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chapter 17
Protein and Amino Acid Metabolism
The overall reaction is:
o
H
II
I
'O — C — CH — (CH2)3 — N
NH3-
Arginine
Nitric O xide
S yn th ase
NH„
I
- C = N H + 0
2
+ NADPH + H +
FMN, FAD, Tetrahydrobiopterin
F e2+, H em e com plex
O
H O
II
I
II
- O - C - CH - (CH2)3 — N - C - NH
2
+ NO
NH3+
Citrulline
The NOS activity is inhibited by NG-substituted ana-
logues of arginine, such as NG-nitroarginine and NG-
monomethyl-L-arginine.
Isoforms (Also Known as Isozymes)
o f Nitric Oxide Synthase
There
are three
major isoforms
of
Nitric
Oxide
Synthase (NOS)
ranging in molecular size from 130 to
160 kDa. Amino acid similarity between any two isoforms
is about 50-60%. Isoforms of NOS exhibit differences in
tissue distribution, transcriptional regulation, and activa-
tion by intracellular Ca2+. Two of the three isoforms of
NOS are constitutive enzymes (cNOS) and the third iso-
forin is an inducible enzyme (iNOS). The cNOS isoforms
are found in the vascular endothelium (eNOS), neuronal
cells (nNOS), and many other cells, and are regulated
by Ca2+ and calmodulin. In the vascular endothelium,
agonists such as acetylcholine and bradykinin activate
eNOS by enhancing intracellular Ca2+ concentrations
via the production of inositol 1,4,5-trisphosphate, which
activates the phosphoinositide second-messenger system
(Chapter 30 ). The NO produced in the vascular endothe-
lium maintains basal vascular tone by vasodilation which
is mediated by vascular smooth muscle cells. Organic
nitrates used in the management of ischemic heart disease
act by denitration with the subsequent formation of NO.
Sodium nitroprusside, an antihypertensive drug, is an NO
donor. Thus, organic nitrates and sodium nitroprusside
are prodrugs, and the exact mechanism by which these
prodrugs yield NO is not yet understood. Inhaled NO can
produce pulmonary vasodilation. This property of NO
has been used in the management of hypoxic respiratory
failure associated with primary pulmonary hypertension
in neonates. NO produced by cNOS in neuronal tissue
functions as a neurotransmitter.
The inducible class of NOS (iNOS) is found in
macrophages and neutrophils and is Ca2+-independent.
Bacterial
endotoxins,
cytokines
(e.g.,
interleukin-1,
interferon-y), or bacterial lipopolysaccharides can induce
and cause expression of NOS in many cell types. Glu-
cocorticoids inhibit the induction of iNOS. In stimulated
macrophages and neurophils, NO and superoxide radi-
cal (Oj ) react to generate peroxynitrite, a powerful ox-
idant, and hydroxyl radicals. These reactive intermedi-
ates are involved in the killing of phagocytized bacteria
(Chapter 14). Excessive production of NO due to endotox-
inemia produces hypotension and vascular hyporeactivity
to vasoconstrictor agents, and leads to septic shock. NOS
inhibitors have potential therapeutic application in the
treatment of hypotensive crisis.
Signal Transduction of NO
NO is lipophilic and diffuses readily across cell mem-
branes. It interacts with molecules in the target cells pro-
ducing various biological effects. One mechanism of ac-
tion of NO is stimulation of guanylate cyclase, which
catalyzes the formation of cyclic guanosine monophos-
phate (cGMP) from GTP, resulting in increased intracel-
lular cGMP levels (Figure 17-9). NO activates guanylate
cyclase by binding to heme iron. The elevation of cGMP
levels may activate cGMP-dependent protein kinases.
o
O H
OH
G uanosine Triphosphate (G TP )
PP i
Guanylate C ycla se
NO
O
HN'
Derived from
Nerve im pulse
Derived from a
Ligand-cell interaction
H2N
O
HN^
h 2n
^
n
O H
OH
5 '-G M P
FIGURE 17-9
NO-mediated synthesis of cGMP from GTP in the corpus cavernosum that
leads to smooth muscle relaxation. Sildenafil potentiates the effects of NO
by inhibiting cGMP phosphodiesterase.
