Keywords
Uric acid - metabolism - hyperuricemia - pathogenesis of disease - oxidative stress
- chronic inflammation - insulin resistance - essential hypertension - chronic kidney
disease
Introduction
Uric acid (UA/urate; 2,6,8-trihydroxy purine, C5H4N4O3; Mwt 168 dalton), which is
a metabolic end product, was first found in kidney stones in 1776[1] and later was identified in normal human urine and was given its current name.[2] However, some 750 years earlier, the disease UA is most associated with gout was
described in great detail by Avicennia (Ibn Sina; 980–1037 AD) in his famous encyclopedic
work, the Canon of Medicine, published in 1025 AD. In these volumes, Avicennia described
gout's clinical features (excruciating joint pain, redness, and swelling) and its
potential complications, including kidney stones. He also advised to help alleviate
these symptoms and hinted at the presence in the circulation of a toxic substance
as a potential causative agent.
Presently most clinical aspects of gout, including prevention and treatment, are pretty
well-defined. However, while our current knowledge about the impact of UA on human
health, in general, has expanded enormously in the past several decades, our understanding
of most aspects of its involvement in the pathophysiology of human diseases still
needs to be completed.
Uric Acid Metabolism
In humans, UA is the end-product of the oxidative breakdown of purine nucleotides.
There are two sources of purines: (1) senescent cells and the breakdown of their nucleic
acids; and (2)diet: purine-rich food, urate molecules, and dietary fructose. In mammals
except for primates, UA is further oxidized in the liver by UA oxidase (uricase),
converting it to allantoin. Unlike UA, allantoin is a nontoxic, highly soluble substance
easily eliminated in the urine. During evolution, most primates, including humans,
lost the ability to oxidize UA due to the loss of uricase activity. As a consequence,
UA became the terminal waste product of purine metabolism.
Purines (adenine, inosine, and guanine) are nucleic bases derived from the nucleotides
adenylate (AMP) and guanylate (GMP) breakdown and are produced endogenously and derived
from dietary sources. Red meat (beef, lamb, etc.) and organ meat (e.g., liver and
kidney) are particularly rich in purines, and so is seafood (tuna, sardines, shrimp,
etc.). Purines are also found in alcoholic drinks, especially beer. Vegetables (asparagus,
cauliflower, brussel sprout, etc.) contain much lower amounts of purines than animal
sources. Purines can also be generated in the course of fructose metabolism. Therefore,
fructose-rich food tends to promote endogenous UA production. Also, prolonged hyperglycemia
can, via the polyol pathway, stimulate endogenous fructose production and its subsequent
metabolism, increasing the production of purines and UA. Evidence is mounting that
the negative influence of fructose on metabolic health is due primarily to its ability
to cause ATP depletion, nucleotide turnover, and UA production.[3]
[4] The liver is responsible for most (>80%) of UA production. At the same time, the
small intestine makes a more minor but significant contribution ([Fig. 1]).
Fig. 1 Production and elimination of uric acid. The liver produces most of it, but the intestine
contributes significantly to its production (<20 %). The kidneys eliminate nearly
70% of the daily uric acid production. The intestine is responsible for eliminating
about 30%. In the intestine, uric acid is broken down by the resident bacteria (urinalysis).
The metabolic pathway from nucleotides to UA involves several enzymatic reactions
starting with nucleotidase and the conversion of the AMP and GMP to the nucleosides
adenosine and guanosine. A specific phosphorylase converts guanosine to guanine, while
adenosine must first be deaminated to inosine before it is converted to hypoxanthine
by nucleotide phosphorylase. The remaining two reactions are catalyzed by xanthine
oxidoreductase (XOR), which is made up of xanthine dehydrogenase (XDH) and xanthine
oxidase (XO). XDH converts hypoxanthine to xanthine, while XO catalyzes the final
step converting xanthine to UA ([Fig. 2]). XOR is expressed in the liver and the intestine. When released into the plasma,
it is converted to XO. The two drugs used to lower serum urate levels, allopurinol,
and febuxostat, are both inhibitors of OX.
Fig. 2 The purine nucleotide pathway: the sequence of enzymatic reactions breaking nucleotides
into uric acid. The key enzyme in this pathway is xanthine oxidase catalyzing the
final step. Here is a list of the reactions and the corresponding enzymes:A. From
Guanosine Triphosphate (GTP) to uric acid: 1. GTP to guanosine diphosphate (GDP):
Nucleoside Triphosphate Diphospho-hydrolase (NTPDase) 2. GDP to guanosine monophosphate
(GMP): Nucleoside Diphosphate Kinase (NDPK) 3. GMP to guanosine: 5′-nucleotidase 4.
Guanosine to guanine: Purine Nucleoside Phosphorylase (PNP) 5. Guanine to xanthine:
Guanine Deaminase (aka, guanase) 6. Xanthine to uric acid: Xanthine oxidaseB. From
Adenosine Triphosphate (ATP) to uric acid: 1. ATP to adenosine diphosphate (ADP):
Nucleoside Triphosphate Diphospho-hydrolase (NTPDase). 2. ADP to adenosine monophosphate
(AMP): Nucleoside Diphosphate Kinase (NDPK) 3. AMP to adenosine: 5′-nucleotidase 4.
Adenosine to inosine: Adenosine Deaminase (ADA) 5. Inosine to hypoxanthine: Purine
Nucleoside Phosphorylase (PNP) 6. Hypoxanthine to xanthine: Xanthine Dehydrogenase
(XDH) 7. Xanthine to uric acid: Xanthine Oxidase (XO)
The enzymatic activity of XOR is particularly elevated in the liver, intestine, and
vascular endothelium. Furthermore, the reactions catalyzed by the XOR enzymes are
accompanied by the production of two major reactive oxygen species (ROS), hydrogen
peroxide (H2O2), and the free radical superoxide anion (O2
•−). Thus, the potential for oxidative stress is particularly elevated in these tissues.
These observations lend support to the suggestion that oxidative stress and the accompanying
inflammation and mitochondrial and endothelial dysfunctions may explain the association
between elevated UA and several chronic conditions, including chronic inflammation,
metabolic syndrome, cardiovascular disease (CVD), hypertension (HTN), and chronic
kidney disease (CKD). These associations and related mechanisms will be expanded further
below.
The Renal Handling of Uric Acid
The Renal Handling of Uric Acid
UA is a weak acid with two dissociable protons. In other words, it is a diprotic acid
with two pKa values: at 37°C, pKa1 = 5.35, and pKa2 = 10.3. Consequently, at the normal pH of extracellular fluid (ECF) of 7.4, over
99% of UA in the ECF exists as the anion urate. This percentage drops dramatically
in the renal tubular fluid, especially in the distal tubules and collecting ducts,
as the medium becomes more acidic. As the urine pH falls, more filtered urate becomes
the undissociated form of UA, which is markedly less soluble in water than urate.
Therefore, the lower the urine pH, the greater the tendency for UA to precipitate,
forming kidney stones. UA begins to crystallize when the pH falls below 5.75. In plasma,
under physiological conditions (37°C; pH 7.4), the solubility of urate is several
times higher than that of the undissociated form of UA, but both solubilities are
relatively low (approximately 400 vs. 75 mg/dL). Thus, in the case of severe hyperuricemia
(serum levels > 7.5 mg/dL or >450 micromoles/L), both urate and UA tend to precipitate,
the first as urate crystals in the joints and the second mostly as kidney stones.
Gout typically emerges when the serum urate level exceeds 6.8 mg/dL (360 μmol/L),
and its prevalence rises with increases in serum urate above this threshold. Thus,
hyperuricemia is defined as serum urate concentration >6.8 mg/dL.
UA homeostasis depends on the balance between its production and elimination rates.
Its production (often referred to as load or burden) is dictated primarily by the
rate of purine catabolism, which may vary from one individual to another but is relatively
constant for each individual. Thus, UA balance depends primarily on its elimination
rate. The kidneys eliminate approximately two-thirds, while the gastrointestinal tract
eliminates one-third of the daily UA load. Under normal conditions (pH = 7.4; 37°C),
virtually all of the UA in the circulation exists as urate anions, and over 95% of
the circulating urate exists free (not bound to plasma proteins), and the remainder
(<5%) is bound almost exclusively to albumin.[5] Therefore, virtually all the urate in the plasma is freely filterable at the glomerulus
level. As the tubular fluid flows along the renal tubules, urate ions are transported
in both absorptive and secretory directions, with the balance determining the amount
that is ultimately excreted in the urine, thereby regulating its level in circulation.
The renal handling of urate is a complex process involving multiple transporters located
in the proximal tubules' luminal and antiluminal aspects. Regulation of these transporters
is critical for maintaining homeostasis and preventing the development of hyperuricemia
and associated disorders.
Given the relatively high pH in the ultrafiltrate, most of the UA in the proximal
tubule fluid exists as the urate anion, as is the case in plasma. Current evidence
indicates that most urate transport (both reabsorption and secretion) takes place
almost exclusively in the proximal tubule, with almost all of the reabsorption occurring
in the early (S1) segment and secretion in the more distal (S2) segment of the proximal
tubule. However, there is also the suggestion that both reabsorption and secretion
may co-occur in the same segment. The concept of postsecretory reabsorption occurring
in the late proximal tubule is becoming increasingly questionable for lack of convincing
evidence.[6] The net result of the activity of the various transporters is that only about 10%
of the filtered urate ends up in the urine, with the remaining 90% reabsorbed back
into the circulation.
Urate transport in the proximal tubule is best described as a tertiary active transport
process driven ultimately by the active reabsorption of Na+. The process begins with the Na+/K+-ATPase activity in the basolateral membrane generating the Na+ gradient across the cell membrane (primary active transport). This gradient drives
many Na+-coupled organic anion transporters (secondary active transport) in the apical (aka
luminal or brush-border) membrane. These transporters, in turn, provide the driving
force for urate reabsorption. The primary transporter responsible for translocating
urate anions across the apical membrane (lumen ⇨ cell interior) is the urate-anion
exchanger (URAT1), which mediates the exchange of urate for organic monocarboxylate
anions such as lactate. On the basolateral side, GLUT9 is the primary transporter
responsible for transferring the urate anions from the cell interior to the peritubular
fluid (ICF ⇨ ISF). BCRP (encoded by ABCG2), an essential apical transporter, carries
the urate anion in a secretory direction (ICF ⇨ lumen). Genetic variations can affect
the transport activity of BCRP and, consequently, urate levels in the body fluids.
Specific variants of the BCRP gene have been associated with an increased risk of
gout, while others have been linked to a reduced risk.[7] At least half a dozen additional transport proteins have been identified, including
OAT4 and OAT10, but they appear to play a minor role in urate homeostasis. Rare genetic
mutations that result in the inactivation of URAT1 can lead to a dramatic drop in
renal urate reabsorption and a corresponding rise in its fractional excretion (from
the normal level of about 10% to as high as 90%), resulting in marked hypo-uricemia
(serum urate ⪯ 1 mg/dL). GLUT9 (encoded by SLC2A9) is a member of a large and widely
distributed family of proteins dedicated to transporting hexoses such as glucose and
fructose.[8] It is expressed in many tissues but mainly in the kidney, liver, and intestine.
Despite its name, GLUT9 is not known to participate significantly in glucose transport.
It is almost exclusively dedicated to transporting urate anions and fructose to a
lesser extent. Loss-of-function mutation in SLC2A9 leads to a marked drop in urate
reabsorption, a marked increase in its secretion, and a dramatic drop in serum level,
as observed in some cases of familial hypo-uricemia.[9]
The Role of Uric Acid in the Pathogenesis of Disease
The Role of Uric Acid in the Pathogenesis of Disease
The functional significance of UA beyond its role in purine catabolism has been debated
for decades and remains uncertain. However, the association between hyperuricemia
and human diseases has been recognized since the 1800s.[2]
The normal range of serum UA (or urate) level in adults can vary slightly depending
on the laboratory and the method used for the measurement. However, the generally
accepted normal range in adult males is 3.4 to 7.2 mg/dL, while for adult females,
2.4 to 6.0 mg/dL. It is important to note that, like glucose, UA levels can fluctuate
throughout the day, influenced by diet, medication, and medical conditions. The review
defines relative hyperuricemia as a serum UA level >6.4 mg/dL and severe hyperuricemia
as a UA level >7.0 mg/dL.
Elevated serum UA level is commonly observed in patients with metabolic syndrome.
It is widely accepted as a significant risk factor for gout, kidney stones, HTN, non-alcoholic
fatty liver disease (NAFLD), CKD, and CVD.
From a pure chemical standpoint, at least in the extracellular environment, UA may
be regarded as an antioxidant. At relatively low levels (<6 mg/dL), UA is thought
to exert a protective antioxidant effect, particularly in the plasma and interstitial
fluid. It is believed to account for nearly half the antioxidant capacity in the plasma.
This property has given rise to an evolutionary perspective. It is believed that primates,
including homo sapiens, had gradually lost the ability to break down UA due to a series
of genetic mutations that ultimately led to the complete loss of uricase (UA oxidase)
activity. The uricase mutations occurred after and perhaps as a compensatory adaptation
to the loss of another key enzyme, L-gluconolactone oxidase, that is responsible for
the ability to synthesize ascorbic acid (vitamin C) endogenously, a potent antioxidant.[10]
[11] Because UA is a significantly weaker antioxidant than vitamin C, much higher concentrations
of UA would be required to compensate for the absence of endogenous vitamin C. This
was achieved by losing uricase activity and the resultant buildup of UA levels in
body fluids. Although this narrative sounds plausible, it still needs more definitive
evidence.
Further, it is now well established that under certain conditions in the intracellular
environment, UA acts as a prooxidant agent.[12] Thus, UA is best thought of as a redox agent, acting as an antioxidant under certain
conditions and as prooxidant under a different set of conditions. A rise in serum
UA level is often observed together with oxidative stress and chronic low-grade inflammation,
conditions that are, in turn, linked to multiple chronic diseases other than gout
and nephrolithiasis. These include CVD, HTN, CKD, and most features of the metabolic
syndrome (obesity, insulin resistance, etc.)[13]
[14] The association of hyperuricemia with these disorders has been documented in both
children and adults.
Uric Acid and Oxidative Stress
Uric Acid and Oxidative Stress
Oxidative stress occurs when the production of ROS overwhelms the body's antioxidant
defenses, leading to an excess of ROS that can cause damage to cellular components
such as DNA, proteins, and lipids. ROS are highly reactive molecules generated during
normal metabolic processes but can also be produced in response to environmental stressors
such as radiation, toxins, or infections.[15]
[16] ROS are highly unstable, powerful oxidizing agents. They can, over time, damage
tissues, causing inflammation, cell dysfunction or cell death, and disease. UA increases
ROS production and, at the same time, limits the body's antioxidant defenses. O2
•− is produced during purine metabolism and UA production. It is also formed through
an autooxidative process when UA is exposed to XO. While XDH uses NAD+ to oxidize substrates producing NADH, XO oxidizes substrates using O2 and producing
H2O2 plus the O2
•−. The activity of XO is enhanced by NADPH oxidase (NOX), a powerful ROS-generating
complex of multiple enzymes that catalyze the transfer of electrons from NADPH to
oxygen, producing O2
•−:
NADPH + 2 O2 → NADP+ + 2 O2
•− + H+
The NOX enzymes can also catalyze the dismutation of the O2
•−, generating H2O2: 2 O2
•− + 2 H+ → H2O2 + O2. The H2O2 thus generated can react with the ferrous ion (Fe++) to produce other ROS such as the hydroxyl radicals (OH•):
H2O2 + Fe++ → OH• + OH− + Fe+++. Both O2
•− and H2O2 are powerful oxidizing agents produced in several hot spots in the cell, including
the mitochondria and the endoplasmic reticulum (ER). ROS and associated oxidative
stress can damage lysosomes impairing their function (the breakdown of cellular waste
and recycling of cellular components), resulting in the accumulation of cellular debris.
In the endothelial cells of blood vessels, XO is the predominant form of XOR, and
through the production of ROS, it is thought to promote inflammation and the formation
of atherosclerotic plaques.[17] UA increases mitochondrial ROS production and oxidative stress, which can promote
mitochondrial dysfunction and can also interfere with protein folding in the ER triggering
the unfolded protein response.[18] Besides promoting ROS production, UA impairs the body's antioxidant defenses by
inhibiting superoxide dismutase and glutathione peroxidase. Also, UA tends to stimulate
the production of specific cytokines, such as interleukin-1 (IL-1) and tumor necrosis
factor-alpha (TNFα), that activate inflammatory pathways, further promoting ROS generation
and developing inflammatory diseases. Further, UA promotes hepatic fat accumulation
via the ROS/JNK/AP-1 pathway,[19] leading to NAFLD and metabolic syndrome.
Uric Acid and Inflammation
Uric Acid and Inflammation
The exact mechanism by which UA causes inflammation has yet to be understood entirely.
However, when the hyperuricemia is severe enough to trigger gout and/or kidney stones,
urate crystals activate the immune system and trigger the release of proinflammatory
cytokines, such as IL-1β, IL-6, and TNF-α. These cytokines promote the influx of immune
cells like neutrophils and macrophages to the site of tissue injury, triggering an
inflammatory response. The NLRP3 inflammasome, which stimulates the production of
pro-inflammatory cytokines, is thought to mediate the activation of the immune system.
UA can bind to and activate the NLRP3 inflammasome, leading to the release of cytokines,
which then trigger inflammation.[20]
[21] More recent findings suggest that the activation of the NLRP3 inflammasome may be
triggered at levels of UA well below the threshold of hyperuricemia.
UA can also trigger inflammation by activating the toll-like receptors (TLRs), protein
macromolecules that play a vital role in the immune system's response to pathogens.
However, endogenous ligands can also activate these receptors, such as UA.[22]
[23] Activation of TLRs can lead to the production of proinflammatory cytokines and the
recruitment of immune cells to the site of inflammation.
In addition, a significant bridge between UA and inflammation is provided by oxygen
radicals. The ROS produced during UA metabolism can trigger several signaling pathways
promoting inflammation in various body parts, thereby promoting the development of
inflammatory disorders, including CVD and metabolic syndrome.
Uric Acid and Insulin Resistance
Uric Acid and Insulin Resistance
Chronically elevated UA level is linked to obesity and metabolic syndrome, a cluster
of biochemical and clinical abnormalities including abdominal obesity, HTN, elevated
triglyceride level, low high-density lipoprotein, and elevated fasting blood glucose
level.[24]
[25]
[26] It is mainly associated with insulin resistance, a condition in which the cells
do not respond adequately to insulin, leading to persistently high blood glucose levels.
Thus, with environmental exposure, individual lifestyle habits, and genetic predisposition,
elevated UA is considered a significant risk factor for developing obesity, fatty
liver, insulin resistance, and type 2 diabetes (T2D). While the exact mechanism by
which UA contributes to the development of insulin resistance is not entirely understood,
several possible pathways have been proposed, including inflammation, oxidative stress,
and endothelial dysfunction. Also, UA can directly interfere with the insulin receptor
and downstream signaling molecules, leading to impaired glucose uptake and metabolism.
The inflammatory response triggered by relative hyperuricemia can impair insulin signaling.
Oxidative stress caused by ROS can cause oxidative damage to cellular components,
including proteins, impairing the insulin signaling pathway, and contributing to insulin
resistance. Endothelial dysfunction can cause a drop in nitric oxide (NO) production
reducing blood flow to insulin-sensitive tissues, further exacerbating insulin resistance.[26]
Uric Acid and Essential (Primary) Hypertension
Uric Acid and Essential (Primary) Hypertension
Studies in humans have documented elevated serum UA levels years before the onset
of HTN, obesity, T2D, and CKD.[4] Pilot studies under various conditions showed improvement in blood pressure following
reductions in serum urate levels.[24] A recent study in a Chinese hospitalized population demonstrated a dose–response
relationship between serum urate level and HTN.[27] Cross-sectional studies and clinical trials in children with essential HTN showed
a close association between elevated UA and new-onset essential HTN. Further, lowering
UA levels with allopurinol or febuxostat appears to lower BP, at least in some patients.[28]
Several mechanisms through which UA may induce HTN have been proposed mainly based
on laboratory studies involving mostly rodents or isolated cells. (1) Endothelial
dysfunction: elevated serum urate can interfere with NO generation reducing its bioavailability.
This may increase oxidative stress, vascular endothelial cell dysfunction, vasoconstriction,
and elevated blood pressure.[29]
[30] (2) Renal vasoconstriction: UA may directly cause renal vasoconstriction, reducing
renal blood flow and contributing to the activation of the renin-angiotensin-aldosterone
system (RAAS). This can lead to increased sodium retention, ECF volume expansion,
and ultimately, HTN.[31](3) Activation of the renin-angiotensin system: UA may directly stimulate the production
of renin and angiotensin II, which can lead to renal vasoconstriction, sodium retention,
and HTN.[32] (4) Inflammation: the inflammation associated with high serum urate can lead to
vascular damage, arterial stiffness, and, ultimately, HTN.[33]
Uric Acid and Chronic Kidney Disease
Uric Acid and Chronic Kidney Disease
CKD is when the kidneys gradually lose function over time. CKD is a common and serious
health problem affecting millions of people worldwide. Many factors can contribute
to the development of CKD, including high blood pressure, diabetes, and other medical
conditions. Recent studies have suggested that UA may also play a role in the development
and progression of CKD.[34]
[35] Like the case of HTN, several mechanisms are thought to mediate the role of UA in
the progression of CKD. Elevated serum urate level can trigger an inflammatory response
in renal tissue leading to the release of proinflammatory cytokines and chemokines,
causing renal injury and promoting fibrosis. UA-induced tubular damage and interstitial
fibrosis can lead to a decline in kidney function. This process is associated with
activating the renin-angiotensin system and increased expression of inflammatory markers.
While the inflammatory response is easily triggered by the tissue injury caused by
the accumulation of urate crystals, there are also crystal-independent mechanisms,
such as activation of the renin-angiotensin system, increased oxidative stress, and
endothelial dysfunction. In addition to its effects on the tubules, UA may also contribute
to CKD through its effects on the blood vessels in the kidneys. UA has been shown
to impair endothelial function and promote oxidative stress, which can contribute
to the development of atherosclerosis and other vascular diseases. These processes
can lead to vasoconstriction of the renal blood vessels, glomerular HTN, reduced renal
blood flow, and a decline in kidney function. Ultimately this leads to the kidney's
inability to process the ultrafiltrate, regulate the composition of body fluids, and
excrete waste products, leading to a buildup of toxins in the body and the development
of CKD. Finally, some studies have suggested that the main contribution of UA to the
development and progression of CKD is through its effects on inflammation and fibrosis,
which are processes involved in the development of kidney damage. As stated above,
UA has been shown to promote the production of pro-inflammatory cytokines and the
activation of the RAAS, both of which can contribute to renal inflammation and fibrosis.
Inhibiting XOR activity with allopurinol and febuxostat can reduce UA production and
ROS generation, slowing CKD progression and improving kidney function.[36]
[37]
Conclusions
UA is no longer regarded solely as the culprit behind the excruciating pains of gout
and kidney stones. Due to its ability to induce oxidative stress and trigger inflammation,
it is now being investigated as a potentially critical risk factor for several chronic
diseases, including obesity, insulin resistance, T2D, NAFLD, HTN, CKD, and CVD. The
kidney is responsible for excreting over 70% of the daily load of UA. The remainder
is secreted into the intestine, where the gut bacteria degrade it. Virtually all urate
reabsorption occurs in the early segment of the proximal tubule (S1), and URAT1 and
GLUT9 are the principal transport proteins responsible for their reabsorption. Some
secretory activity also occurs in the proximal tubules, carried out by the BCRP transporter
in the apical membrane.
Typically, the net result of the various transport activities is that the amount of
UA excreted in the urine is equivalent to about 10% of the filtered load. UA transporters
are subject to genetic variations, which can affect their circulating level and may
lead to reduced elimination and the development of diseases, including gout. Drugs
are currently being used to treat symptomatic and asymptomatic hyperuricemia to reduce
the burden of chronic diseases. Most of these treatments are still experimental, and
further clinical trials are needed. Rare mutations can also lead to enhanced elimination
via the kidneys and/or the small intestine and the development of hypouricemia.
