ABSTRACT
This chapter focuses on the long-term regulation of glutamine catabolism within the liver through changes in the activities of phosphate-activated glutaminase and the urea cycle enzymes. It discusses the coordinate regulation of hepatic glutaminase with the urea cycle enzymes. There are two isozymes of glutaminase such as liver-type and kidney-type; the latter is sometimes referred to as brain-type in the literature. Both hepatic glutamine metabolism and the urea cycle are subject to regulation by both short-term and long-term mechanisms. Since the pathways of glutamine catabolism and urea synthesis within the liver are tightly regulated in the short-term, the capacity of these pathways is in excess under normal circumstances. Liver glutaminase is encoded by a mRNA of approximately 2.8 kb in length which is expressed only in postnatal liver. Evidence from experiments in vivo and in vitro indicate that glutaminase will have similar elements for the tissue-specific and hormonal regulation of expression.
