Cytosolic nucleoside phosphorylase (NP) trimer catalyzes the reversible reaction of inosine or deoxyinosine with orthophosphate to form hypoxanthine and ribose 1-phosphate or deoxyribose 1-phosphate (Ealick et al. 1990; Wiginton et al. 1980). While NP is active with either nuckeotide in vitro, levels of deoxyinosine are normally low in vivo, limiting the extent of this reaction. NP deficiency in vivo is associated with defects in purine nucleotide salvage and leads to immunodeficiency (Williams et al. 1987).