Ornithine from arginine

Essential amino acids are indispensable in that the body is incapable of replacing them simply by using its own synthesizing facilities they must either be supplied in adequate quantities from outside or generated by degradation of body proteins. Non-essential amino acids are synthesized in the liver, muscles, kidneys and intestine. Only the synthesis of arginine from ornithine and the hydroxylation of phenylalanine to tyrosine are liver-specific reactions. 

The formation of arginine from ornithine, via citruUine, has long been known. In fact, the ornithine cycle of Krebs and Henseleit was one of the first biosynthetic pathways ever proposed. 

S-Citrulline (2>amino-5-ureidopentanoic acid) [372-75-8] M 175.2, m 222°, [o] +24.2° (in 5M HCl), pK 9.71. Likely impurities are arginine, and ornithine. Crystd from water by adding 5 volumes of EtOH. Also crystd from water by addn of MeOH. 

There are numerous abnormalities of cysteine metabolism. Cystine, lysine, arginine, and ornithine are excreted in cystine-lysinuria (cystinuria), a defect in renal reabsorption. Apart from cystine calculi, cystinuria is benign. The mixed disulfide of L-cysteine and L-homocysteine (Figure 30-9) excreted by cystinuric patients is more soluble than cystine and reduces formation of cystine calculi. Several metabolic defects result in vitamin Bg-responsive or -unresponsive ho-mocystinurias. Defective carrier-mediated transport of cystine results in cystinosis (cystine storage disease) with deposition of cystine crystals in tissues and early mortality from acute renal failure. Despite... 

Plasma should be separated from the blood cells within a few hours. For most amino acids the levels in plasma and red cells are comparable, but glutamate, aspartate, and taurine have extremely high intracellular levels and thus tend to rise in plasma upon hemolysis. A second effect of red cell degradation is the liberation of the enzyme arginase, which will convert arginine into ornithine. 

Cysteine and cystine are relatively insoluble and are toxic in excess.450 Excretion is usually controlled carefully. However, in cystinuria, a disease recognized in the medical literature since 1810,451 there is a greatly increased excretion of cystine and also of the dibasic amino acids.451 452 As a consequence, stones of cystine develop in the kidneys and bladder. Patients may excrete more than 1 g of cystine in 24 h compared to a normal of 0.05 g, as well as excessive amounts of lysine, arginine, and ornithine. The defect can be fatal, but some persons with the condition remain healthy indefinitely. Cystinuria is one of several human diseases with altered membrane transport and faulty reabsorption of materials from kidney tubules or from the small intestine. Substances are taken up on one side of a cell (e.g., at the bottom of the cell in Fig. 1-6) and discharged into the bloodstream from the other side of the cell. In another rare hereditary condition, cystinosis, free cystine accumulates within lyso-somes.453... 

Synthesis of arginine by the salvage pathway found in vertebrates and by the de novo pathway found in plants and bacteria. The final steps from ornithine to arginine are also part of the urea cycle (see fig. 22.7). 

Figure 8.1. Chromatograms produced on a 100 mm column of an amino acid analyser with 0.3 N sodium citrate solution (pH 7.00) as eluting agent (a) standard solution containing 1 unol/ml of each amino acid (b) 30 g of cabbage extract purified with Dowex 50-X8 in the ammonium form (0.97 mg of S-methylmethionine) and (c) the same sample as for (b) but after treatment at pH 10.0 for 30 minutes at 120 C. MMS S-methylmethionine, fils histidine, lys lysine, orn ornithine, arg arginine. From [12]...

Alkaloids derived from ornithine, arginine Pyrrolizidine alkaloids Miscellaneous alkaloids X X X X X ... 

The nonessential amino acids are synthesized by quite simple reactions, whereas the pathways for the formation of the essential amino acids are quite complex. For example, the nonessential amino acids alanine and aspartate are synthesized in a single step from pyruvate and oxaloacetate, respectively. In contrast, the pathways for the essential amino acids require from 5 to 16 steps (Figure 24.8). The sole exception to this pattern is arginine, inasmuch as the synthesis of this nonessential amino acid de novo requires 10 steps. Typically, though, it is made in only 3 steps from ornithine as part of the urea cycle. Tyrosine, classified as a nonessential amino acid because it can be synthesized in 1 step from phenylalanine, requires 10 steps to be synthesized from scratch and is essential if phenylalanine is not abundant. We begin with the biosynthesis of nonessential amino acids. 

A daily dosage of 0.3 g/kg BW is recommended (approx. 3 X 10 g). The use of BCAA has become established for latent and manifest HE. It has even been possible to render patients fit to drive again. No side effects are known. The efficacy of branched-chain amino acids as parenteral i.v. therapy is well validated for the severe stages II—IV of HE. The concomitant intake of arginine and ornithine aspartate has proved to be particularly effective. (127) Eatty emulsions are to be avoided since they release tryptophan from the albumin binding and inhibit the utilization of branched-chain amino acids in the musculature. (123, 126, 129, 131, 133, 138, 140, 145, 148-151, 170) (s. p. 860)... 

Arginine from the diet or from protein breakdown is cleaved by the cytosolic enzyme arginase, generating urea and ornithine. 

Arginine, via three reactions of the urea cycle, can be derived from ornithine, which is produced by transamination of glutamate semialdehyde. 

The first key step in the biosynthesis of tropane alkaloids is the formation of the intermediate putrescine. Polyamines and, therefore putrescine, are found in plant cells and are implicated in growth, root, fruit and flower development, and in different stress phenomena. It is well known that plants synthesize polyamines from ornithine and arginine, unlike other eukaryotes like mammals, which only synthesize polyamines from ornithine. In plants putrescine is synthesized directly from ornithine, a reaction catalysed by ornithine decarboxylase (ODC, EC 4.1.1.17) and indirectly from arginine via agmatine catalysed by arginine decarboxylase (ADC, EC 4.1.1.19), Fig. (1). In Arabidopsis, it is known that the adc gene is required for the production of polyamines that are essential for normal seed development [103]. 

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