Introduction
Renal aminoaciduria is a condition in which high levels of amino acids are present in the urine. The term aminoaciduria is used if more than five percent of the filtered load is present in the urine. Aminoaciduria may occur due to an inherited abnormality or an acquired abnormality that may be transient or become permanent. Various types of amino acids exist, and aminoaciduria is a common term that describes the excretion of one or more amino acids in greater quantities.
What Are Amino Acids?
Amino acids are organic molecules that unite to form proteins. Amino acids and proteins are essential for life because they form the building blocks of the human body. Amino acids are classified into essential, nonessential, and conditional amino acids. Essential amino acids cannot be produced in the body and are obtained from food.
Essential amino acids are histidine, leucine, lysine, isoleucine, methionine, tryptophan, threonine, phenylalanine, and valine. Nonessential amino acids can be produced in the body, namely arginine, alanine, aspartic acid, asparagine, cysteine, glutamic acid, glycine, glutamine, glutamic acid, proline, serine, and tyrosine. Conditional amino acids are those amino acids that are essential only during illness and stress. It includes arginine, cysteine, glutamine, tyrosine, glycine, ornithine, serine, and proline.
What Are the Types of Aminoaciduria?
Congenital disorders of amino acid metabolism are due to the impairment in the synthesis and degradation of amino acids. This includes the following:
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Alkaptonuria (an inherited disorder that prevents the body from breaking down two proteins namely tyrosine and phenylalanine).
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Aspartylglucosaminuria (a lysosomal storage disorder that causes skeletal and connective tissue abnormalities).
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Methylmalonic acidemia (a condition in which the body cannot breakdown certain proteins and fats).
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Maple syrup disease (a condition in which the body cannot breakdown certain amino acids).
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Tyrosinemia (a disorder in which the synthesis of tyrosine is disrupted).
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Hartnup disease (a condition in which the body is unable to absorb certain amino acids).
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Renal aminoaciduria is due to failure in tubular reabsorption of amino acids or secondary to liver diseases. Renal aminoaciduria includes cystinuria, Hartnup disease, and Fanconi syndrome.
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Overflow aminoaciduria is characterized by abnormally high levels of amino acids in the blood plasma exceed the resorptive capacity of the renal tubules, resulting in increased levels of amino acids in the urine. This form of aminoaciduria is called overflow aminoaciduria. This occurs in severe hepatic cell damage. Overflow aminoaciduria or secondary aminoaciduria can also be present in conditions in which hyperaminoacidemia occurs.
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Transient aminoaciduria is observed in the diuretic phase after acute renal insufficiency or due to a potassium deficiency. Longer aminoacidurias are seen in heavy metal poisoning with cadmium, uranium, lead, or mercury due to their effect on renal tubules. Aminoaciduria is also observed in Wilson’s disease due to the toxic effect of copper on the renal tubules. The affected renal tubules fail to resorb the amino acids.
What Is Renal Aminoaciduria?
Amino acids in the blood are filtered in the glomeruli of the normally functioning kidneys. Ninety-five percent of the filtered amino acids are reabsorbed by the renal tubules back into the blood. Renal aminoaciduria is a condition in which the renal tubules fail to reabsorb the filtered amino acids, resulting in increased concentrations of amino acids in the urine.
How Does Renal Aminoaciduria Occur?
Amino aciduria can be caused by congenital abnormalities of amino acid metabolism or can be secondary to liver disease. A defect in the transport present in the renal tubules can disrupt the reabsorption of the filtered amino acids in the renal tubules. It can also be due to damage in the renal tubules that interfere with reabsorption.
What Are the Types of Renal Aminoaciduria?
Renal aminoaciduria is seen in the following conditions:
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Fanconi’s Syndrome - It is characterized by inherited aminoaciduria. It presents with generalized aminoaciduria and renal aminoaciduria due to renal tubular defects that affect the reabsorption of amino acids. In Fanconi’s syndrome, the ability to break down cystine, fructose, galactose, and glycogen is affected. Fanconi’s syndrome is associated with symptoms like passing large amounts of urine, excessive thirst, bone pain, muscle weakness, and fracture due to weakness.
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Cystinuria - It is an autosomal recessive disorder in which the glomerulus fails to resorb cystine, ornithine, lysine, and arginine, which are excreted in the urine. Three types of cystinuria are presently based on the mode of inheritance and the pattern of tubular amino-acid transport. Cystinuria usually causes cystine stone formation.
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Hartnup Disease - Hartnup disease is an autosomal recessive disorder that is characterized by defective renal tubular transport. This causes renal amino aciduria. The small intestines fail to absorb tryptophan and other amino acids. They are converted by gut bacteria into compounds that are toxic to the central nervous system. Renal aminoaciduria and poor absorption from the gut result in protein malnutrition.
How Is Renal Aminoaciduria Diagnosed?
The diagnosis of renal aminoaciduria is based on urinalysis. A clear-catch urine sample is obtained and tested for increased levels of amino acids in urine. A clear-catch sample is performed by collecting the midstream urine. The urine sample is examined for increased levels of individual amino acids. Increased amino acids may be due to various amino acid disorders. Screening infants help in the early identification of aminoaciduria. Early treatment may help in the prevention of complications.
How Is Renal Aminoaciduria Treated?
Renal aminoaciduria can be managed by reducing the intake of particular amino acids in the diet. This helps the body to utilize the existing amino acids in a more efficient way and allows the kidneys to effectively clear the amino acids. Increased intake of liquids helps in reducing the concentration of amino acids, thereby allowing the kidneys to absorb the amino acids effectively. Medications like sodium bicarbonate and sodium citrate are given to alkalinize the urine. Altering the pH (potential of hydrogen) of urine improves the tubular reabsorption of amino acids and thereby allowing the excretion of amino acids.
Conclusion
Renal aminoaciduria is a condition in which excessive concentrations of amino acids are present in urine due to abnormalities in the renal tubules. Amino acids are required for various functions in the body. Excess excretion hampers essential functions and results in malnutrition and the buildup of toxic substances in the body. Hence, it is essential to diagnose the presence of amino acids in urine and treat them to prevent various complications. Renal aminoaciduria is usually an inherited abnormality. Screening babies for aminoaciduria help in the early management of amino acid metabolism disorders.
