Purine nucleoside phosphorylase from rabbit liver

Catabolism of exogenous deoxyinosine in cultured epithelial amniotic cells

Uptake and catabolism of purine nucleosides have been commonly considered as means to salvage the purine ring for nucleic acid synthesis, usually neglecting the destiny of the pentose moiety. With the aim to ascertain if deoxyribose derived from exogenous DNA can be utilised as a carbon and energy source, we studied the catabolism of exogenous deoxyinosine in a cell line derived from human amnion epithelium (WISH). Intact WISH cells catabolise deoxyinosine by conversion into hypoxanthine. The nucleoside enters the cell through a nitrobenzylthioinosine-insensitive equilibrative transport. Deoxyinosine undergoes a phosphorolytic cleavage inside the cell. The purine base diffuses back to the external medium, while the phosphorylated pentose moiety can be further catabolised to glycolysis and citric acid cycle intermediates. Our results indicate that the catabolism of the deoxynucleoside can be considered mainly as a means to meet the carbon and energy requirements of growing cells.

Pentatrichomonas hominis: purine salvage pathway

1. Pentatrichomonas hominis was found incapable of de novo synthesis of purines. 2. Pentatrichomonas hominis can salvage adenine, guanine, hypoxanthine, adenosine, guanosine and inosine, but not xanthine for the synthesis of nucleotides. 3. HPLC tracing of radiolabelled purines or purine nucleosides revealed that adenine, adenosine and hypoxanthine are incorporated into adenine nucleotides and IMP through a similar channel while guanine and guanosine are salvaged into guanine nucleotides via another route. There appears to be no direct interconversion between adenine and guanine nucleotides. Interconversion between AMP and IMP was observed. 4. Assays of purine salvage enzymes revealed that P. hominis possess adenosine kinase; adenosine, guanosine and inosine phosphotransferases; adenosine, guanosine and inosine phosphorylases and AMP deaminase.

Enzymes of nucleotide metabolism: the significance of subunit size and polymer size for biological function and regulatory properties

The 72 enzymes in nucleotide metabolism, from all sources, have a distribution of subunit sizes similar to those from other surveys: an average subunit Mr of 47,900, and a median size of 33,300. The same enzyme, from whatever source, usually has the same subunit size (there are exceptions); enzymes having a similar activity (e.g., kinases, deaminases) usually have a similar subunit size. Most simple enzymes in all EC classes (except class 6, ligases/synthetases) have subunit sizes of less than 30,000. Since structural domains defined in proteins tend to be in the Mr range of 5,000 to 30,000, it may be that most simple enzymes are formed as single domains. Multifunctional proteins and ligases have subunits generally much larger than Mr 40,000. Analyses of several well-characterized ligases suggest that they also have two or more distinct catalytic sites, and that ligases therefore are also multifunctional proteins, containing two or more domains. Cooperative kinetics and evidence for allosteric regulation are much more frequently associated with larger enzymes: such complex functions are associated with only 19% of enzymes having a subunit Mr less than or equal to 29,000, and with 86% of all enzymes having a subunit Mr greater than 50,000. In general, larger enzymes have more functions. Only 20% of these enzymes appear to be monomers; the rest are homopolymers and rarely are they heteropolymers. Evidence for the reversible dissociation of homopolymers has been found for 15% of the enzymes. Such changes in quaternary structure are usually mediated by appropriate physiological effectors, and this may serve as a mechanism for their regulation between active and less active forms. There is considerable structural organization of the various pathways: 19 enzymes are found in various multifunctional proteins, and 13 enzymes are found in different types of multienzyme complexes.

Chromate, molybdate, tungstate and vanadate behave as substrates of yeast diadenosine 5',5'''-p1,p4-tetraphosphate alpha, beta-phosphorylase

Diadenosine 5',5'''-p1,p4-tetraphosphate (Ap4A) alpha, beta-phosphorylase from yeast Saccharomyces cerevisiae catalyzes two reactions: Ap4A cleavage and nucleoside diphosphate--phosphate (NDP-Pi) exchange. In both reactions phosphate can be substituted by arsenate, chromate, molybdate, tungstate or vanadate. In the presence of each anion, nucleoside 5'-monophosphate (NMP) always accumulates as a product of the reaction. This indicates that an unstable NMP anion is formed as an intermediate.

A second purine nucleoside phosphorylase in Escherichia coli K-12. II. Properties of xanthosine phosphorylase and its induction by xanthosine

The presence of a second purine nucleoside phosphorylase in wild-type strains of E. coli K-12 after growth on xanthosine has been demonstrated. Like other purine nucleoside phosphorylase it is able to carry out both phosphorylosis and synthesis of purine deoxy- and ribonucleosides whilst pyrimidine nucleosides cannot act as substrates. In contrast to the well characterised purine nucleoside phosphorylase of E. coli K-12 (encoded by the deoD gene) this new enzyme could act on xanthosine and is hence called xanthosine phosphorylase. Studies of its substrate specificity showed that xanthosine phosphorylase, like the mammalian purine nucleoside phosphorylases, has no activity towards adenine and the corresponding nucleosides. Determinations of Km and gel filtration behaviour was carried out on crude dialysed extracts. The presence of xanthosine phosphorylase enables E. coli to grow on xanthosine as carbon source. Xanthosine was the only compound found which induce xanthosine phosphorylase. No other known nucleoside catabolising enzyme was induced by xanthosine. The implications of non-linear induction kinetics of xanthosine phosphorylase is discussed.