Calf spleen purine nucleoside phosphorylase was purified to homogeneity and its amino acid sequence was determined. The complex of the enzyme with an N(7)-acycloguanosine inhibitor crystallized in the cubic space group P213, with unit cell dimension 00540-P/asset/equation/feb2001457939500540p-math-si1.gif?v=1&s=b5cf4aa5a1666d8ee25306fc810e780d49d34f01)
and one monomer in the asymmetric crystal unit. The biologically active trimer is formed by the crystallographic three-fold axis. The structure was solved by molecular replacement methods, using the model of the human erythrocyte enzyme, and refined at a resolution of 2.9 Å to an R-factor of 0.21. The orientation of the inhibitor at the active site is examined in relation to the catalytic activity of the enzyme in the phosphorolysis of 00540-P/asset/equation/feb2001457939500540p-math-si2.gif?v=1&s=ef291bdae08901cef8ec7231f6ad092c307ff755)
nucleosides.
