① Defect of enzyme
The normal catabolism of one or more amino acids in the body is blocked, and the blood concentration of amino acids is obviously increased, resulting in hyperaminoacidemia. The level of amino acids in plasma is too high, which makes the amino acids filtered into renal tubules exceed the reabsorption capacity of renal tubules, and the level of amino acids in urine is too high, leading to hyperaminoaciduria. The nature and content of amino acids in blood or urine can be determined by chemical method, microbiological method, electrophoresis method and chromatography method.
② Defects of amino acid transport system
Rarely seen. Hereditary defects in amino acid transport system can cause some diseases with abnormal amino acid transport. People with renal tubular amino acid reabsorption dysfunction may have hyperaminoaciduria when the blood amino acid level is normal. Heat malnutrition in protein can reduce the concentration of tryptophan, leucine, isoleucine and valine in blood, but increase the concentration of tyrosine, glycine and proline. Long-term hunger can reduce the concentration of alanine and increase the concentration of threonine and glycine in blood. The concentration of leucine, isoleucine, valine, phenylalanine and tyrosine in the blood of obese people increased. Physical and chemical factors can also affect the membrane transport of amino acids, such as neomycin can hinder the absorption of amino acids in small intestine, and renal damage caused by X-ray irradiation, heavy metal poisoning and expired tetracycline can inhibit the absorption of amino acids in renal tubules.