Aminoacidurias

The term aminoaciduria is applied when more than 5% of the filtered load is detected in the urine.¹
Aminoacidurias may result from an inherited metabolic abnormality, in which case the aminoaciduria is a permanent finding, or it may be an acquired abnormality which may either be transient or become permanent.

Three distinct groups of inherited aminoacidurias are distinguished based on the net charge of the target amino acids at neutral pH: ²

• Acidic (negative charge)
• Basic (positive charge)
• Neutral (no charge)

Transient aminoacidurias may occur during the diuretic phase after acute renal insufficiency, or as a result of a deficiency of potassium.

Aminoacidurias of longer duration occur as a result of poisoning with heavy metals, particularly cadmium and uranium, but also with lead or mercury.

There is aminoaciduria found in patients with Wilson's disease; it is associated with the toxic effect on the renal tubule of copper, which characteristically accumulates in patients with this disease.

• Under normal circumstances, the renal tubules reabsorb in excess of 93% of the amino acids filtered from the plasma, influenced by the glomerular filtration rate. When the filtered load of amino acids is increased there is an increase in both the amounts reabsorbed and those excreted. However, the ability of the renal tubule to respond to an increased filtered load of amino acids is so great that a maximum rate of reabsorption has not been found in the human.
• In some instances the aminoaciduria is generalised; there is increased excretion of all of the amino acids occurring in the plasma. In other instances, the aminoaciduria is more specific in that there are increased amounts of some amino acids in the urine while all others are excreted in normal amounts.
• Secondary or 'overflow' aminoaciduria can also be seen in conditions in which there is hyperaminoacidaemia.

Major clinical problems are found in cystinuria and lysinuric protein intolerance:³

1. Fanconi syndrome is the most frequently studied inherited aminoaciduria. The syndrome is characterised by a generalised aminoaciduria and by other renal tubular defects affecting reabsorption of phosphate and glucose. Frequently the renal handling of potassium and water, as well as the secretion of hydrogen ions and the manufacture of ammonia is also affected. The aminoaciduria itself, although generalised, is minimal and of no metabolic consequence.
2. Cystinuria - see Cystinuria article.
3. Glycinuria - a rare inherited renal tubular defect producing oxalate stones.
4. Hartnup (H) disease - see Hartnup disease article.
5. Lysinuric Protein Intolerance (LPI) - see LPI article.