Vitamin B6 is present in foods mainly as the PN, PLP
and PMP vitamers. In many fruits and vegetables,
30% or more of the total vitamin B6 is present as PNglucoside.
The binding of PLP to protein through aldimine
(Schiff base) and substituted aldamine linkages
is reversibly dependent on pH, the vitamin–protein
complexes being readily dissociated under normal
gastric acid (low pH) conditions. The release of PLP
from its association with protein is an important step
in the subsequent absorption of vitamin B6, as binding
to protein inhibits the next step, hydrolysis of PLP by
alkaline phosphatase (Middleton, 1986). It would appear,
therefore, that the widespread practice of raising
the post-prandial gastric and upper small intestinal
pH by the use of pharmaceutical antacids may impair
vitamin B6 absorption.
Physiological amounts of PLP and PMP are largely
hydrolysed by alkaline phosphatase in the intestinal
lumen before absorption of free PL and PM (Hamm
et al., 1979; Mehanso et al., 1979). When present in
the lumen at non-physiological levels which saturate
the hydrolytic enzymes, substantial amounts of PLP
and PMP are absorbed intact, but at a slower rate than
their non-phosphorylated forms.
The absorption of PN, PL and PM takes place
mainly in the jejunum and is a dynamic process involving
several interrelated events. The vitamers cross
the brush-border membrane by simple diffusion as
shown, for example, in everted intestinal sacs (Tsuji et al., 1973), brush-border membrane vesicles (Yoshida
et al., 1981) and isolated intestinal loops (Middleton,
1979). In humans, PM is absorbed more slowly or
metabolized differently, or both, than either PL or
PN (Wozenski et al., 1980). Middleton (1983) noted a
signifi cant positive correlation between PLP luminal
disappearance and both alkaline phosphatase activity
and net water absorption in perfused segments of rat
jejunum. It is conjectural that increased water absorption
results in a greater concentration of PL within the
lumen, allowing absorption to proceed more rapidly.
Within the enterocyte PN, PL and PM are converted
to their corresponding phosphates by the catalytic action
of cytoplasmic pyridoxal kinase, and transaminases
interconvert PLP and PMP. The conversion of
a particular vitamer to other forms by intracellular
metabolism creates a concentration gradient across
the brush border for that vitamer, thus enhancing
its uptake by diffusion (Middleton, 1985). The phosphorylated
vitamers formed in the cell are largely
dephosphorylated by non-specific phosphatases, thus
permitting easy diffusion of vitamin B6 compounds
across the basolateral membrane. The major form
of vitamin B6 released to the portal circulation is the
non-phosphorylated form of the vitamer predominant
in the intestinal lumen.
Absorption capacity in rats was not affected directly
by dietary vitamin B6 supply (Roth-Maier et al., 1982)
and so it is supposed that homeostatic regulation of
vitamin B6 is not due to a variation of absorption.
Tuesday, July 3, 2007
Bioavailability of Vitamin B6
Losses of vitamin B6 content caused by thermal instability
occur during food processing, but the remaining
vitamin B6 does not necessarily exhibit incomplete
bioavailability.
The bioavailability of vitamin B6 in foods is highly
variable, owing largely to the presence of poorly utilized
PN-glucoside in plant tissues. As expected, vitamin
B6 generally has a lower availability from plantderived
foods than from animal tissues (Nguyen &
Gregory, 1983). Based on plasma PLP levels in male
human subjects, the bioavailability of the vitamin in
an average American diet ranged from 61% to 81%,
with a mean of 71% (Tarr et al., 1981).
Gregory et al. (1991) determined the bioavailability
of PN-glucoside in humans through the use
of a stable-isotope method. The utilization of orally
administered deuterated PN-glucoside was 58 ± 13%
(mean ± SEM) relative to that of deuterated PN.
Intravenously administered PN-glucoside underwent
approximately half the metabolic utilization
of oral PN-glucoside, which suggested a role of β-
glucosidase(s) of the intestinal mucosa, microfl ora, or
both, in the release of free PN from dietary PN-glucoside.
Stable isotope methodology provided evidence
that PN-glucoside weakly retards the metabolic utilization
of non-glycosylated forms of vitamin B6 in humans
(Gilbert et al., 1991). Despite the relatively high
consumption of glycosylated vitamin B6, vegetarian
women did not demonstrate any signifi cant difference
in vitamin B6 status compared with non-vegetarian
women (Shultz & Leklem, 1987; Löwik et al.,
1990). In addition, the intake of glycosylated vitamin
B6 had little, if any, effect upon maternal plasma PLP
concentration and maternal urinary excretion of total
vitamin B6 and 4-pyridoxic acid in lactating women
(Andon et al., 1989). These observations suggest that
there may be little practical signifi cance to the human
consumption of glycosylated vitamin B6.
14.4 Absorption, transport and metabolism
Humans cannot synthesize vitamin B6 and thus must
obtain the vitamin from exogenous sources via intestinal
absorption. The intestine is exposed to vitamin
B6 from two sources: (1) the diet and (2) the bacterially
synthesized vitamin B6 in the large intestine. Whether
the latter source of vitamin B6 is available to the host
tissues (apart from the colonic epithelial cells) in nutritionally
signifi cant amounts is unknown.
occur during food processing, but the remaining
vitamin B6 does not necessarily exhibit incomplete
bioavailability.
The bioavailability of vitamin B6 in foods is highly
variable, owing largely to the presence of poorly utilized
PN-glucoside in plant tissues. As expected, vitamin
B6 generally has a lower availability from plantderived
foods than from animal tissues (Nguyen &
Gregory, 1983). Based on plasma PLP levels in male
human subjects, the bioavailability of the vitamin in
an average American diet ranged from 61% to 81%,
with a mean of 71% (Tarr et al., 1981).
Gregory et al. (1991) determined the bioavailability
of PN-glucoside in humans through the use
of a stable-isotope method. The utilization of orally
administered deuterated PN-glucoside was 58 ± 13%
(mean ± SEM) relative to that of deuterated PN.
Intravenously administered PN-glucoside underwent
approximately half the metabolic utilization
of oral PN-glucoside, which suggested a role of β-
glucosidase(s) of the intestinal mucosa, microfl ora, or
both, in the release of free PN from dietary PN-glucoside.
Stable isotope methodology provided evidence
that PN-glucoside weakly retards the metabolic utilization
of non-glycosylated forms of vitamin B6 in humans
(Gilbert et al., 1991). Despite the relatively high
consumption of glycosylated vitamin B6, vegetarian
women did not demonstrate any signifi cant difference
in vitamin B6 status compared with non-vegetarian
women (Shultz & Leklem, 1987; Löwik et al.,
1990). In addition, the intake of glycosylated vitamin
B6 had little, if any, effect upon maternal plasma PLP
concentration and maternal urinary excretion of total
vitamin B6 and 4-pyridoxic acid in lactating women
(Andon et al., 1989). These observations suggest that
there may be little practical signifi cance to the human
consumption of glycosylated vitamin B6.
14.4 Absorption, transport and metabolism
Humans cannot synthesize vitamin B6 and thus must
obtain the vitamin from exogenous sources via intestinal
absorption. The intestine is exposed to vitamin
B6 from two sources: (1) the diet and (2) the bacterially
synthesized vitamin B6 in the large intestine. Whether
the latter source of vitamin B6 is available to the host
tissues (apart from the colonic epithelial cells) in nutritionally
signifi cant amounts is unknown.
Dietary sources of Vitamin B6
Vitamin B6 is present in all natural unprocessed foods,
with yeast extract, wheat bran and liver containing
particularly high concentrations. Other important
sources include whole-grain cereals, nuts, pulses, lean
meat, fi sh, kidney, potatoes and other vegetables. In
cereal grains over 90% of the vitamin B6 is found in
the bran and germ (Polansky & Toepfer, 1969), and
75–90% of the B6 content of the whole grain is lost in
the milling of wheat to low-extraction fl our (Sauberlich,
1985). Thus, white bread is considerably lower in
vitamin B6 content than is whole wheat bread. Milk,
eggs and fruits contain relatively low concentrations
of the vitamin.
In raw animal and fi sh tissue the major form of
vitamin B6 is PLP. Apart from very low concentrations
in liver, PN and PNP are virtually absent in animal
tissues.
Plant tissue contains mostly PN, a proportion of
which may be present as PN-glucoside and/or other
conjugates. PN-glucoside has not been found in
animal products. No generalizations can be made
as to one group of foods consistently having a high PN-glucoside content. Typical sources of PN-glucoside
(expressed as a percentage of the total vitamin B6
present) are bananas (5%), raw broccoli (35%), raw
green beans (58%), raw carrots (70%) and orange juice
(69%) (Gregory & Ink, 1987). PN-glucoside accounted
for 10–15% of the total vitamin B6 in the typical mixed
diets used in an American human study (Gregory et al.,
1991), but would be proportionally higher in vegetarian
diets.
with yeast extract, wheat bran and liver containing
particularly high concentrations. Other important
sources include whole-grain cereals, nuts, pulses, lean
meat, fi sh, kidney, potatoes and other vegetables. In
cereal grains over 90% of the vitamin B6 is found in
the bran and germ (Polansky & Toepfer, 1969), and
75–90% of the B6 content of the whole grain is lost in
the milling of wheat to low-extraction fl our (Sauberlich,
1985). Thus, white bread is considerably lower in
vitamin B6 content than is whole wheat bread. Milk,
eggs and fruits contain relatively low concentrations
of the vitamin.
In raw animal and fi sh tissue the major form of
vitamin B6 is PLP. Apart from very low concentrations
in liver, PN and PNP are virtually absent in animal
tissues.
Plant tissue contains mostly PN, a proportion of
which may be present as PN-glucoside and/or other
conjugates. PN-glucoside has not been found in
animal products. No generalizations can be made
as to one group of foods consistently having a high PN-glucoside content. Typical sources of PN-glucoside
(expressed as a percentage of the total vitamin B6
present) are bananas (5%), raw broccoli (35%), raw
green beans (58%), raw carrots (70%) and orange juice
(69%) (Gregory & Ink, 1987). PN-glucoside accounted
for 10–15% of the total vitamin B6 in the typical mixed
diets used in an American human study (Gregory et al.,
1991), but would be proportionally higher in vegetarian
diets.
Vitamin B6
In 1934 Paul György observed the appearance of a
scaly dermatitis (acrodynia) in rats fed on diets free
from the whole vitamin B complex and supplemented
with thiamin and ribofl avin. This observation led to
the establishment of a ‘rat acrodynia-preventative factor’
and its designation as vitamin B6. The isolation of the pure crystalline vitamin was first reported by Lepkovsky in 1938, and the synthesis of pyridoxine
was accomplished by Harris and Folkers in the following
year. Discovery of the existence of pyridoxal
and pyridoxamine and the recognition of their phosphorylated
forms as coenzymes is largely credited to
Esmond E. Snell during 1944–1948.
Vitamin B6 is the generic descriptor for all 3-hydroxy-
2-methylpyridine derivatives which exhibit qualitatively
in rats the biological activity of pyridoxine.
Six B6 vitamers are known, namely pyridoxine or
pyridoxol (PN), pyridoxal (PL) and pyridoxamine
(PM), which possess, respectively, alcohol, aldehyde
and amine group in the 4-position; their respective
5´-phosphate esters are designated as PNP, PLP and
PMP (Fig. 14.1).
In its role as a coenzyme, PLP is attached to the
apoenzyme by a Schiff base (aldimine) linkage
(–N=CH–) formed through condensation of the 4-
carbonyl group with the ε-amino group of specifi c
lysine residues A ubiquitous bound form of PN that occurs in
plant tissues is a glucoside conjugate, 5´-O-(β-Dglucopyranosyl)
pyridoxine (Fig. 14.3), designated
in this text as PN-glucoside. A more complex derivative
of PN-glucoside containing cellobiose and
5-hydroxydioxindole-3-acetic acid moieties has been
identifi ed as a major form of vitamin B6 in rice bran
and a minor form in wheat bran and legumes (Tadera
& Orite, 1991).
All of the six B6 vitamers are considered to have approximately
equivalent biological activity in humans
as a result of their ultimate conversion to coenzymes.
scaly dermatitis (acrodynia) in rats fed on diets free
from the whole vitamin B complex and supplemented
with thiamin and ribofl avin. This observation led to
the establishment of a ‘rat acrodynia-preventative factor’
and its designation as vitamin B6. The isolation of the pure crystalline vitamin was first reported by Lepkovsky in 1938, and the synthesis of pyridoxine
was accomplished by Harris and Folkers in the following
year. Discovery of the existence of pyridoxal
and pyridoxamine and the recognition of their phosphorylated
forms as coenzymes is largely credited to
Esmond E. Snell during 1944–1948.
Vitamin B6 is the generic descriptor for all 3-hydroxy-
2-methylpyridine derivatives which exhibit qualitatively
in rats the biological activity of pyridoxine.
Six B6 vitamers are known, namely pyridoxine or
pyridoxol (PN), pyridoxal (PL) and pyridoxamine
(PM), which possess, respectively, alcohol, aldehyde
and amine group in the 4-position; their respective
5´-phosphate esters are designated as PNP, PLP and
PMP (Fig. 14.1).
In its role as a coenzyme, PLP is attached to the
apoenzyme by a Schiff base (aldimine) linkage
(–N=CH–) formed through condensation of the 4-
carbonyl group with the ε-amino group of specifi c
lysine residues A ubiquitous bound form of PN that occurs in
plant tissues is a glucoside conjugate, 5´-O-(β-Dglucopyranosyl)
pyridoxine (Fig. 14.3), designated
in this text as PN-glucoside. A more complex derivative
of PN-glucoside containing cellobiose and
5-hydroxydioxindole-3-acetic acid moieties has been
identifi ed as a major form of vitamin B6 in rice bran
and a minor form in wheat bran and legumes (Tadera
& Orite, 1991).
All of the six B6 vitamers are considered to have approximately
equivalent biological activity in humans
as a result of their ultimate conversion to coenzymes.
Nutritional aspects for Niacin
Human requirement
Requirements for niacin are related to energy intake
because of the involvement of NAD and NADP as
coenzymes in the oxidative release of energy from
food. Estimation of niacin requirement is complicated
by the conversion of tryptophan to the vitamin. The
effi ciency of the conversion is affected by a variety of
infl uences, including the amounts of tryptophan and
niacin ingested, protein and energy intake, hormonal
status, and vitamin B6 and ribofl avin nutriture. A
normal intake of protein will probably provide more
than enough tryptophan to meet the body’s requirement
for niacin without the need for any preformed
niacin in the diet.
A notable exception to the 60:1 conversion ratio of
L-tryptophan to niacin is the state of pregnancy, in
which the conversion is about twice as efficient. This
increased conversion is presumably due to the stimulation
by oestrogen of tryptophan oxygenase, which
is a rate-limiting enzyme in the biosynthetic pathway.
Conversion is also increased when contraceptive pills
are used.
Effects of high intake
Nicotinic acid administered orally at doses as low
as 100 mg per day causes peripheral vasodilatation,
with the appearance of skin fl ushing. In high doses,
nicotinic acid competes with uric acid for excretion,
leading to an increase in the incidence of gouty arthritis.
Of greatest concern is possible liver damage,
and in one report severe jaundice occurred at doses
of 750 mg per day for only 3 months. Nicotinamide
does not cause vasodilatation, but is otherwise two to
three times as toxic as the acid (Miller & Hayes, 1982;
Alhadeff et al., 1984).
Requirements for niacin are related to energy intake
because of the involvement of NAD and NADP as
coenzymes in the oxidative release of energy from
food. Estimation of niacin requirement is complicated
by the conversion of tryptophan to the vitamin. The
effi ciency of the conversion is affected by a variety of
infl uences, including the amounts of tryptophan and
niacin ingested, protein and energy intake, hormonal
status, and vitamin B6 and ribofl avin nutriture. A
normal intake of protein will probably provide more
than enough tryptophan to meet the body’s requirement
for niacin without the need for any preformed
niacin in the diet.
A notable exception to the 60:1 conversion ratio of
L-tryptophan to niacin is the state of pregnancy, in
which the conversion is about twice as efficient. This
increased conversion is presumably due to the stimulation
by oestrogen of tryptophan oxygenase, which
is a rate-limiting enzyme in the biosynthetic pathway.
Conversion is also increased when contraceptive pills
are used.
Effects of high intake
Nicotinic acid administered orally at doses as low
as 100 mg per day causes peripheral vasodilatation,
with the appearance of skin fl ushing. In high doses,
nicotinic acid competes with uric acid for excretion,
leading to an increase in the incidence of gouty arthritis.
Of greatest concern is possible liver damage,
and in one report severe jaundice occurred at doses
of 750 mg per day for only 3 months. Nicotinamide
does not cause vasodilatation, but is otherwise two to
three times as toxic as the acid (Miller & Hayes, 1982;
Alhadeff et al., 1984).
Niacin deficiency
A deficiency in niacin results in pellagra, which is a
nutritional disease endemic among poor communities
who subsist chiefl y on maize. The classical features
of endemic pellagra are dermatitis, infl ammation of
the mucous membranes, diarrhoea and psychiatric
disturbances. The dermatitis often appears after exposure
to sunlight and resembles sunburn. The skin
becomes red and blistered and frequently peels off in
large areas. In chronic cases the skin becomes rough
and thickened with a brown pigmentation. In acute
pellagra, the mucous membranes of the gastrointestinal
and genitourinary tracts are severely infl amed.
The mouth becomes extremely sore and the tongue
is swollen and scarlet in colour. Chewing and swallowing
are painful and even liquids may be refused.
Infl ammation of the small and large intestine is manifested
by diarrhoea, abdominal pain and soreness of
the rectum. Hypermotility of the gastrointestinal
tract and the loss of appetite lead to profound loss of
weight. Infl ammation of the lower urinary tract causes
urethritis with increased micturition accompanied
by a burning sensation. In the female, severe vaginitis
is observed and amenorrhoea is common. Bender
(1984) vividly described neurological and neuropsychiatric
signs. Early signs include tremor, irritability,
anxiety and depression, with delirium and dementia
sometimes occurring in severe and chronic cases.
Unless the disease is treated, the inevitable outcome
is death. Fortunately, the response to nicotinamide
therapy is rapid and dramatic.
The prognosis is complicated by signs of proteinenergy
malnutrition and by an imbalance of amino
acid intake, particularly low levels of tryptophan and
high levels of leucine. Because most proteins contain
at least 1.0% tryptophan, it is theoretically possible
to maintain adequate niacin status on a diet devoid
of niacin but containing >100 g of protein. Primary
defi ciencies are rare (at least in industrialized countries),
but secondary defi ciencies may arise from
gastro intestinal disorders or alcoholism.
nutritional disease endemic among poor communities
who subsist chiefl y on maize. The classical features
of endemic pellagra are dermatitis, infl ammation of
the mucous membranes, diarrhoea and psychiatric
disturbances. The dermatitis often appears after exposure
to sunlight and resembles sunburn. The skin
becomes red and blistered and frequently peels off in
large areas. In chronic cases the skin becomes rough
and thickened with a brown pigmentation. In acute
pellagra, the mucous membranes of the gastrointestinal
and genitourinary tracts are severely infl amed.
The mouth becomes extremely sore and the tongue
is swollen and scarlet in colour. Chewing and swallowing
are painful and even liquids may be refused.
Infl ammation of the small and large intestine is manifested
by diarrhoea, abdominal pain and soreness of
the rectum. Hypermotility of the gastrointestinal
tract and the loss of appetite lead to profound loss of
weight. Infl ammation of the lower urinary tract causes
urethritis with increased micturition accompanied
by a burning sensation. In the female, severe vaginitis
is observed and amenorrhoea is common. Bender
(1984) vividly described neurological and neuropsychiatric
signs. Early signs include tremor, irritability,
anxiety and depression, with delirium and dementia
sometimes occurring in severe and chronic cases.
Unless the disease is treated, the inevitable outcome
is death. Fortunately, the response to nicotinamide
therapy is rapid and dramatic.
The prognosis is complicated by signs of proteinenergy
malnutrition and by an imbalance of amino
acid intake, particularly low levels of tryptophan and
high levels of leucine. Because most proteins contain
at least 1.0% tryptophan, it is theoretically possible
to maintain adequate niacin status on a diet devoid
of niacin but containing >100 g of protein. Primary
defi ciencies are rare (at least in industrialized countries),
but secondary defi ciencies may arise from
gastro intestinal disorders or alcoholism.
Role of NAD in ADP-ribosylation
NAD functions in ADP-ribosylation, a reversible
post-translational modifi cation of proteins in which
the ADP-ribose moiety of NAD is transferred to
acceptor proteins, thereby altering their function.
ADP-ribosylation reactions are classifi ed into two
major groups: mono-ADP-ribosylation and poly-
ADP-ribosylation.
Mono-ADP-ribosylation by bacterial toxins
The transfer of ADP-ribose to the acceptor protein
(Fig. 13.5) is catalysed by ADP-ribosyltransferases,
which are found in the cytosol, plasma membrane and
nuclear envelope of eukaryotic cells. The ADP-ribose
reacts with specifi c amino acid residues on the acceptor
protein to form N-glycosides. Certain bacterial
toxins also possess ADP-ribosyltransferase activity
(Ueda & Hayaishi, 1985) and, since more is known
about them than eukaryotic ADP-ribosyltransferases,
they will be selected as examples.
Two bacterial exotoxins, diphtheria toxin and
Pseudomonas aeruginosa exotoxin A, prevent protein
synthesis in bacterially infected eukaryotic cells by
inactivating elongation factor 2, a protein required
for polypeptide chain elongation. The uncontrolled
action of these exotoxins results in death of the host
cells. A mammalian cellular ADP-ribosyltransferase
also inactivates elongation factor 2 (Iglewski, 1994),
but this is a controlled action required for normal
protein synthesis.
Cholera toxin and Escherichia coli heat-labile enterotoxin
ADP-ribosylate the α subunit of the stimulatory
G protein, Gs, which relays the signal from a
hormone-activated cell surface receptor to an intracellular
effector, in this case adenylyl cyclase (see Section
3.7.5). Cholera toxin-catalysed ADP-ribosylation
inhibits the intrinsic GTPase activity of Gsα, resulting
in stabilization of an active GTP-bound subunit and
persistent activation of adenylyl cyclase. ADP-ribosyltransferase
activity of cholera toxin is enhanced
by ADP-ribosylation factor (ARF), a GTP-dependent
eukaryotic protein that functions in intracellular vesicular
transport (Moss & Vaughn, 1995).
Pertussis toxin ADP-ribosylates the α subunit of
the inhibitory G protein, Gi. The modifi ed G protein
uncouples from the receptor, thereby maintaining
the protein as its inactive heterotrimer. Because this
inhibitory G protein is inactivated, inhibition of
adenylyl cyclase is removed and the result is increased
cyclase activity.
post-translational modifi cation of proteins in which
the ADP-ribose moiety of NAD is transferred to
acceptor proteins, thereby altering their function.
ADP-ribosylation reactions are classifi ed into two
major groups: mono-ADP-ribosylation and poly-
ADP-ribosylation.
Mono-ADP-ribosylation by bacterial toxins
The transfer of ADP-ribose to the acceptor protein
(Fig. 13.5) is catalysed by ADP-ribosyltransferases,
which are found in the cytosol, plasma membrane and
nuclear envelope of eukaryotic cells. The ADP-ribose
reacts with specifi c amino acid residues on the acceptor
protein to form N-glycosides. Certain bacterial
toxins also possess ADP-ribosyltransferase activity
(Ueda & Hayaishi, 1985) and, since more is known
about them than eukaryotic ADP-ribosyltransferases,
they will be selected as examples.
Two bacterial exotoxins, diphtheria toxin and
Pseudomonas aeruginosa exotoxin A, prevent protein
synthesis in bacterially infected eukaryotic cells by
inactivating elongation factor 2, a protein required
for polypeptide chain elongation. The uncontrolled
action of these exotoxins results in death of the host
cells. A mammalian cellular ADP-ribosyltransferase
also inactivates elongation factor 2 (Iglewski, 1994),
but this is a controlled action required for normal
protein synthesis.
Cholera toxin and Escherichia coli heat-labile enterotoxin
ADP-ribosylate the α subunit of the stimulatory
G protein, Gs, which relays the signal from a
hormone-activated cell surface receptor to an intracellular
effector, in this case adenylyl cyclase (see Section
3.7.5). Cholera toxin-catalysed ADP-ribosylation
inhibits the intrinsic GTPase activity of Gsα, resulting
in stabilization of an active GTP-bound subunit and
persistent activation of adenylyl cyclase. ADP-ribosyltransferase
activity of cholera toxin is enhanced
by ADP-ribosylation factor (ARF), a GTP-dependent
eukaryotic protein that functions in intracellular vesicular
transport (Moss & Vaughn, 1995).
Pertussis toxin ADP-ribosylates the α subunit of
the inhibitory G protein, Gi. The modifi ed G protein
uncouples from the receptor, thereby maintaining
the protein as its inactive heterotrimer. Because this
inhibitory G protein is inactivated, inhibition of
adenylyl cyclase is removed and the result is increased
cyclase activity.
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