Purine nucleoside phosphorylases: properties, functions, and clinical aspects

The ubiquitous purine nucleoside phosphorylases (PNPs) play a key role in t he purine salvage pathway, and PNP deficiency in humans leads to an impairm ent of T-cell function, usually with no apparent effects on B-cell function . This review updates the properties of the enzymes from eukaryotes and a w ide range of prokaryotes, including a tentative classification of the enzym es from various sources, based on three-dimensional structures in the solid state, subunit composition, amino acid sequences, and substrate specificit ies. Attention is drawn to the compelling need of quantitative experimental data on subunit composition in solution, binding constants, and stoichiome try of binding; order of ligand binding and release; and its possible relev ance to the complex kinetics exhibited with some substrates. Mutations resp onsible for PNP deficiency are described, as well as clinical methods, incl uding gene therapy, for corrections of this usually fatal disease. Substrat e discrimination between enzymes from different sources is also being profi ted from for development of tumour- directed gene therapy. Detailed account s are presented of design of potent inhibitors, largely nucleosides and acy clonucleosides, their phosphates and phosphonates, particularly of the huma n erythrocyte enzyme, some with Ki values in nanomolar and picomolar range, intended for induction of the immunodeficient state for clinical applicati ons. such as prevention of host-versus-graft response in organ transplantat ions. Methods of assay of PNP activity are reviewed. Also described are app lications of PNP from various sources as tools for the enzymatic synthesis of otherwise inaccessible therapeutic nucleoside analogues, as coupling enz ymes for assays of orthophosphate in biological systems in the micromolar a nd submicromolar ranges, and for coupled assays of other enzyme systems. (C ) 2001 Elsevier Science Inc. All rights reserved.