Tyrosine-Derived Neurotransmitters
The majority of tyrosine that does not get incorporated into proteins is
catabolized for energy production. One other significant fate of tyrosine is
conversion to the catecholamines. The catecholamine neurotransmitters
are dopamine, norepinephrine, and epinephrine (see also Biochemistry
of Nerve Transmission).
Norepinephrine is the principal neurotransmitter of sympathetic
postganglionic endings. Both norepinephrine and the methylated derivative,
epinephrine are stored in synaptic knobs of neurons that secrete it,
however, epinephrine is not a mediator at postganglionic sympathetic
endings.
Tyrosine is transported into catecholamine-secreting neurons and
adrenal medullary cells where catechaolamine synthesis takes place. The
first step in the process requires tyrosine hydroxylase, which like
phenylalanine hydroxylase requires tetrahydrobiopterin (H4B, or written as
BH4) as cofactor. The dependence of tyrosine hydroxylase on H4B
necessitates the coupling to the action of dihydropteridine reductase
(DHPR) as is the situation for phenylalanine hydroxylase and tryptophan
hydroxylase (see below).
The
hydroxylation
reaction
generates
DOPA
(3,4-dihydrophenylalanine). DOPA decarboxylase converts DOPA to
dopamine, dopamine β-hydroxylase converts dopamine to norepinephrine
and phenylethanolamine N-methyltransferase converts norepinephrine to
epinephrine. This latter reaction is one of several in the body that uses
SAM as a methyl donor generating S-adenosylhomocysteine. Within the
substantia nigra and some other regions of the brain, synthesis proceeds
only to dopamine. Within the adrenal medulla dopamine is converted to
norepinephrine and epinephrine.
Synthesis of the Catecholamines from Tyrosine.
Once synthesized, dopamine, norepinephrine and epinephrine are
packaged in granulated vesicles. Within these vesicles, norepinephrine
and epinephrine are bound to ATP and a protein called chromogranin A.
The actions of norepinephrine and epinephrine are exerted via
receptor-mediated signal transduction events. There are three distinct
types of adrenergic receptors: α1, α2, β. Within each class of adrenergic
receptor there are several sub-classes. The α1 class contains the α1A, α1B,
and α1D receptors. The α1 receptor class are coupled to Gq-type G-proteins
that activate PLCγ resulting in increases in IP3 and DAG release from
membrane PIP2. The α2 class contains the α2A, α2B, and α2C receptors. The
α2 class of adrenergic receptors are coupled to Gi-type G-proteins that
inhibit the activation of adenylate cyclase and therefore, activation results
in reductions in cAMP levels. The β class of receptors is composed of three
subtypes: β1, β2, and β3 each of which couple to Gs-type G-proteins
resulting in activation of adenylate cyclase and increases in cAMP with
concomitant activation of PKA.
Dopamine binds to dopamineric receptors identified as D-type
receptors and there are four subclasses identified as D1, D2, D4, and D5.
Activation of the dopaminergic receptors results in activation of adenylate
cyclase (D1 and D5) or inhibition of adenylate cyclase (D2 and D4).
Epinephrine and norepinephrine are catabolized to inactive
compounds
through
the
sequential
actions
of
catecholamine-O-methyltransferase (COMT) and monoamine oxidase
(MAO). Compounds that inhibit the action of MAO have been shown to
have beneficial effects in the treatment of clinical depression, even when
tricyclic antidepressants are ineffective. The utility of MAO inhibitors was
discovered serendipitously when patients treated for tuberculosis with
isoniazid showed signs of an improvement in mood; isoniazid was
subsequently found to work by inhibiting MAO.
Metabolism of the catecholamine neurotransmitters. Only clinically
important enzymes are included in this diagram. The catabolic byproducts
of the catecholamines, whose levels in the cerebrospinal fluid are
indicative of defects in catabolism, are in blue underlined text.
Abbreviations: TH = tyrosine hydroxylase, DHPR = dihydropteridine
reductase, H2B = dihydrobiopterin, H4B = tetrahydrobiopterin, MAO =
monoamine oxidase, COMT = catecholamine-O-methyltransferase, MHPG
= 3-methoxy-4-hydroxyphenylglycol, DOPAC = dihydroxyphenylacetic
acid.
Tryptophan-Derived Neurotransmitters
Tryptopan serves as the precursor for the synthesis of serotonin
(5-hydroxytryptamine, 5-HT, see also Biochemistry of Nerve Transmission)
and melatonin (N-acetyl-5-methoxytryptamine).
Pathway for serotonin and melatonin synthesis from tryptophan.
Abbreviations: THP = tryptophan hydroxylase, DHPR = dihydropteridine
reductase, H2B = dihydrobiopterin, H4B = tetrahyrobiopterin, 5-HT =
5-hydroxytryptophan, AADC = aromatic L-amino acid decarboxylase, SNA
= serotonin N-acetylase, HOMT = hydroxyindole-O-methyltransferase.
Serotonin is synthesized through 2-step process involving a
tetrahydrobiopterin-dependent hydroxylation reaction (catalyzed by
tryptophan-5-monooxygenase, also called tryptophan hydroxylase) and
then a decarboxylation catalyzed by aromatic L-amino acid decarboxylase.
The hydroxylase is normally not saturated and as a result, an increased
uptake of tryptophan in the diet will lead to increased brain serotonin
content.
Serotonin is present at highest concentrations in platelets and in the
gastrointestinal tract. Lesser amounts are found in the brain and the retina.
Serotonin containing neurons have their cell bodies in the midline raphe
nuclei of the brain stem and project to portions of the hypothalamus, the
limbic system, the neocortex and the spinal cord. After release from
serotonergic neurons, most of the released serotonin is recaptured by an
active reuptake mechanism. The function of the antidepressant, Prozac®,
and related drugs called selective serotonin re-uptake inhibitors (SSRIs), is
to inhibit this reuptake process, thereby, resulting in prolonged serotonin
presence in the synaptic cleft.
The function of serotonin is exerted upon its interaction with specific
receptors. Several serotonin receptors have been cloned and are identified
as 5HT1, 5HT2, 5HT3, 5HT4, 5HT5, 5HT6, and 5HT7. Within the 5HT1 group
there are subtypes 5HT1A, 5HT1B, 5HT1D, 5HT1E, and 5HT1F. There are
three 5HT2 subtypes, 5HT2A, 5HT2B, and 5HT2C as well as two 5HT5
subtypes, 5HT5a and 5HT5B. Most of these receptors are coupled to
G-proteins that affect the activities of either adenylate cyclase or
phospholipase Cν (PLCγ). The 5HT3 class of receptors are ion channels.
Some serotonin receptors are presynaptic and others postsynaptic.
The 5HT2A receptors mediate platelet aggregation and smooth muscle
contraction. The 5HT2C receptors are suspected in control of food intake as
mice lacking this gene become obese from increased food intake and are
also subject to fatal seizures. The 5HT 3 receptors are present in the
gastrointestinal tract and are related to vomiting. Also present in the
gastrointestinal tract are 5HT4 receptors where they function in secretion
and peristalsis. The 5HT6 and 5HT7 receptors are distributed throughout
the limbic system of the brain and the 5HT 6 receptors have high affinity for
antidepressant drugs.
Melatonin is derived from serotonin within the pineal gland and the
retina, where the necessary N-acetyltransferase enzyme is found. The
pineal parenchymal cells secrete melatonin into the blood and
cerebrospinal fluid. Synthesis and secretion of melatonin increases during
the dark period of the day and is maintained at a low level during daylight
hours. This diurnal variation in melatonin synthesis is brought about by
norepinephrine secreted by the postganglionic sympathetic nerves that
innervate the pineal gland. The effects of norepinephrine are exerted
through interaction with β-adrenergic receptors. This leads to increased
levels of cAMP, which in turn activate the N-acetyltransferase required for
melatonin synthesis. Melatonin functions by inhibiting the synthesis and
secretion of other neurotransmitters such as dopamine and GABA.
Creatine Biosynthesis
Creatine is synthesized in the liver by methylation of guanidoacetate
using SAM as the methyl donor. Guanidoacetate itself is formed in the
kidney from the amino acids arginine and glycine.
Synthesis of Creatine and Creatinine
Creatine is used as a storage form of high energy phosphate. The
phosphate of ATP is transferred to creatine, generating creatine phosphate,
through the action of creatine phosphokinase. The reaction is reversible
such that when energy demand is high (e.g. during muscle exertion)
creatine phosphate donates its phosphate to ADP to yield ATP.
Both creatine and creatine phosphate are found in muscle, brain and
blood. Creatinine is formed in muscle from creatine phosphate by a
nonenzymatic dehydration and loss of phosphate. The amount of
creatinine produced is related to muscle mass and remains remarkably
constant from day to day. Creatinine is excreted by the kidneys and the
level of excretion (creatinine clearance rate) is a measure of renal function.
Glutathione Functions
Glutathione (abbreviated GSH) is a tripeptide composed of glutamate,
cysteine and glycine that has numerous important functions within cells.
Glutathione serves as a reductant; is conjugated to drugs to make them
more water soluble; is involved in amino acid transport across cell
membranes (the γ-glutamyl cycle); is a substrate for the
peptidoleukotrienes; serves as a cofactor for some enzymatic reactions
and as an aid in the rearrangement of protein disulfide bonds. GSH is
synthesized in the cytosol of all mammalian cells. The rate of GSH
synthesis is dependent upon the availability of cysteine and the activity of
the rate-limiting enzyme, γ-glutamylcysteine synthetase (GCS). In the liver,
major factors that determine the availability of cysteine are diet, membrane
transport activities of the three sulfur amino acids cysteine, cystine and
methionine, and the conversion of methionine to cysteine (see the Amino
Acid Metabolism page). Numerous conditions can alter the level of GSH
synthesis via changes in GCS activity and GCS gene expression such as
oxidative stress, antioxidant levels, hormones, cell proliferation, and
diabetes mellitus.
Synthesis of Glutathione
(GSH)
Structure of GSSG
The role of GSH as a reductant is extremely important particularly in
the highly oxidizing environment of the erythrocyte. The sulfhydryl of GSH
can be used to reduce peroxides formed during oxygen transport.
Endogenously produced hydrogen peroxide (H2O2) is reduced by GSH in
the presence of selenium-dependent GSH peroxidase. Hydrogen peroxide
can also be reduced by catalase, which is present only in the peroxisomes.
In the mitochondria, GSH is particularly important because mitochondria
lack catalase. The resulting oxidized form of GSH consists of two
molecules disulfide bonded together (abbreviated GSSG). The enzyme
glutathione reductase utilizes NADPH as a cofactor to reduce GSSG back
to two moles of GSH. Hence, the pentose phosphate pathway is an
extremely important pathway of erythrocytes for the continuing production
of the NADPH needed by glutathione reductase. In fact as much as 10% of
glucose consumption, by erythrocytes, may be mediated by the pentose
phosphate pathway. Detoxification of xenobiotics or their metabolites is
another major function of GSH. These compounds form conjugates with
GSH either spontaneously or enzymatically in reactions catalyzed by GSH
S-transferase. The conjugates formed are usually excreted from the cell
and, in the case of the liver they are excreted in the bile.
Several mechanisms exist for the transport of amino acids across cell
membranes. Many are symport or antiport mechanisms that couple amino
acid transport to sodium transport. The γ-glutamyl cycle is an example of a
group transfer mechanism of amino acid transport. Although this
mechanism requires more energy input, it is rapid and has a high capacity.
The cycle functions primarily in the kidney, particularly renal epithelial cells.
The enzyme γ-glutamyl transpeptidase is located in the cell membrane and
shuttles GSH to the cell surface to interact with an amino acid. Reaction
with an amino acid liberates cysteinylglycine and generates a
γ-glutamyl-amino acid which is transported into the cell and hydrolyzed to
release the amino acid. Glutamate is released as 5-oxoproline and the
cysteinylglycine is cleaved to its component amino acids. Regeneration of
GSH requires an ATP-dependent conversion of 5-oxoproline to glutamate
and then the 2 additional moles of ATP that are required during the normal
generation of GSH.
Polyamine Biosynthesis
One of the earliest signals that cells have entered their replication
cycle is the appearance of elevated levels of mRNA for ornithine
decarboxylase (ODC), and then increased levels of the enzyme, which is
the first enzyme in the pathway to synthesis of the polyamines. Because of
the latter, and because the polyamines are highly cationic and tend to bind
nucleic acids with high affinity, it is believed that the polyamines are
important participants in DNA synthesis, or in the regulation of that
process.
The key features of the pathway are that it involves putrescine, an
ornithine catabolite, and S-adenosylmethionine (SAM) as a donor of 2
propylamine residues. The first propylamine conjugation yields
spermidine and addition of another to spermidine yields spermine.
The function of ODC is to produce the 4-carbon saturated diamine,
putrescine. At the same time, SAM decarboxylase cleaves the SAM
carboxyl
residue,
producing
decarboxylated
SAM
(S-adenosymethylthiopropylamine), which retains the methyl group usually
involved in SAM methyltransferase activity. SAM decarboxylase activity is
regulated by product inhibition and allosterically stimulated by
putrescine. Spermidine synthase catalyzes the condensation reaction,
producing spermidine and 5'-methylthioadenosine. A second propylamine
residue is added to spermidine producing spermine.
The signal for regulating ODC activity is unknown, but since the
product of its activity, putrescine, regulates SAM decarboxylase activity, it
appears that polyamine production is principally regulated by ODC
concentration.
The butylamino group of spermidine is used in a posttranslational
modification reaction important to the process of translation. A specific
lysine residue in the translational initiation factor eIF-4D is modified.
Following the modification the residue is hydroxylated yielding a residue in
the protein termed hypusine.
Nitric Oxide Synthesis and Function
Vasodilators, such as acetylcholine and bradykinin, do not exert their
effects upon the vascular smooth muscle cell in the absence of the
overlying endothelium. When acetylcholine (or bradykinin) binds its
receptor on the surface of endothelial cells, a signal cascade, coupled to
the activation phospholipase C-γ (PLCγ), is initiated. The PLCγ-mediated
release of inositol trisphosphate, IP3 (from membrane associated
phosphatidylinositol-4,5-bisphosphate, PIP2), leads to the release of
intracellular stores of Ca2+. In turn, the elevation in Ca2+ leads to the
liberation of endothelium-derived relaxing factor (EDRF) which then
diffuses into the adjacent smooth muscle. Within smooth muscle cells,
EDRF reacts with the heme moiety of a soluble guanylyl cyclase, resulting
in activation of the latter and a consequent elevation of intracellular levels
of cGMP. The net effect is the activation of cGMP-dependent protein
kinase (PKG) and the phosphorylation of substrates leading to smooth
muscle cell relaxation. The coronary artery vasodilator, nitroglycerin, acts
to increase intracellular release of EDRF and thus the activation of the
cGMP signal cascade.
Quite unexpectedly, EDRF was found to be the free radical diatomic
gas, nitric oxide, NO. So stunning was the elucidation of the pathway to
and actions of NO that Drs. Murad, Ignarro and Furchgott were awarded
the Nobel Prize in 1998 for their work on this system. NO is formed by the
action of NO synthase, (NOS) on the amino acid arginine.
arginine ——> citrulline + NO
There are 3 isozymes of NOS in mammalian cells:
Neuronal NOS (nNOS), also called NOS-1
Inducible or macrophage NOS (iNOS), also called NOS-2
Endothelial NOS (eNOS), also called NOS-3.
Nitric oxide synthases are very complex enzymes, employing five
redox cofactors: NADPH, FAD, FMN, heme and tetrahydrobiopterin (H4B).
NO can also be formed from nitrite, derived from vasodilators such as
glycerin trinitrate (nitroglycerin) during their metabolism. The half-life of NO
is extremely short, lasting only 2-4 seconds. This is because it is a highly
reactive free radical and interacts with oxygen and superoxide. NO is
inhibited by hemoglobin and other heme proteins which bind it tightly.
Both eNOS and nNOS are constitutively expressed and regulated by
The calcium regulation is imparted be the associated calmodulin
subunits, thus explaining how vasodilators such as acetylcholine effect
smooth muscle relaxation as a consequence of increasing intracellular
endothelial cell calcium levels. Although iNOS contains calmodulin
subunits, its activity is unaffected by changes in Ca2+ concentration. iNOS
is transcriptionally activated in macrophages, neutrophils, and smooth
muscle cells.
Ca2+.
The major functions of NO production through activation of iNOS are
associated with the bactericidal and tumoricidal actions of macrophages.
Overproduction of NO via iNOS is associated with cytokine-induced septic
shock such as occurs post-operatively in patients with bacterial infections.
Bacteria produce endotoxins such as lipopolysaccharide (LPS) that
activate iNOS in macrophages.
Nitric oxide is involved in a number of other important cellular
processes in addition to its impact on vascular smooth muscle cells.
Events initiated by NO that are important for blood coagulation include
inhibition of platelet aggregation and adhesion and inhibition of neutrophil
adhesion to platelets and to the vascular endothelium. NO is also
generated by cells of the immune system and as such is involved in
non-specific host defense mechanisms and macrophage-mediated killing.
NO also inhibits the proliferation of tumor cells and microorganisms.
Additional cellular responses to NO include induction of apoptosis
(programmed cell death), DNA breakage and mutation.
Chemical inhibitors of NOS are available and can markedly decrease
production of NO. The effect is a dramatic increase in blood pressure due
to vasoconstriction. Another important cardiovascular effect of NO is
exerted through the production of cGMP, which acts to inhibit platelet
aggregation.