Amino acid ornithine aminotransferase

Among the numerous enzymes that utilize pyridoxal phosphate (PLP) as cofactor, the amino acid racemases, amino acid decarboxylases (e.g., aromatic amino acids, ornithine, glutamic acid), aminotransferases (y-aminobutyrate transaminase), and a-oxamine synthases, have been the main targets in the search for fluorinated mechanism-based inhibitors. Pharmaceutical companies have played a very active role in this promising research (control of the metabolism of amino acids and neuroamines is very important at the physiological level). 

Amination of aldehydes by an amino acid aldehyde aminotransferase from Mercurialis perennis was inhibited strongly by pyruvate, oxaloacetate, and oxoglutarate, and this inhibition was suspected to be competitive (Hartmann et al., 1972). Formation of coniceine by an alanine 5-keto-octanal aminotransferase was inhibited competitively by pyruvate and uncompetitively by glyoxylate (Roberts, 1978). With the L-ornithine 2-oxoacid aminotransferase from Cucurbita pepo, severe inhibition by valine, leucine, and isoleucine could be observed, but there was no inhibition by lysine or proline (Lu and Mazelis, 1975). Cucurbita maxima ornithine aminotransferase, however, was reported to be inhibited by proline (Splittstoesser and Fowden, 1973), as well as by canavanine and diaminobutyrate. 

This enzyme catalyzes the reaction of an w-amino acid with pyruvate to produce L-alanine and an aldehyde-containing carboxyhc acid. See Lysine 6-Aminotransferase Ornithine Aminotransferase... 

A transaminase patented by Celgene Corporation (Warren, NJ), called an co-aminotransferase [(co-AT)E.C. 2.6.1.18] does not require an a-amino acid as amino donor instead it requires a primary amine and hence has the ability to produce chiral amines.125 126 A similar co-AT from Vibrio fluvialis has been described for the production of chiral amines along with chiral alcohols when coupled with AdH or chiral amino acids when coupled with an a-amino acid aminotransferase.127130 Another co-AT, ornithine (lysine) aminotransferase (E.C. 2.6.1.68), has been described for the preparation of a chiral pharmaceutical intermediate used in the synthesis of Omapatrilat, a vasopep-tidase inhibitor developed by Bristol-Myers Squibb, as well as the UAA A1 -piperidinc-6-carboxylic acid.131-132... 

GH has been shown to induce a number of enzymes concerned with amino acid metabolism in the liver of the hypophysectomized rat (in vivo or in perfused liver [78]). Induced enzymes include tyrosine aminotransferase, tryptophan oxygenase and ornithine decarboxylase. The effects are complex, particularly in relation to interaction with glucocorticoids, and in some experiments GH lowered enzyme levels induced by glucocorticoids, although given alone it led to induction of these enzymes. 

A (UsorAex hyperornithinemia of the retina and choroid with gyrate atrophy and progressive degeneration is due to the deficiency of the enzyme omithine-5-aminotransferase (OAT). OAT deficiency is inherited as an autosomal recessive disorder and illustrates the metabolic importance of ornithine, a nonprotein amino acid (Chapter 17). Ornithine participates either as a substrate or a product of five enzymatic reactions. Two biochemical mechanisms have been proposed to explain the pathophysiology of gyrate atrophy of the choroid and the retina. One is that a high ornithine concentration causes reduced formation of... 

In a recent optimization study, an alternative coupling system has been developed [29 c], The chemical yield in the synthesis of two non-proteinogenic a-amino acids was remarkably improved by applying a coupled transamination process using additionally an ornithine co-aminotransferase to couple L-ornithine co-trans-amination to L-glutamate a-transamination [29 c],... 

A closely related group of pyridoxal-P-dependent aminotransferases act not at the chiral a-carbon atom of an amino acid, but instead at the prochiral w-aminomethy-lene carbon of amino acids such as lysine [42], ornithine [43] and y-aminobutyrate... 

Ornithine is an amino acid. However, it is not incorporated into proteins during the process of protein synthesis because no genetic codon exists for this amino acid. Although ornithine is normally regenerated by the urea cycle (one of the products of the arginase reaction), ornithine also can be synthesized de novo if needed. The reaction is an unusual transamination reaction catalyzed by ornithine aminotransferase under specific conditions in the intestine (Fig. 38.14). The usual direction of this reaction is the formation of glutamate semialdehyde, which is the first step of the degradation pathway for ornithine. 

Amino acid metabolism A artate aminotransferase Alanine aminotransferase Cysteine aminotransferase Tyrosine aminotransferase Leucine aminotransferase Alanine-ketoacid aminotransfoase Ornithine-ketoacid aminotransferase A artate carbamoyl transferase Methionine adenosyl transferase Glutamate decarboxylase Glutamate dehydrogenase Serine hydroxymethyltransferase Aminoacyl-sRNA synthetases... 

Now that P5C metabolism is linked to nucleotide synthesis, the induction of ornithine aminotransferase by estrogens (60), the presence of P5C synthase in cultured cells (90) and in proliferating tissue, i.e., intestine (110), may be considered as mechanisms to augment the production of ribonucleotides for DNA and RNA synthesis. The inhibition of P5C dehydrogenase by amino acids (58,59), may be a proline-sparing mechanism, but it also may route P5C to P5C reductase in order to augment the production of ribonucleotides. 

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