Arginine metabolic fate

List several metabolic fates of the amino acid arginine in the human. What is the difference between essential and non-essential amino acids How is this relevant to the treatment of urea cycle defects ... 

Creatine is an amino acid derivative synthesized from arginine, glycine, and methionine in the kidneys, liver, and pancreas (see Figure 9.1). The metabolic fate of creatine is conversion to creatinine and excretion in the uiine." ... 

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.

The ammonia enters the portal vein and mixes with the metabolic nitrogen pool of the body. The fixation of this ammonia nitrogen into arginine, glutamic acid and utamine and aspartic acid represents a net contribution to nitrogen balance. The alternative fate of this ammonia is its conversion to urea prior to renal excretion. Thus the use of N-labelled ammonium salts or [ N]urea forms the basis of several common lines of clinical investigation. 

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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