Aminohydrolase

An even more elegant approach for the production of D-phydroxyphenylglydne on an industrial scale uses foe bacterium. Agrobacterium radiobacter (Figure A8.8). The organism is able to produce both D-hydantoinase and a second enzyme, N-carbamoyl-D-amino acid aminohydrolase, which catalyse the hydrolysis of N-carbamoyl-D-amino add. 

Treatment of 9-(/ -D-ribofuranosyluronic acid)adenine with diphenylphosphoro-chloridate and orthophosphate or tripolyphosphate yields (62) and (63), which, although unstable, inhibit rabbit AMP aminohydrolase and pyruvate kinase, respectively, with behaviour characteristic of active-site-specific reagents.98 Adenylate kinases from several sources are inactivated by iV6-[2- and 4-fluorobenzoyl]-adenosine-5 -triphosphates, with kinetics characteristic of active-site labelling, although these compounds were without effect on yeast hexokinase and rabbit pyruvate kinase.99... 

Dexrazoxane was also hydrolyzed enzymatically in the liver and kidney by dihydropyrimidine aminohydrolase. This enzyme could hydrolyze one but not a second ring of this molecule. Levrazoxane, the enantiomer of dexrazoxane, was also hydrolyzed enzymatically by DHPase in liver homogenates, but at a rate 4.5-fold slower [136], However, in vivo studies in rats dosed with razoxane (the racemic mixture of levrazoxane and dexrazoxane) revealed only a relatively small difference in elimination of the two enantiomers. This suggests that distribution and excretion reduced the impact of stereoselective biotransformation on the pharmacokinetics of these two enantiomers [137]. 

The heterocyclic ring of hydantoins, like that of succinimides (see Sect. 4.4.2), is hydrolytically cleaved by dihydropyrimidine aminohydrolase (DHPase, EC 3.5.2.2). Since both hydantoins and succinimides are hydrolyzed by the same enzyme, it is not surprising that structural features, such as absolute configuration, ring-substitution, and TV-substitution, exhibit comparable influence on catalysis. 

The antitumor agent 5-fluorouracil (4.236) is rapidly metabolized to 2-l luoro-/ -alanine (4.237) according to the sequence depicted in Fig. 4.10 [150][151]. The degradation of 5-fluorouracil occurs in all tissues, but tumor tissues contain very small amounts of dihydropyrimidine aminohydrolase. 

Adenine aminohydrolase has been found in micro-organisms, but not in mammalian cells, and the substrate specificities of the enzymes from Azotobacter vinelandii and Candida utilis were found to be similar [55, 56], Among other purines, 2-aminoadenine, A -aminoadenine, and 6-chloropurine were found to be substrates [55]. ... 

Adenosine aminohydrolase (adenosine deaminase) is found in all types of cells and is apparently an important catabolic enzyme for the regulation of cellular metabolism. It has been isolated from a number of sources and the substrate specificities of the various enzymes are similar, since a low degree of specificity R... 

This enzyme [EC 3.S.4.2], also called adenase, adenine aminohydrolase, and adenine aminase, catalyzes the hydrolysis of adenine to hypoxanthine and ammonia. 

This enzyme [EC 3.5.4.4], also known as adenosine aminohydrolase, catalyzes the hydrolysis of adenosine to yield inosine and ammonia. 

AMP AMINOHYDROLASE ADENYLOSUCCINATE LYASE ADENYLOSUCCINATE SYNTHETASE Adenylyl cyclase,... 

OXYGEN, OXIDES 0X0 ANIONS NITRIC OXIDE SYNTHASE AUTOINHIBITION Nitrile aminohydrolase,... 

Muscular work is accompanied by the production of ammonia, the immediate source of which is adenosine 5 -phosphate (AMP).301 302 This fact led to the recognition of another substrate cycle (Chapter 11) that functions by virtue of the presence of a biosynthetic pathway and of a degradative enzyme in the same cells (cycle A, Fig. 25-17). This purine nucleotide cycle operates in the brain303 304 as well as in muscle. The key enzyme 5-AMP aminohydrolase (AMP deaminase step a, Fig. 25-17) also occurs in erythrocytes and many other tissues.304 305 Persons having normal erythrocyte levels but an absence of this enzyme in muscles suffer from muscular weakness and cramping after exercise.306... 

The existence of two separate enzymes in animal tissues responsible for the liberation of ammonia from each of the two aminopurines, adenine and guanine, the latter specific for the free purine and the former for the nucleosides, was initially presented by Jones and his colleagues 11, 12). In 1928, Schmidt 13-15) demonstrated that AMP aminohy-drolase was responsible for the appearance of inosinic acid in muscle and for at least a portion of ammonia liberated during contraction. He showed not only a marked specificity for deamination of 5 -AMP but also provided the first clue that muscle adenylic acid (5 -AMP) and yeast adenylic acid (3 -AMP) were different compounds. Initial evidence for guanine and adenosine aminohydrolase including aspects of the specificity were also described by Schmidt 16). Additional details regarding development of interest in purine aminohydrolases are available in several excellent reviews 17-20). 

The virtual lack of adenine aminohydrolase in animal tissues has been confirmed in several laboratories (21-25). The reported presence of this enzyme in milk (21) has not been confirmed (26). Evidence for adenine aminohydrolase in Saccharomyces cerevisiae and Candida utilis based on enhanced growth on adenine (27) has been supported by Rousch and his colleagues (28, 29). The direct deamination of adenine by extracts of E. coli (30-32) has not been verified (33). 

Adenosine aminohydrolase occurs in tissues of both vertebrates and invertebrates. The enzyme has been observed in the larvae of Drosophila melanogaster (34), the blowfly (35), sea urchin eggs (36), the hapato-pancreas of both crayfish and lobster (37), and a variety of animal tissues (38 41b). Brady and O Donovan (42) examined the distribution... 

AMP aminohydrolase, an enzyme relatively specific for AMP, has been observed in reptiles (44), erythrocytes (38), snail (45), unfertilized fish eggs (46), invertebrates (47), a variety of mammalian tissues (20), and a particulate fraction of pea seeds (48). Evidence suggests that the frog muscle AMP aminohydrolase is located within or just beneath the sarcolemma (49). The rabbit skeletal and heart muscle enzymes were found in the cytoplasm and mitochondria (20, Jfi, 50, 51), while the enzyme of kidneys and gills of freshwater fish was located in the cytoplasmic fraction (52). The enzyme occurs in most areas of the rat (53) and rabbit brain (54). The nonspecific enzyme from several microbial sources deaminates adenosine triphosphate (ATP) and adenosine diphosphate (ADP) as well as AMP (see Section V). 

Guanine aminohydrolase is present in a variety of animal tissues (55-58), in lobster hepatopancreas (37), in certain bacteria (59), and... 

Purine aminohydrolases may be assayed by measuring release of ammonia (13, 14) or directly by the much more convenient spec-trophotometric method developed by Kalckar (61) and Rousch and Norris (62). Absorbancy changes resulting from enzymic hydrolysis of the purines or purine derivatives are summarized in Table I (62-75). 

The partially purified adenine aminohydrolase (EC 3.5.4.2) from Azotobacter vinelandii catalyzes the anaerobic conversion of adenine to... 

Substrates for Adenine Aminohydrolase of Candida uiilis and Azulobader vinelandii... 

Adenine aminohydrolase of A. vinelandii does not catalyze the back incorporation of products, hypoxanthine or chloride, into 6-chloropurine during the course of hydrolysis when examined over a wide range of pH in contrast to the back incorporation of oxygen-18 into hypoxanthine catalyzed by adenosine aminohydrolase (80) (see Section III). These results are consistent with a direct displacement of the 6 substituent by water rather than the intermediate formation of purinyl enzyme or chloroenzyme during catalysis. 

Homogeneous preparations of adenosine aminohydrolase (EC 3.5.4.4) have been obtained from mucosa of calf duodena (81, 82), chicken... 

The molecular weight of the calf duodenal adenosine aminohydrolase determined from sedimentation and diffusion data (91) and by comparative elution from Sephadex gels (93, 94) ranges from 31,000-35,000 (84,91, 93, 95). A molecular weight of 52,000 from sedimentation velocity and sedimentation equilibrium data has not been confirmed (93). 

Molecular weight data for other preparations are consistent with additional forms of the enzyme for example, chromatography of hepatic extracts of amphibia on Sephadex G-200 gave three peaks of adenosine aminohydrolase activity labeled type A, B, and C (96). Types corre-... 

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