The urinary excretion of free amino acids by the normal person is slight; urine amino nitrogen accounts for only about 1 to 2% of total urine nitrogen. Together with free amino acids, the so-called combined amino acids, which are converted to free amino acids by acid hydrolysis, are present in urine. The amino acids that are found in increased amounts after acid hydrolysis are mainly aspartic acid, glutamic acid, and glycine, which come from the hydrolysis of asparagine, glutamine, and hippuric acid. In addition, other amino acids are released from the small amounts of peptides present in the urine.
The free amino acids of plasma are ultrafiltrable, and their concentration in glomerular filtrate can be assumed to be the same as that in plasma water. If the quantity of amino acids estimated to be present in the total glomerular filtrate formed is compared with the free amino acids in urine,
Harrison HE, Harrison HC. AMINOACIDURIA IN RELATION TO DEFICIENCY DISEASES AND KIDNEY FUNCTION. JAMA. 1957;164(14):1571-1577. doi:10.1001/jama.1957.62980140006008