: Inhibition of nitric oxide synthase isoforms by porphyrins.
abstract: Protoporphyrin IX inhibits citrulline formation by all three nitric oxide synthase isoforms in a manner reversible by dilution. Zinc protoporphyrin IX, by contrast, produces a time- and concentration-dependent inactivation of all three nitric oxide synthase isoforms, not reversible by dilution. The inhibition of citrulline formation by protoporphyrin IX occurs with IC50 values of 0.8, 4, and 5 microM for the nNOS, iNOS, and eNOS isoforms, respectively. Inhibition by N-methyl-protoporphyrin IX occurs at IC50 values of 6, 5, and 8 microM for the nNOS, iNOS, and eNOS isoforms, respectively. Inhibition of nitric oxide synthase by protoporphyrin IX is a multisite, positively cooperative inhibition that exhibits a Hill coefficient of 2.3 for the iNOS isoform. Protoporphyrin IX reduces the maximal velocity of citrulline formation for both the iNOS and nNOS isoforms without altering the K(m) for the arginine substrate or the EC50 value for the tetrahydrobiopterin cofactor. Protoporphyrin IX inhibits the arginine-independent NADPH oxidase activity of nNOS with an IC50 value of 1 microM but has no effect on cytochrome c reductase activity at concentrations as high as 30 microM. At concentrations of 10 and 20 microM, protoporphyrin IX inhibits NO formation by cytokine-induced murine RAW 264.7 cells; however, these inhibitions are accompanied by significant cellular cytotoxicity. Coproporphyrins I and III, uroporphyrins I and III, and porphobilinogen, intermediates in the biosynthesis of heme that accumulate in hepatic porphyrias, are ineffective as inhibitors of the nitric oxide synthase isoforms. Since protoporphyrin IX is the immediate biosynthetic precursor of heme that accumulates in hepatic protoporphyria, iron deficiency anemia, and lead poisoning, protoporphyrin IX inhibition of nitric oxide synthase may contribute to the pathophysiology of these conditions.
authors.affiliation: Department of Pharmacology, University of Medicine and Dentistry of New Jersey, Robert Wood Johnson Medical School, Piscataway 08854, USA.
authors.forename: D J
authors.initials: DJ
authors.lastname: Wolff
authors.forename: R A
authors.initials: RA
authors.lastname: Naddelman
authors.forename: A
authors.initials: A
authors.lastname: Lubeskie
authors.forename: D A
authors.initials: DA
authors.lastname: Saks
mesh_terms: D000818:Animals; D001120:Arginine / Q000378:metabolism; D002417:Cattle; D002460:Cell Line; D002956:Citrulline / Q000096:biosynthesis; D004791:Enzyme Inhibitors / Q000737:chemistry / Q000494:pharmacology*; D006801:Humans; D066298:In Vitro Techniques; D007371:Interferon-gamma / Q000494:pharmacology; D007527:Isoenzymes / Q000037:antagonists & inhibitors*; D007700:Kinetics; D051379:Mice; D015394:Molecular Structure; D009245:NADH Dehydrogenase / Q000378:metabolism; D019255:NADPH Oxidases / Q000378:metabolism; D019001:Nitric Oxide Synthase / Q000037:antagonists & inhibitors*; D011166:Porphyrins / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology*; D011524:Protoporphyrins / Q000494:pharmacology; D051381:Rats
chemical_list: D004791:Enzyme Inhibitors; D007527:Isoenzymes; D011166:Porphyrins; D011524:Protoporphyrins; C017803:zinc protoporphyrin; D002956:Citrulline; D007371:Interferon-gamma; D001120:Arginine; D019001:Nitric Oxide Synthase; D019255:NADPH Oxidases; D009245:NADH Dehydrogenase