Adenosine aminohydrolase

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

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). 

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... 

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-... 

All adenosine aminohydrolase preparations have typical absorption spectra consistent with the absence of tightly bound prosthetic groups. No dissociable cofactors are necessary for catalysis since extensive dialysis of enzyme or addition of the metal chelator ethylenediaminete-traacetate (EDTA) did not cause significant inhibition. 

The kinetic parameters Km, V, ax, pH optima, and energy of activation (2 act) for deamination of adenosine catalyzed by homogeneous preparations of adenosine aminohydrolase are summarized in Table III 82-86, 98). Except for the unusually low specific activities observed for both the chicken duodenal and calf serum enzymes, and the significantly higher energy of activation reported for the former, the reaction param-... 

Kinetic Properties0 op Homogeneous Preparations1 op Adenosine Aminohydrolase... 

The reversible and competitive inhibition of calf duodenal adenosine aminohydrolase by relatively low concentrations of urea, methyl urea, and 1,3-dimethyl urea is similar to the urea (104) inhibition of several other enzymes (105). The inhibition appears to involve 1-2 molecules of inhibitor in the inhibition complex (106) determined from log[ (V0/Vi) —... 

Calf duodenal adenosine aminohydrolase affects enzymic hydrolysis of a wide variety of 6-substituted purine derivatives as well as analogs of adenosine with alteration in the purine ring and sugar moiety (Table IV) (65, 65a, 67, 70, 71, 112). Although not noted in Table IV, AMP, ADP, and ATP are not substrates. The hydrolysis of the 6-methoxy-purine derivative in H2180 occurs between C-6 and oxygen (113) consistent with the observed back incorporation of 180 from H2180 into inosine as catalyzed by both calf duodenal and Takadiastase non-... 

The reverse reaction, i.e., the direct conversion of inosine to adenosine catalyzed by both the calf duodenal and the Takadiastase nonspecific adenosine aminohydrolase (Section V) and measured as a function of pH with the calf enzyme, was defined by a theoretical curve for the equilibrium, K,= ([inosine] [NH i])/([adenosine] H20]) =38, with water concentration taken as one and pKa values of 8.8 and 9.2 for inosine and ammonium ion, respectively. The calculated AF = —5400 cal/mole at pH 7.0 was in reasonable agreement with —6000 cal/mole estimated from the summation of a series of partial reactions (114). 

The hydrolytic deamination catalyzed by rabbit and rat muscle AMP aminohydrolase may be facilitated by Zn2+ (133, 154a) in contrast to the mediation of a common purinyl enzyme intermediate for adenosine aminohydrolase catalysis (see Section III). 

Of the two homogeneous preparations of a nonspecific adenosine aminohydrolase from Aspergillus oryzae (Takadiastase) (92,179), that described by Wolfenden et al. (92) appears to be more facile and concise. Both procedures yield enzyme with turnover numbers near 105 moles adenosine deaminated per minute and molecular weights near 215,000. The mo-... 

Aspects of the mechanism of deamination catalyzed by the nonspecific Takadiastase enzyme are discussed in conjunction with calf duodenal adenosine aminohydrolase (see Section III). 

A guanosine aminohydrolase preparation from Pseudomonas convexa No. 149 free from guanine, adenine and adenosine aminohydrolase and nucleoside phosphorylase activities has been described (63). Of the 22... 

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