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.