Urea production with gluconeogenesis

Reviews by Ruderman (19) and Adibi (20,21) indicate that the branched-chain amino acids, particularly leucine, have an important role along with alanine in gluconeogenesis. Leucine and the other two branched-chain amino acids are catabolized in skeletal muscle. The nitrogen that is removed from the branched-chain amino acids in skeletal muscle is combined with pyruvate and returned to the liver as alanine. In the liver the nitrogen is removed for urea production and the carbon chain is utilized as substrate for synthesis of glucose. Adibi et al. (22) reported that during the catabolic conditions of starvation, oxidation of leucine and fatty acids increases in skeletal muscles. While glucose oxidation is reduced, the capacity for oxidation of the fatty acid palmltate more than doubled, and leucine oxidation increased by a factor of six. 

In a differentiated organism, each tissue must be provided with fuels that it can utilize for its own energy needs to perform its function. For example, muscles need to generate adenosine triphosphate (ATP) for their mechanical work of contraction, and the liver needs ATP for the synthesis of plasma proteins and fatty acids, gluconeogenesis, or for the production of urea for the excretion of nitrogenous compounds. 

When taken up into the human body from the diet, the 22 standard amino acids are either used to synthesize proteins and other biomolecules, or are oxidized to urea and carbon dioxide as a source of energy. The oxidation pathway starts with the removal of the amino group by a transaminase, the amino group is then fed into the urea cycle. The other product of transamidation is a keto acid that enters the citric acid cycle. Glucogenic amino acids can also be converted into glucose, through gluconeogenesis. 

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