Histidine metabolic fate

The histidine catabolic pathway is discussed under Folate in Chapter 9. The material reveals that histidine is catabolized to produce glutamate. Glutamate in turn, can be converted to a-ketoglutarate and completely oxidized to CO in the Krebs cycle. In the study depicted in Figure 8,26, the dietary histidine was spiked with I Cjhistidine, The term "spiked" means that only a very small proportion of the histidine contained carbon-14. The metabolic behavior of the radioactive histidine, which can be followed, mirrors the metabolic fate of nonradioactive histidine in the diet. All of the CQz exhaled by the rats can be easily collected, The " COj present in the rat s breath can be measured by use of a liquid scintillation counter. The amount of CO2 produced directly mirrors the proportion of histidine, absorbed from the diet that was degraded the rat s body. 

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.

In considering amino acid catabolism, one must distinguish the catabolism of the carbon chain from that of the nitrogen moiety. The breakdown of the carbon chain of the amino acids yields carbon units that can be used in carbohydrate metabolism, acetate metabolism, or the metabolism of single carbon units. The fate of the carbon units of the individual amino acids has been discussed in other sections of this book, and only a synopsis of the results will be presented here. The carbon skeletons of isoleucine, phenylalanine, threonine, tryptophan, valine, histidine, alanine, arginine, aspartic acid, glycine, proline, glutamic acid, and hydroxyproline are ultimately converted to pyruvic acid. 

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