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3 Urea Cycle in Fish: Molecular and Mitochondrial Studies

1995; Academic Press; Linguagem: Inglês

10.1016/s1546-5098(08)60242-3

1557-8011

Paul M. Anderson,

Virus-based gene therapy research

Publisher Summary This chapter discusses the molecular and mitochondrial studies of the urea cycle in fish. A significant proportion of energy production in fish involves the catabolism and oxidation of proteins and amino acids. Consistent with their water habitat, the major end product of nitrogen metabolism in most fish is ammonia. Carbamoyl phosphate is the precursor for two major metabolic pathways: the urea cycle and pyrimidine nucleotide biosynthesis. The first step of the urea cycle in mammalian and amphibian ureotelic species is catalyzed by carbamoyl-phosphate synthetase I. The function of CPSase II is to catalyze carbamoyl phosphate formation as the first step in pyrimidine nucleotide biosynthesis. The utilization of the amide group of glutamine for the biosynthesis of carbamoyl phosphate in the glutamine-dependent CPSases involves the reaction of glutamine with a cysteine SH group on the enzyme to form a γ-glutamyl thioester intermediate, releasing ammonia, which reacts with an activated intermediate common to all CPSase. The unique co-functioning of glutamine synthetase and CPSase III in ammonia assimilation in the mitochondrial matrix probably reflects the adaptation of urea synthesis for the dual role of ureoosmotic and ureotelic functions.