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Uric Acid

• Uric acid is the end product of purine metabolism.
• Purines are common chemical compounds.
• Two types of purines:
  • Exogenous: absorbed from food.
  • Endogenous: produced by the body from simple precursors or breakdown of nucleoproteins.
• Purines like Adenine and Guanine are broken down into uric acid, mainly in the liver.
• Adenine and guanine are found in DNA, RNA, ATP, and ADP.

Uric Acid Transport and Excretion

• Uric acid is transported from the liver to the kidney.
• 98-100% of uric acid is reabsorbed in the proximal tubules of the kidney.
• Small amounts are secreted by the distal tubules into the urine.
• Most uric acid in the plasma is present as monosodium urate.
• Urate is relatively insoluble at the pH of plasma (pH 7).
• Renal excretion accounts for about 70% of uric acid elimination.
• The remaining uric acid passes into the gut and is broken down by bacterial enzymes into carbon dioxide and ammonia.

Uric Acid Homeostasis and Clinical Significance

• Uric acid homeostasis is balanced by production, renal excretion, and intestinal secretion.
• Kidney regulates uric acid levels.
• The rate of purine synthesis or nucleoprotein breakdown affects uric acid levels.
• Normal levels of uric acids vary between males and females.
• High or low uric acid levels can cause health problems.

Hyperuricemia

• Hyperuricemia is excess uric acid in the blood.
• If the body can't excrete uric acid through the kidneys, it builds up in the blood, tissues, and can form kidney stones or gout.
• Crystals formed by uric acid can cause inflammation and intense pain (gout attack)
• Elevated uric acid levels can lead to increased uric acid production, decreased excretion, or both.
• Kidney stone volume can be ≥ 1.6 mm³.

Causes of Hyperuricemia

• Impaired renal excretion: renal failure, drug therapy, alcohol, or ketoacidosis
• Increased nucleic acids turnover: leukemia or polycythemia
• Specific enzyme deficiency (e.g., HGPRT deficiency in Lesch-Nyhan syndrome)

Causes of Hypouricemia

• Decreased uric acid production
• Decreased renal tubular reabsorption (due to inherited or acquired disorders)
• Liver disease (like cirrhosis)
• Renal disease affecting tubular reabsorption
• Drugs like salicylates

Measurement of Uric acid

• Uricase converts uric acid into allantoin.
• Reaction also produces hydrogen peroxide and carbon dioxide
• Hydrogen peroxide reacts with chromogen to form a colored complex.
• Absorbance is measured at 520 nm.

Laboratory Procedure

• Specific amounts of reagents, standards and samples are mixed.
• Absorbance readings are taken.
• Formula are used to interpret and calculate uric acid concentration in samples.