Gut and Liver as Metabolic Organs

The gut and liver facilitate absorption and delivery of dietary amino acids to the systemic blood and other tissues in the body. During this process, the gut and liver see all nutrients being absorbed and may sequester on the first pass during absorption any fraction of the dietary amino acids before these amino acid ever enter the systemic circulation. The liver has a natural role in this process as it is the organ that inactivates/modifies toxic substances in the blood. Therefore, the liver would be expected to regulate the flow of dietary amino acids into the systemic circulation following a meal. In addition, the liver is the only site in the body for metabolizing essential amino acids, except the BCAAs (which are metabolized by several tissues, most notably muscle). As mentioned above and discussed below, in terms of determining amino acid requirements, amino acids consumed during feeding in excess of the body's current needs would be expected to be oxidized. Therefore, a potential role for the liver is removal of excess amino acids, especially essential amino acids that cannot be oxidized in other tissues, on the first pass during amino acid absorption. The role of the gut and liver as active metabolic organs that metabolize dietary amino acids was shown in catheterized dogs by Elwyn et al. in 1968 (143). They demonstrated the active metabolism of the key nonessential amino acids, alanine, glutamine, and glutamate, and described how the BCAAs passed intact through the splanchnic bed. Several groups have shown similar results with respect to passage of amino acids following meal feeding in humans, using splanchnic bed and/or leg/arm A-V catheterization (51,, 144, 145 and 146). These studies have made several consistent observations.

As discussed above, the first observation was that most of the BCAAs in a meal pass through the splanchnic bed to be extracted and used by peripheral tissues. Later studies using labeled amino acid tracers administered enterally and intravenously ( Fig, 2,14) showed that the BCAA leucine has the lowest fractional extraction by the gut and liver on the first pass when delivered enterally (I§b]e...2J..1.). The amount of leucine oxidized by liver in the postabsorptive state is minimal (<2%) ( 93). Under conditions of feeding, the amount of enteral leucine extracted by the gut and liver on the first pass rose only slightly ( 94). However, the liver was shown to be a significant producer of KIC, indicating that the first step in BCAA metabolism is active, i.e., transamination. This step does not irreversibly remove leucine, rather it only forms a ketoacid that can be reconverted into leucine in a peripheral tissue such as muscle. Alternatively, the KIC produced by liver and released can be oxidized.

Of particular interest is the observation that the gut extracts glutamate and glutamine and releases alanine and ammonia ( 143). Early experiments with enteral glutamate infusion in dogs suggested that the glutamate sequestered is converted directly to alanine on the basis of glutamate uptake with resulting alanine release (147). Unfortunately, any interpretation of such an observation without proof from isotopic tracer studies would fall under the "gumball machine" analogy of Hgure.^S. Nonetheless, the observation is intriguing, especially in light of the consistent observation of a large A-V gradient for glutamine across the gut, indicating uptake ( 148, 149, 150 and 151).

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