Leucine metabolic fate

After orally ingested, L-theanine is absorbed into the blood circulation through the small intestinal tract s brush-border membrane and then distributed to tissues." " It is easily transported into the brain through the blood-brain barrier s leucine-preferring amino acid transporter system L-Theanine does not appear to accumulate. The metabolic fate of theanine after its oral administration was verified to be enzymatically hydrolyzed to glutamic acid and ethylamine in the blood, kidney, liver, and brain then most of the ethylamine generated was immediately excreted into urine, with only a part circulated in plasma. It is completely absent 24 h after administration. 

In addition to the fatty acids with straight (unbranched) chains and an even number of C atoms, small amounts of acids with branched methyl groups or with an odd number of C atoms are also found in nature. Their metabolic fate in jS-oxi-dation is of considerable interest and can best be studied on the examples of the methyl branched C4- and the Ce-carboxylic acids. These two acids arise from the catabolism of the amino acids leucine, isoleucine, and valine by transamination and oxidative decarboxylation, as described in Chapt. VIII-10. Basically only two situations need to be discussed one with a methyl group in the a-position, or potentially in a-position (i.e. by repeated shortening of the chain by two C atoms the methyl group eventually ends up in a-position) the other with a methyl group in the -position, or at least potentially in /3-position. 

Selective labelling of the two diastereotopic methyl groups of i-leucine (144) has enabled their fates during secondary metabolic reactions to be elucidated [66]. Moreover, in the context of protein interactions, differentiation of the leucine pro-R and pro-S methyl groups in protein NMR spectra allows molecular recognition phenomena to be studied [67]. Recently, efficient routes to both forms of Relabeled leucine, based on application of an auxiliary-controlled stereoselective conjugate addition reaction (Scheme 6.27) have been described [68]. Thus, starting... 

Figure 9-3. Fates of the carbon skeletons upon metabolism of the amino acids. Points of entry at various steps of the tricarboxylic acid (TCA) cycle, glycolysis and gluconeogenesis are shown for the carbons skeletons of the amino acids. Note the multiple fates of the glucogenic amino acids glycine (Gly), serine (Ser), and threonine (Thr) as well as the combined glucogenic and ketogenic amino acids phenylalanine (Phe), tryptophan (Trp), and tyrosine (Tyr). Ala, alanine Cys, cysteine lie, isoleucine Leu, leucine Lys, lysine Asn, asparagine Asp, aspartate Arg, arginine His, histidine Glu, glutamate Gin, glutamine Pro, proline Val, valine Met, methionine.

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