Acetylornithine

This enzyme [EC 2.6.1.11] catalyzes the pyridoxal-phos-phate-dependent reaction of 2-acetylornithine with a-... 

This enzyme [EC 3.5.1.16], also known as acetylornithi-nase and A -acetylornithinase, catalyzes the reaction of water with A -acetylornithine to produce acetate and ornithine. The enzyme also catalyzes the hydrolysis of A -acetylmethionine. 

Animal and bacterial enzymes that utilize or synthesize carbamyl phosphate have activity with acetyl phosphate. Acyl phosphatase hydrolyzes both substrates, and maybe involved in the specific dynamic action of proteins. Ornithine and aspartic transcarbamylases also synthesize acetylornithine and acetyl aspartate. Finally, bacterial carbamate kinase and animal carbamyl phosphate synthetase utilize acetyl phosphate as well as carbamyl phosphate in the synthesis of adenosine triphosphate. The synthesis of acetyl phosphate and of formyl phosphate by carbamyl phosphate synthetases is described. The mechanism of carbon dioxide activation by animal carbamyl phosphate synthetase is reviewed on the basis of the findings concerning acetate and formate activation. 

As indicated in Reactions 1 and 2, both carbamyl-P and acetyl-P can react with ornithine to form either citrulline or 6-acetylornithine with ornithine transcarbamylase (15). 

While the a-amino group of ornithine could also react with acetyl -P, this possibility was eliminated in a number of ways. Chromatography in the automatic recording amino acid analyzer by the procedure of Spackman, Stein, and Moore (41) yields excellent separation of 6-acetylornithine and a-acetylornithine. Only traces of a -acetylornithine have been detected with ornithine transcarbamylase and acetyl-P the product is always better than 95% 6-acetylornithine. The separation of a- and 6-acetylornithine by paper chromatography is not practically feasible, although both acetyl derivatives are very readily separated from ornithine (15). 6-Acetylornithine is a natural product first isolated from a Siberian plant 26 years ago (28) and now known to be present in many plants (39). 

The product of the stoichiometric reaction of acetyl-P with ornithine, catalyzed by ornithine transcarbamylase, has been shown unequivocally to be 6-acetylornithine the transcarbamylases from rat liver, frog liver, and bacteria, however, even though yielding the same product, appear to differ in their ratios of activity with carbamyl-P and acetyl-P (Table n). While it is possible that the synthesis of 6-acetylornithine is catalyzed by other enzymes (16), the different ratios may be due to species differences we know now that the ratios of activity with carbamyl-P and acetyl-P of all ornithine transcarbamylases thus far tested remain constant with purification. Further, the ratio of citrulline to acetylornithine formation does not change with a number of treatments, such as heat inactivation of preparations containing orni-... 

Rajaram, V., Ratna Prasuna, P, Savithri, H.S., and Murthy, M.R. (2008) Structure of biosynthetic N-acetylornithine aminotransferase from Salmonella typhimu-rium Studies on substrate specificity and inhibitor binding. Proteins 70, 429-441. 

In 1993, a thermostable aminoacylase from Bacillus stearothermophilus was characterized by Sakanyan et al.l51. The enzyme hydrolyzes N-acyl derivatives of aromatic amino acids preferentially and even has some dipeptidase activity. Its optimal reaction temperature is 70 °C after incubation for 15 min, 90% of the original activity was retained. The authors write that the similarity of the B. stearothermophilus enzyme sequence with that of other enzymes such as aminoacylase I, acetylornithine deacetylase and carboxypeptidase G2 suggests a common origin. The aminoacylase from B. stearothermophilus is well characterized the gene has been completely sequenced1511, cloned into E. coli and overexpressed[51> 671 and studied for catalytic and stability properties16 1 the intrinsic one Zn2+ ion per subunit seems to have a predominantly structural role and activity can be restored to the apo-enzyme by Co2+ and particularly by Cd2+ (3-fold activity ) but not by Zn2+. 

Ornithine aminotransferase Acetylornithine aminotr. Succinylornithine aminotr. Ala-glyoxylate aminotransferase... 

The oxidation product has been isolated as its 2,4-dinitrophenylhydrazone and corresponds with -keto-6-aminovaleric acid. Putrescine, arginine, Manske s 1 (+) acetylornithine (118) and other related amino acids are oxidized much less readily if at all. The same authors have extracted an ornithine dehydrogenase from the young roots of Datura tatula. It requires the cooperation of a coenzyme not yet identified, and appears unable to oxidize putrescine and amino acids other than ornithine and to a lesser extent glutamic. Either of these systems, or the two linked into a H-transfer chain, would seem able to catalyze the oxidation of ornithine in the living tissues. No carbon dioxide was released from ornithine by the poly-phenolase system but on addition of an unwashed belladonna tissue-suspension carbon dioxide was liberated, presumably by decarboxylation of the a-keto-5-aminovaleric acid formed by the oxidation. 

C. cornuta Royle C. cryatallina Engelm. Acetylornithine (36) Bicuculline (37) Capnoidine (37) d-Stylopine (36) Protopine (36) Protopine (37)... 

Fig. 4. Outline of synthetic and degradative pathways of arginine and proline in plant cells. This figure emphasizes the synthetic and degradative aspects of arginine and proline metabolism. Protein is considered the end-product of synthesis and the starting point of degradation. Compounds on both sides of center line are considered to be in separate pools (possibly separated by membranes). Subscript "ex refers to ornithine supplied from outside the cell. The dotted arrows indicate that,a small proportion of the ornithine metabolized goes to KAV. Abbreviations not explained in the legend to Fig. 3 are as follows AcGLU, acetylglutamic acid AcORN, acetylornithine.

Zacharius, R. M. Composition of the nitrogenous components of the bush bean seed (Phaseolus vulgaris) including isolation of 5-acetylornithine. Phytochemistry 9, 2047 (1970). 

A -acetylglutamic y-semialdehyde dehydrogenase [4, 7], acetylornithine 5-transaminase [1, 8, 9], acetylornithinase [1, 10, 11], ornithine trans-carbamylase [12], and argininosuccinase [13] have been partially purified and characterized. A -Acetylglutamate synthetase activity has been exhibited in extracts [14], and this enzyme, which catalyzes the first step of the pathway, has been shown in cell suspension experiments to be subject to feedback inhibition by arginine [15],... 

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