NPNs 2 Flashcards
Uric acid
product of nucleic acid catabolism ( filtered by glomerulus )
most uric acid is reabsorbed by the proximal tubules of the kidney & recycled
not very soluble in plasma
- at high concentrations it will deposit in joints & tissues
sources of uric acids
sources of uric acid in the body :
Breakdown of tissue nucleic acids ****
ingestion of nucleic acids
synthesis from glycine,NH3 & CO2
Most reabsorbed by proximal tubules
of the amount excreted :
75% is excreted by kidneys
25% degraded by bacterial enzymes in the GI tract
Catabolism of Nucleic acids
Purines from the diet & tissue breakdown are converted to uric acid
occurs primairly in th eliver **`
Uric acid biochemistry
Uric acid is transported in plasma from the liver to the kidneys
in the kidneys:
- filtered by glomerulus
- 98-100% of the filtrate id reabsorbed by the proximal tubules
- small amount is secreted by the distal tubules in to the urine( left over id degraded)
most uric acid in plasma is present as monosodium urate
at concentration s> 400µmol/L the plasma becomes saturated
- crystals can form & precipitate in tissue & joints
- crystals may form in acidic urine
Uric acid specimen requirements
serum, heparinized plasma, urine
stable for 3-5 days refrigerated ( once centrifuged)
to prevent crystals from precipitating out in acid urine: make alkaline ( adjust pH to 8)
false decreases
- high bilirubin ( peroxidase methods)
- hemolysis( due to glutathione release)
false increases certain drugs ( salicylates, thiazides)
uric acids - analytical methods
enzymatic methods
- uricase method
- coupled enzymatic method
chemical method
- Phosphotungstic acid (PTA) method
isotope dilution mass spectrometry **
- proposed reference methos
Uricase method
uric acid is oxidized to allantoin by uricase
uric acid absorbs light at 293nm ; allantoin doesn’t
- a decrease in absorbance is measured & is proportional to the uric acid concentration
can be preformed as an endpoint or a kinetic reaction
notes on uricase method
endpoint reactions must be allowed to go to completion ( cant read early)
inly guanine, xanthine & other structural anaolgs of uric acid will interfere ( but their quantities are usually low )- typically dont
Coupled uricase method
measure H2O2 produced as uric acid is converted to allantoin
in second step, peroxidase or catalase is used to catalyze a chemical indicator reaction
uricase uric acid+O2+H2O--> allantoin +CO2+H2O2
peroxidase H2O2+ 4-aminoantipyrine +DHB--> quinoneimine dye +H2O
the intensity of color produced is proportional to the amount of uric acid in sample
Interferneces in uricase methods
bilirubin & ascorbic acid interfere
- destroy peroxidase
Many commercial reagent preparations will include potassium ferricyanide & ascorbate oxidase to minimize interferences
Phosphotungstic acid method
AKA caraway method -> one of first methods used
uric acid is oxidized while phosphotungstic acid is reduced in an alkaline solution to tungsten blue
the intensity of blue colour is proportional to the concentration of uric acid in sample
protein causes turbidity, so protein free filtrate (PFF) should be used
Uric acid reference range
plasma/serum: 120-420µmol/L
males have higher uric acid levels than females
concentration of uric acid increases with age
Uric acid clinical significance
Hyperuricemia
- increased serum or plasma uric acid (>420µmol/L)
- due to increased formation or decreased
excretion
Gout
- increased uric acid in the blood can cause deposits of urate crystals in joints & tissue( esp. big toe joint ) painful inflammatory
- can be primary or secondary
Primary gout
cause usually unknown but could be caused by a combination of :
- over production or purines
- decreased renal excretion
- increased dietary intake
Defects of enzymes are rare casues:
Lesch-Nyhan syndrome(rare)
-deficiency of the major enzyme of purine salvage
pathways
Glucose-6- phosphatase deficiency
-leads to overproduction & under-excretion of uric
acid
Secondary gout
hyperuricemia due to other identifiable causes
- acute or chronic renal disease
- administration of drugs ( diuretics)
- diabetic acidosis/lactic acidosis
- interferences with tubular secretion of urate
