Nitric oxide synthase (NOS) catalyzes the conversion of argmine to nitric oxide (NO) and citrulline through a series of monooxygenation reactions. All three isoforms (neuronal, inducible, and endothelial) contain heme and flavin prosthetic groups and require the cofactors calmodulin, tetrahydrobiopterin (BH4), and NADPH for activity. The role of BH4 has remained controversial, with arguments for both conformational and redox functions. The expression of these isoforms in E. coli, which does not synthesize BH4, makes it possible to produce high levels of purified NOSs deplete of BH4; these enzymes are easily reconstituted with BH4, if desired. The ability to produce either BH4-replete or -deplete enzyme allows us to examine the effects of BH4 on NO production, Superoxide formation, and NADPH utilization. The Km for BH4 in NO production by the BH4-deplete iNOS is approximately twice that of the BH4-replete form, 150 nM and 75 nM, respectively. The Km for BH4 of the BH4-replete nNOS is approximately 125 nM. The formation of Superoxide by BH4-replete iNOS is completely obliterated in the presence of 5 jiM BH4, indicating that BH4 promotes the very tight coupling of electron transfer to NO production. In the BH4-deplete iNOS or the BH4-replete nNOS, the effect is much less dramatic, but the trend is the same, i.e., 75% and 25% reduction, respectively, in superoxide formation in the presence of 5 μM BH4.
|Original language||English (US)|
|State||Published - Dec 1 1997|
ASJC Scopus subject areas
- Molecular Biology