1256 PHYSIOLOGY 



protein diet. The two acids are generally found associated in the 

 urine. 



The presence of aceto-acetic acid may be detected as follows : 



(1) To some urine add ferric chloride as long as a precipitate of ferric phos- 

 phate continues to form. Filter this off and to the filtrate add a few more drops 

 of ferric chloride. If the acid be present a claret colour is produced. 



(2) On heating with dilute alkali, aceto-acetic acid is decomposed, with 

 the production of acetone. This may be detected by its odour or by distilling 

 off a small proportion of the fluid and testing the distillate in the following 

 ways : 



(a) On the addition of sodium hydrate and iodine and warming, iodoform 

 is formed. 



(6) Legal's test. A few drops of freshly precipitated sodium nitroprusside 

 solution is added and the mixture rendered alkaline with sodium hydrate. A 

 deep red colour is formed. On acidifying with acetic acid this colour is changed 

 to a reddish purple. 



CYSTINE. This substance, which is a normal product of the 

 hydrolysis of proteins, is found as a constant constituent to the amount 

 of half a gramme a day in the urine of certain individuals. The con- 

 dition of cystinuria represents, like alcaptonuria, an inborn error of 

 metabolism. It is found in the child and persists throughout life. 

 In such cases the cystine may give rise to urinary deposits or even to a 

 urinary calculus. 



HOMOGENTISIC ACID. This is an aromatic acid having the 

 composition of dioxyphenyl acetic acid. Its formula is as follows : 



OH 



icH 2 .COOH 



OH 



It occurs as a constituent of the urine of certain individuals, who 

 are said to be affected with alcaptonuria. The urine of these cases is 

 remarkable for its resistance to putrefactive changes. It slowly 

 darkens on exposure to the air, and on the addition of alkali and 

 shaking with air it becomes rapidly brown or black. It reduces 

 Fehling's solution, so that the presence of sugar may be suspected. 

 Such urine contains homogentisic acid in a quantity of 3 to 6 grm. per 

 day. The amount of the acid excreted varies with the protein food 

 taken. It seems that in these cases the power of the organism to 

 break up tyrosine and phenylalanine is entirely absent. If either of 

 these substances be administered by the mouth it is converted almost 

 quantitatively into homogentisic acid, which appears in the urine. 

 Individuals with alcaptonuria continue to secrete homogentisic acid 

 during starvation, so that the tyrosine and phenylalanine set free in 

 the course of tissue disintegration undergo the same fate as when they 



