Glutamine as a precursor of urea: The role of the liver and the small intestine

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Abstract

Liver possesses a unique isozyme of phosphate activated glutaminase which shows 86% similarity in amino acid sequence to the glutaminase expressed in other tissues. Expression of hepatic enzyme is only seen after birth and is restricted to a small population of periportal hepatocytes. In contrast, hepatic glutamine synthetase expression is limited to a very small population of perivenous hepatocytes. Transcription of the hepatic glutaminase gene is increased during diabetes, starvation and high protein feeding, and is decreased with low protein diets. The region -1022 to +48 upstream of the hepatic glutaminase gene contains a functional promoter whose activity is increased 10-fold by glucocorticoids and decreased by ammonium chloride. Responsive elementsmapped to -252 to -103 for glucocorticoids and -103 to +48 for ammonium chloride. Hepatic glutaminase functions to provide substrates (ammonia and glutamate) directly for urea synthesis. Glutamine can also function as an indirect precursor of urea synthesis via initial catabolism in the the small intestine. Within the enterocyte glutamine undergoes partial oxidation with the major end products being, ammonia, alanine and lactate with some synthesis of proline and citrulline. The alanine and ammonia produced are delivered to the liver where they are used for urea synthesis.

Original languageEnglish (US)
JournalFASEB Journal
Volume12
Issue number5
StatePublished - Mar 20 1998

All Science Journal Classification (ASJC) codes

  • Agricultural and Biological Sciences (miscellaneous)
  • Biochemistry, Genetics and Molecular Biology(all)
  • Biochemistry
  • Cell Biology

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