310 UNITY AND DIVERSITY IN BIOCHEMISTRY 



arginosuccinic acid which is converted into arginine and fumaric acid in the 

 presence of another enzyme system 



H 

 H„N— C— N— CHa— CH2— CH2— CH— COOH HOOC— CH^— CH— COOH 



II I + I 



O NHa NH, 



citruUine aspartic acid 



enzyme H 



>HN= C N— CHa— CH2— CH2— CH— COOH 



Mg++,ATP 1 I I 



HN— H— COOH NHa 



C 



I 

 CHa 



I 

 COOH 



arginosuccinic acid 

 H2O enzyme 



H 

 H,N— C—N—CH2—CH2—CH2— CH— COOH H-HOOC—CH=CH— COOH 



II i 



NH NH2 



arginine fumaric acid 



The arginine is hydrolysed to urea and ornithine in the presence of arginase 



H 

 H,N— C— N— CHj— CH2— CH2— CH— COOH 



11 I 



NH NHa 



arginine 



NHj 



arginase / 



>0=C +NH2— CH2— CHjj— CH2— CH— COOH 



\ I 



NH3 NH2 



ornithine 



The fumaric acid passes into the tricarboxyHc acid cycle or in the presence 

 of fumarase it is transformed to malic acid w^hich also enters the Krebs 

 cycle. 



In mammalian liver parenchyma cells, the synthesis of the steroid 

 skeleton which was described on p. 236 is extended by acquisition of an 

 enzyme system leading to the production of the substances known as bile 

 acids. In man, this synthesis yields cholic acid, desoxycholic acid and 

 chenodesoxycholic acid. The hepatic parenchyma in mammals also con- 

 jugates these bile acids with taurine and with glycine. 



The pathway from cholesterol to cholic acid implies that there are a 

 number of stages and a number of enzymes involved. Another character- 

 istic of the liver cell is its ability to bring about detoxication reactions, in 

 man the most important being conjugation with glucuronic acid. Liver 

 contains UDP-glucuronate and an enzyme which catalyses its formation 

 fronri glucuronic acid. This UDP-glucuronate appears to be one of the 

 substances involved in the biosynthesis of mucopolysaccharides. In the 



