NPNs 2

Question 1

Uric acid

Answer 1

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

Question 2

sources of uric acids

Answer 2

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

Question 3

Catabolism of Nucleic acids

Answer 3

Purines from the diet & tissue breakdown are converted to uric acid

occurs primairly in th eliver **`

Question 4

Uric acid biochemistry

Answer 4

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

Question 5

Uric acid specimen requirements

Answer 5

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)

Question 6

uric acids - analytical methods

Answer 6

enzymatic methods

• uricase method
• coupled enzymatic method

chemical method
- Phosphotungstic acid (PTA) method

isotope dilution mass spectrometry **
- proposed reference methos

Question 7

Uricase method

Answer 7

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

Question 8

notes on uricase method

Answer 8

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

Question 9

Coupled uricase method

Answer 9

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

Question 10

Interferneces in uricase methods

Answer 10

bilirubin & ascorbic acid interfere
- destroy peroxidase

Many commercial reagent preparations will include potassium ferricyanide & ascorbate oxidase to minimize interferences

Question 11

Phosphotungstic acid method

Answer 11

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

Question 12

Uric acid reference range

Answer 12

plasma/serum: 120-420µmol/L

males have higher uric acid levels than females

concentration of uric acid increases with age

Question 13

Uric acid clinical significance

Answer 13

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

Question 14

Primary gout

Answer 14

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

Question 15

Secondary gout

Answer 15

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

Question 16

Ammonia

Answer 16

product of deamination of amino acids during protein metabolism ( removal of amino group)

mostly found as NH4+ions in plasma

free ammonia is toxic , but only low concentrations are present in plasma

the liver converts ammonia to urea, which is excreted in urine