Carbon Metabolism

For most amino acids, removal of the amino nitrogen group generates their ketoacid analogues. Many of these are already in a form for entry into the pathways of oxidative metabolism (Figure 10-3). For example, both a-ketoglutarate (from glutamate) and pyruvate (from alanine) are intermediates of the glycolysis-tricarboxylic acid (TCA) pathway of glucose oxidation. All the others have specific degradation systems that give rise to intermediates that can be metabolized in these oxidative pathways. Thus, protein can make a significant contribution to the body's energy supply. This is particularly true in non-growing adults, who on average consume, and therefore oxidize, about 10 to 15 percent of their dietary energy as protein (Appendix Table E-17).

The contribution of protein to energy needs may be significant during periods of energy restriction or following the utilization of the body's limited endogenous carbohydrate stores. Protein oxidation also has been shown to rise considerably in highly traumatized or septic individuals, which results in large amounts of body protein loss; this loss can compromise recovery or even lead to death (see below) (Klein, 1990). It is much less in periods of chronic starvation because of various metabolic adaptations related to ketone utilization, or on protein-restricted diets.

606 DIETARY REFERENCE INTAKES

tryptophan tryptophan

FIGURE 10-3 Metabolism of the carbon skeletons of the amino acid chains (light arrows) and their points of entry into the general pathways of metabolism of glucose and fat (bold arrows). SOURCE: Garlick and Reeds (1993).

Once the amino acid deamination products enter the TCA cycle (also known as the citric acid cycle or Krebs cycle) or the glycolytic pathway, their carbon skeletons are also available for use in biosynthetic pathways, particularly for glucose and fat. Whether glucose or fat is formed from the carbon skeleton of an amino acid depends on its point of entry into these two pathways. If they enter as acetyl-CoA, then only fat or ketone bodies can be formed. The carbon skeletons of other amino acids can, however, enter the pathways in such a way that their carbons can be used for gluco-neogenesis. This is the basis for the classical nutritional description of amino acids as either ketogenic or glucogenic (i.e., able to give rise to either ketones [or fat] or glucose).

Some amino acids produce both products upon degradation and so are considered both ketogenic and glucogenic (Figure 10-3). It has been argued that the majority of hepatic amino acid catabolism is directed in an obligatory fashion to glucose synthesis (Jungas et al., 1992). The synthesis of glucose in the liver from amino acids is dominated by alanine and glutamate, which derive much of their carbon from peripheral metabolism of glucose to lactate and TCA cycle intermediates. Thus, much of gluco-

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neogenesis in metabolism is the result of a metabolic cycle of glucose carbon between the peripheral tissues (especially muscle) and the liver and kidney. This cycle also involves the peripheral synthesis of glutamine, an amino acid that is utilized in substantial quantities by the intestinal cells in which it is used for energy and for the synthesis of proline, citrulline, and nucleic acids.

A significant proportion of the glucose synthesized in the liver is due to recapture and recycling via the liver of 3-carbon units in the form of lactate derived from anaerobic glucose breakdown in muscle (the Cori cycle). Hepatic gluconeogenesis also occurs via the glucose-alanine cycle (a direct parallel of the Cori cycle) and the glucose-glutamine cycle. Since the nitrogen donors may be either glucogenic or ketogenic amino acids, these cycles function as mechanisms for transporting nitrogen from the periphery to the liver as well as for glucose production. The cycle involving glutamine transport from the periphery to the gastrointestinal tract is also vital to the synthesis of arginine and proline and is critical to the prevention of the build up of excessive ammonia in the circulation.

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Gaining Weight 101

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