Biochemistry I - Lecture 6
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Transcript of Biochemistry I - Lecture 6
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Biochemistry I
Lesson 5Lesson 5
Gout and Routine Urinalysis
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Uric Acid
Is the final breakdown product of purine metabolism
Purines (e.g. adenosine and guanine) result from breakdown of nucleic acidsfrom breakdown of nucleic acids
They are either ingested or come from the destruction of tissue cells - converted into uric acid by liver
From liver, uric acid are transported to kidney where it is filtered by glomeruli
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Uric Acid
Nearly all filtered uric acid is reabsorbed in the proximal tubules, small amounts are then secreted by distal tubules and finally ending up in urine
Uric acid in plasma is normally in the form of monosodium urate
Urates in this form are relatively insoluble
At high levels (>6.4 mg/dL), plasma is saturated, urate crystals may form and precipitate in the tissues
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Purine Metabolism and Uric Acid
Purines are simple cyclic organic molecules
containing nitrogen - are essential
components of nucleic acids (involved in
energy transformation and phosphorylation energy transformation and phosphorylation
reaction and acts as intracellular messengers)
3 sources of purines in human : diet,
degradation of endogenous nucleotide and de
novo (new) synthesis
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Purine Metabolism and Uric Acid
Since purines are metabolised to uric acid, the body urate pool (and hence plasma concentration) depends on the rate of urate formation and urate excretion
Urate is excreted by kidney ( of the total) and alimentary canal
Urate secreted into alimentary canal is metabolised to CO2 and ammonia by bacterial action (uricolysis)
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Urate Handling by the Kidney
It is filtered at the glomeruli and almost totally reabsorbed in the proximal convoluted tubules
Distally, both secretion and reabsorption Distally, both secretion and reabsorption occur
Normal urate clearance is about 10% of the filtered load
Urate excretion increases if the filtered load is increased
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Urate Handling by the Kidney
In chronic renal failure, the plasma
concentration rises only when the glomerular
filtration rate falls below about 20 ml/min
Dietary purines makes up about 30% of Dietary purines makes up about 30% of
excreted urate
The introduction of a purine-free diet typically
reduces plasma urate concentration by only
10 - 20%
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Metabolic Pathways of Uric Acid Synthesis
De novo synthesis leads to the formation of
inosine monophosphate (IMP), which can be
converted to the nucleotides adenosine
monophosphate (AMP) and guanosine monophosphate (AMP) and guanosine
monophosphate
Nucleotide degradation involves formation of
respective nucleosides (inosine, adenosine
and guanosine), these are then metabolised to
purines
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Metabolic Pathways of Uric Acid Synthesis
Purine derived from IMP is hypoxanthine,
which is then converted by the enzyme
xanthine oxidase first to xanthine and then to
uric aciduric acid
Guanine can be metabolised to xanthine (and
then to uric acid) directly but adenine cannot
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Metabolic Pathways of Uric Acid Synthesis
However, AMP can be converted to IMP by the
enzyme AMP deaminase, and at the
nucleoside level, adenosine can be converted
to inosineto inosine
Thus, surplus GMP and AMP can be converted
to uric acid and excreted
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Plasma Urate Concentration
Plasma urate concentration are generally
higher in men than in women
Marked increase occur at puberty in males
(there is lesser increase in females at this (there is lesser increase in females at this
time) and peri-menopausal (before
menopause) women
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Gout
Acute gout = severe joint
pain of rapid onset
associated with swelling and
rednessredness
Risk of gout increases with
increasing plasma urate
concentration
At any particular urate
concentration, the risk is
similar in males and females
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Gout
Gout can be precipitated by a sudden change
(either increase or decrease) in urate
concentration
When urate concentration has fallen rapidly in When urate concentration has fallen rapidly in
a hyperuricaemic individual, the plasma urate
concentration may not be elevated when the
patient presents with gout
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Classification of Gout
Gout customarily defined as:
1. Primary (idiopathic, of unknown cause)
2. Secondary (when a condition known to cause
hyperuricaemia is present)hyperuricaemia is present)
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1. Primary Gout
Characterised by recurrent attacks of
monoarticular arthritis
It is likely that supersaturation of urate (>0.42
mmol/L) occur in patients with mmol/L) occur in patients with
hyperuricaemia
This cause crystals to accumulate in
connective tissues over a long period prior to
the development of symptoms
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1. Primary Gout
Most patients have impaired renal fractional
excretion of urate (an inappropriately low
urinary urate output if raised plasma urate is
taken into consideration)taken into consideration)
Patients with primary gout often show
deposition of urate as tophi (deposit of urates
in the skin and tissue around a joint) in soft
tissue
Some develop renal stones (from uric acid)
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1. Primary Gout
A genetic component to primary gout appears
to be present but the precise genetic basis in
unclear
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Diagnosis of Primary Gout
Diagnosis is often made clinically on the basis of:
Distribution of the joint involvement
A past history of similar episodes
Presence of a raised plasma urate Presence of a raised plasma urate
Note that not all cases are typical clinically and
must remember that:
A high plasma urate makes the diagnosis probable
but not certain
A small minority of patients with gout have a normal
plasma urate at the time of attack
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Diagnosis of Primary Gout
For definitive diagnosis of gout, it may be
necessary to aspirate joint fluid during an
acute attack
The finding of needle-shaped urate crystals The finding of needle-shaped urate crystals
when examined microscopically establishes
the diagnosis
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Pathogenesis of Gout
Acute symptoms of gout - due to trauma or
local metabolic changes
This cause crystals of monosodium urate to
shed into the joints cavityshed into the joints cavity
The crystals are phagocytosed by leukocytes
and macrophages - cause damage to
membranes within the leukocytes
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Pathogenesis of Gout
Lysosomal contents and other mediators of the
acute inflammatory response (cytokines,
prostaglandins, free radicals, etc.) are released,
causing both systemic and local acute formation
of goutof gout
The solubility of monosodium urate declines
rapidly with decreasing temperature and this
may explain the tendency of more peripheral
joints (hands, feet and knees) to be affected
Reason: those joints have lower intra-articular
temperatures
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Pathogenesis of Gout
The progress of gout may be over many years
Patients may have asymptomatic
hyperuricaemia for many years before the first
acute attackacute attack
This may be followed by periods which are
symptom-free
Then it will lead to frequent acute attacks
If unchecked, will progress to chronic
tophaceous gout
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Treatment of Primary Gout
Anti-inflammatory drugs are normally
prescribed (e.g. indomethacin)
Colchicine is also effective - has prophylactic
(preventive and protective) effect(preventive and protective) effect
Long-term treatment = reduce plasma urate
Avoid factors which increases plasma urate
(e.g. high-protein diet, alcohol and certain
drugs)
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Treatment of Primary Gout
Weight reduction, uricosuric (promoting the
excretion of uric acid in the urine) drugs and
inhibitors of urate synthesis may be required
Because acute changes in plasma urate Because acute changes in plasma urate
(increase or decrease) can increase attacks of
gout, drugs mentioned above should be
avoided within several weeks of an acute
attack
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2. Secondary Gout
Hyperuricaemia may occur as a complication
of several disorders, all of which affect either
urate production or excretion, or both
These conditions, though commonly cause These conditions, though commonly cause
hyperuricaemia, are only uncommonly
associated with formation of gout in joints
They include:
Overproduction
Defective elimination of urate
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Overproduction of Urate
Myeloproliferative
Polycythaemia rubra vera is the most common of
these disorders, may be associated with signs of
goutgout
This is due to increased turnover of red cell
precursors causing hyperuricaemia
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Overproduction of Urate
Cytotoxic Drug Therapy
Increased rates of cell turnover cause
hyperuricaemia
Renal failure may occur due to deposition of urate Renal failure may occur due to deposition of urate
crystals in the collecting ducts and ureters
Maintenance of a high fluid intake and prophylaxis
with inhibitors of urate synthesis can prevent this
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Overproduction of Urate
Psoriasis
Hyperuricaemia is thought to be due to an
increased rate of cell turnover in the skin
Hypercatabolic states and starvation Hypercatabolic states and starvation
There may be both an increased rate of cell
destruction and impaired urate excretion due to
an associated acidosis (a blood condition in which
the bicarbonate concentration is below normal)
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Defective Elimination of Urate
Chronic Renal Disease
Plasma urate rises in uraemia due to reduced GFR,
but clinical gout is very unusual
Diuretic Therapy Diuretic Therapy
Most effective diuretics (increasing the volume of
the urine excreted) cause hyperuricaemia by
reducing distal tubular secretion of urate
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Defective Elimination of Urate
Inherited metabolic disorders
Those that are associated with lactic acidosis,
often cause hyperuricaemia
Hypertension and ischaemic (local deficiency Hypertension and ischaemic (local deficiency
of blood supply produced by vasoconstriction
or local obstacles to the arterial flow) heart
disease
Associated with hyperuricaemia for a variety of
reasons, e.g. obesity and drug treatment
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Assay Method Uric Acid -
Colourimetric Method
Uric acid is converted by uricase to allantoin
and hydrogen peroxide
Under the catalytic influence of peroxidase, Under the catalytic influence of peroxidase,
hydrogen peroxide oxidizes 3,5-dichloro-2-
hydroxybenzenesulfonic acid and 4-
aminophenazone to form a red-violet
quinoneimine compound
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Principle
Uric acid + O2 + 2H2O uricase Allantoin + CO2 + H2O2
2H O + 3,5-dichloro-2-hydroxybenzenesulfonic acid 2H2O2+ 3,5-dichloro-2-hydroxybenzenesulfonic acid
+ 4-aminophenazone peroxidase N-(4-antipyryl)-3-
chloro-5-sulfonate-p-benzo-quinoneimine
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Samples for Uric Acid Test
It is best to use human serum for this test
Besides, other samples that can be used
includes:
Heparinised plasma Heparinised plasma
EDTA-plasma
Urine
When urine is used, it needs to be diluted
1+10 with distilled water
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Calculation of Uric Acid Concentration
Serum or plasma
Uric acid conc. = x conc. of standardA sampleAstandard
Urine
Uric acid conc. = x conc. of standard x 11
Astandard
A sampleAstandard
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Interference
Haemoglobin up to 100 mg/dL and bilirubin
up to 20 mg/dL DO NOT affect the
determination by using this method
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Normal Values
For serum:
mol/L mg/mL
Men 202 - 416 3.4 - 7.0
Women 142 - 339 2.4 - 5.7
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Normal Values
FOR URINE:
24-hour urine sample must be used
Reference ranges - same for both gender Reference ranges - same for both gender
1.5 - 4.5 mmol / 24-hr
250 - 750 mg / 24-hr