Guanosine deaminase

GUANINE DEAMINASE GUANOSINE DEAMINASE GUANYATE CYCASE GUANYATE KINASE GUEST... 

Guanylate reductase, which deaminates this nucleotide, catalyzes a reductive, rather than hydrolytic, deamination and has been discussed in Chapter 9. Like adenylate deaminase, it has a catabolic role and also functions in purine nucleotide interconversion. A guanosine deaminase has recently been identified in a pseudomonad (13), but it is not known to occur in animal cells. 

S.4.3 Guanine aminohydrolase Guanine deaminase Guanosine deaminase... 

Humans convert adenosine and guanosine to uric acid (Figure 34-8). Adenosine is first converted to inosine by adenosine deaminase. In mammals other than higher primates, uricase converts uric acid to the water-soluble product allantoin. However, since humans lack uricase, the end product of purine catabofism in humans is uric acid. 

After purine nucleotides have been converted to the corresponding nucleosides by 5 -nucleotidases and by phosphatases, inosine and guanosine are readily cleaved to the nucleobase and ribose-1-phosphate by the widely distributed purine nucleoside phosphorylase. The corresponding deoxynucleosides yield deoxyribose- 1-phosphate and base with the phosphorylase from most sources. Adenosine and deoxyadenosine are not attacked by the phosphorylase of mammalian tissue, but much AMP is converted to IMP by an aminohydrolase (deaminase), which is very active in muscle and other tissues (fig. 23.20). An inherited deficiency of purine nucleoside phosphorylase is associated with a deficiency in the cellular type of immunity. 

Deamination with adenosine deaminase is widely used for the synthesis of guanosine derivatives, e.g. formation of... 

The phosphorylase can catalyze the formation of inosine or deoxyinosine, and of guanosine or deoxyguano-sine, but not adenosine or deoxyadenosine. However, the last two nucleosides can be converted to inosine and deoxyinosine by adenosine deaminase. The normal function of the phosphorylase appears to be the formation... 

Phosphoribosyltransferase activity was found for hypoxanthine, guanine and xanthine but not for adenine (2). Adenine and guanine deaminase activities are present. Phosphorylase activities were found for adenosine, guanosine and inosine. Also present were adenosine kinase and a guanosine phosphotransferase neither inosine kinase nor phosphotransferase activity was present. The IMP dehydrogenase differs from the mammalian enzyme in that it does not require for activity and it is more sensitive to inhibition by mycophenolic acid (13). 

T.b. gambiense bloodstream forms have APRTase, HGPRTase, adenosine kinase and adenylosuccinate synthetase but lack adenosine deaminase. Two phosphorylase activities have been described for the bloodstream forms of T.b. brucei (42,50). One catalyzes the reversible phosphorolysis of adenosine, inosine and guanosine and the other is specific for adenosine and methylthioadenosine. Guanine deaminase is present whereas both adenosine and adenine deaminase are absent (8). Similar results have been reported for T. congolense (51). T. vivax is unique among the other trypanosomes in that it has an adenine deaminase (51). 

Adenosine and AMP can be deaminated by adenosine deaminase and AMP deaminase, respectively, to form inosine and IMP (see Fig. 41.10). Adenosine is also the only nucleoside to be directly phosphorylated to a nucleotide by adenosine kinase. Guanosine and inosine must be converted to free bases by purine nucleoside phosphorylase before they can be converted to nucleotides by HGPRT. 

Selective quenching allows also to put into evidence and to study a conformational change within a protein. ADAR2 (adenosine deaminase that acts on RNA) is a -80-kDa protein that efficiently deaminates the R/G site of GluR-B pre-mRNA sequences in vitro (O Connell et al. 1997). This enzyme has an RNA binding domain (RBD)l located in the C-terniinal catalytic domain. Deamination of adenosine (A) in the mRNA results in inosine (I) at that position. Because inosine is translated as guanosine (G), the editing reaction causes a functional A to G replacement. 

Figure 3. Compartmentalization of the purine salvage pathway of Leishmania. Abbreviations are as follows AAH, adenine aminohydrolase XPRT, xanthine phosphoribosyltransferase HGPRT, hypoxanthine-guaninephosphoribosyltransferase ADSS, adenylosuccinate synthetase ASL, adenylosuccinate lyase IMPDH, inosine monophosphate dehydrogenase GMPS, gua-nosine monophosphate synthase GDA, guanine deaminase AMPDA, adenosine monophosphate deaminase GMPR, guanosine monophosphate reductase APRT, adenine phosphoribosyltransferase AK, adenosine kinase. Enzymes that have been localized are shown in black and those that are predicted to be in the denoted locations are depicted in gray.

Fig. 173. Interconversion of inosine, adenosine and guanosine monophosphates 1 Inosine monophosphate dehydrogenase 2 guanosine monophosphate synthase 3 guanosine monophosphate reductase 4 adenylosuccinate synthetase 5 adenylosuccinate lyase 6 adenosine (phosphate) deaminase...

Adenosine (deoxyadenosine) is metabolized by erythrocytes (RBC) to either adenylates via adenylate kinase (AK) or to inosine by adenosine deaminase (ADA). Inosine, guanosine and their deoxy forms are converted to the purine base hypoxanthine by purine nucleoside phosphorylase (PNP). 

The 5 -adenyhc acid deaminase (22) found in rabbit muscle has been crystallized (23). It converts adenylic acid to inosinic acid and ammonia [Eq. (7)]. The enzyme does not deaminate adenine, adenosine, adenosine diphosphate, adenosine triphosphate, adenosine 2 -phosphate, adenosine S -phosphate, guanosine, or cytosine but does act upon deoxyadenylic acid (24)-... 

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