ABSTRACT

N H 3 i s th e actua l substrat e fo r th e firs t reactio n i n th e ure a cycle . T h e overal l

process require s an energ y equivalen t o f fou r A T P per molecul e o f ure a formed .

The firs t tw o reaction s i n th e ure a cycl e (Figur e 20.9 ) tak e plac e i n th e mito -

chondria, an d th e res t take s plac e i n th e cytosol . L ive r i s th e onl y orga n tha t

contains al l the ure a cycle enzyme s i n sufficien t quantit y t o generat e substantia l

quantities o f urea . However , othe r organ s ma y hav e individua l ure a cycl e en -

zymes , s o ther e i s a n extensive traffi c of ure a cycle intermediate s fro m on e orga n

to another .