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The induction requires gene transcription and mRNA production, and therefore occurs several
hours after the exposure to the activating agents. However this also means that the production
of NO can go on for days until the enzyme is broken down. Also important is that when
activated, nanomole levels of NO are produced; ie 100 times that of the endothelial or
neuronal isoenzyme production. NO exposure can damage all classes of macromolecules
including DNA, it can destroy mitochondrial enzymes, inhibit protein synthesis, and cause
apoptosis of cells (Hibbs, Taintor et al. 1988; Curran, Ferrari et al. 1991; Kwon, Stuehr et al.
l99l; Stadler, Billiar et al. 1991; Lancaster, Langrehr et al. 1992: Lepoivre, Flaman et al.
1992; Tamir, Irwis et al. 1993; Nakano, Terato et al. 2003). So the regulation of this enzqe
is extremely important given its unique role in host defence but also ability to inflict
significant host damage (Nathan lggT\.
Under certain conditions such as in the absence L-arginine or if BFI+ is limited (or in the
presence of adriamycin which may in part explain the toxicity of this drug), all the NOS
enzymes, but more often seen with this inducible form of the enzyme, can reduce 02 via the
flavin cofactors to O2'- (Xia, Dawson et al. 1996; Cosentino, Patton et al. 1998; Vasquez-
Vivar, Kalyanaraman et al. l99S). The simultaneous generation of 02- and NO then results in
producing ONOO- causing further cellular injury.
3.4.3 Contol of the nitric oxide synthase isoen4tmes
3.4.3 (i) The constitutive forms
All of the enzymes can be regulated at pre-transcriptional, post transcriptional and post
translational levels (Papapetropoulos, Rudic et al. 1999). However the main control factors
appear to be different for each. The control of the constitutive enzymes is predominantly
maintained by the need for the presence of substrate and cofactors, while the control of the
inducible form is predominantly by the need for two signals for transcription.
The constitutive enzymes require both the presence of calmodulin and a certain level of
calcium from 200 to 400 nmols to commence production, and if the calcium concentration
falls below this level then production ceases. The substrate L-arginine is required and L-
arginine exists within and outside the cell. A low substrate amount can occur as a result of the
absolute availability of L-arginine, or the ability of the cell to take up L-arginine, or the ability
of the cell to regenerate L-citrulline back to L-arginine to act as further substrate. Vascular
cells, for example, are able to perform this regeneration with the use of an enzyme
arginosuccinate synthetase. BFI+ is also required, and this is produced by another pathway.
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