Cytosolic nucleoside phosphorylase (NP) trimer catalyzes the reversible reaction of guanine with ribose 1-phosphate or deoxyribose 1-phosphate to form guanosine or deoxyguanosine and orthophosphate (Ealick et al. 1990; Wiginton et al. 1980). While NP is active with either ribose 1-phosphate or deoxyribose 1-phosphate in vitro, levels of deoxyribose 1-phosphate 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).