Amino acid catabolism in liver : glycine and glutamine

Ewart, Harry Stephen (1993) Amino acid catabolism in liver : glycine and glutamine. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The liver removes substantial quantities of glycine and glutamine from the circulation in animals ingesting high-protein diets. Glycine and glutamine, however, do not accumulate in liver tissue during a protein meal, and in fact, their hepatic concentrations may actually decrease. Thus, there appears to be an activation of hepatic glycine and glutamine catabolism at this time. Glycine and glutamine catabolism are initiated within mitochondria via the glycine cleavage system and glutaminase, respectively. Rapid activation of these enzymes has been demonstrated in intact mitochondria from glucagon-injected rats. However, it was unknown whether similar activations occur as part of an hepatic physiological response to protein intake. In this thesis, glycine and glutamine catabolism have been extensively studied in intact mitochondria from rats fed on a high-protein diet or given a single high-protein meal. -- It was discovered that intact liver mitochondria from rats fed on a high-protein diet for six days catabolise glycine and glutamine at enhanced rates compared to mitochondria from rats fed a normal-protein diet. Glycine and glutamine catabolism were also stimulated in normal- protein-fed rats if they ingested a single high-protein meal for 2 hours before being killed. Thus, flux through the glycine cleavage system and through glutaminase is able to respond rapidly, according to the protein intake experienced during an individual meal. Investigations using whole cells or isolated, intact liver from such animals allow similar conclusions. Thus it is proposed that activation of the glycine cleavage system and glutaminase is a normal hepatic response in animals ingesting a high-protein meal.

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