ABSTRACT
Another majo r syste m fo r removin g ammoni a fro m tissue s i s th e glutamin e
cycle. I t is activ e i n the muscle , an d i n the brain , th e astrocyt e cell s ar e especiall y
active i n glutamin e biosynthesis . Glutamin e i s synthesize d fro m glutamat e an d
N H 3 b y glutamin e synthetas e usin g A T P as a n energ y source :
Glutamate + A T P -> y-glutamylphosphat e + A D P (20.15 )
y-glutamyl phosphat e + N H 3 -^glutamine + P ; (20.16 )
A similar , althoug h les s prevalen t reactio n catalyze s th e conversio n o f asparti c
acid t o th e correspondin g amide , asparagine . Th e enzyme involve d i s asparagin e
synthetase. Glutamin e i s exporte d b y th e tissue s i n w h i c h i t i s forme d int o th e
bloodstream, fro m w h i c h i t i s extracte d b y th e kidne y an d intestina l mucosa l cells. Bot h tissue s ar e ric h i n glutaminase , w h i c h cause s th e hydrolysi s o f
glutamine t o glutamat e an d ammonia . Th e ammonia ma y pic k up a proton t o b e
converted t o th e ammoniu m ion , and , i f i n th e kidne y cells , i s excrete d i n th e
urine. T h u s , most , i f no t all , urinar y N H 4 + i s th e produc t o f th e glutaminas e reaction. Excretio n of th e ammoniu m io n provide s a mean s fo r th e organis m t o
excrete proton s an d t o spar e th e sodiu m an d potassiu m electrolytes .
