PROTEIN METABOLISM 483 



which is allributed to an accumulation of sodium urcatc in the 

 affected joints. 



Creatine and ornithine are both formed from arginine and glycine. 

 Creatine phosphate is imjjortant in muscle and appears in the urine 

 in small amounts. Creatinine is the normal urinary waste product 

 probably derived from creatine phosphate by the spontaneous irre- 

 versible process shown here: 



HN=C— NH— PO3H- HN=C NH 



1 -^11 +H,P04- 



H3C— N— CH2— COO- H3C— N— CHo— C=0 



creatine phosphate creatinine 



Although Table 20-3 reports a range of levels for the excretion of 

 creatinine, the value for a given individual is remarkably constant. 

 The actual level depends upon the quantity of muscle tissue possessed 

 by the individual but upon no other known factor, including muscular 

 activity. Hence each person or animal has a characteristic excretory 

 level which fluctuates very little in the absence of any condition that 

 causes muscle wastage. Starvation, diabetes, fever, hyperthyroidism, 

 and progressive muscular dystrophy lower the excretion of creatinine. 

 In such situations it is common to find creatine in amounts compen- 

 sating for the decrease in creatinine. 



HijijDuric acid appears in mine to the extent of about 0.7 g. daily. 

 It is formed during the detoxication of benzoic acid, which occurs in 

 fruits and berries and is widely used in foodstuffs as a preservative. 

 The reaction occurs in liver and is sufficiently characteristic to serve 

 as a test of liver function. 



^ )>— COOH + coenzyme A + ATP ^^ 



benzoic acid 



^ \— CO— CoA + ADP + H2PO4 



glycine 



coenzyme ^ + i )>— CONHCH.COOH 



hippuric acid 



Amino acids are also normal constituents of urine but are limited 

 in quantity (Table 20-3) and not all are found. Since these sub- 

 stances occur in plasma, presumably they may be dialyzed from the 

 blood and incompletely reabsorbed by the kidneys. Although little 



