A pET-15b expression vector containing canonical human IMPDH1 sequence in the XhoI restriction site was kindly donated by Peter Humphries’ laboratory (Trinity College, Dublin, Ireland). IMPDH2 cDNA was amplified from human peripheral blood leukocyte first-strand cDNA (BioChain) using the following primers: 5′-gacgacgacaagatggccgactacctgattagtgg-3′ and 5′-gaggagaagcccgtcagaaaagccgcttctcatacg-3′. PCR product was cloned into a pEK-LIC30 vector (Novagen, Madison, WI) using the manufacturer’s protocol. Expression vectors were transfected into Escherichia coli (BL21) and induced using IPTG (isopropyl-β-d-thiogalactopyranoside; 1 mM final concentration). Recombinant proteins were isolated using an incorporated His tag. Bacterial pellets were sonicated in lysis buffer (50 mM NaH2PO4, 300 mM NaCl, 5 mM imidazole, and 10% glycerol [pH 8.0]), containing 1× protease inhibitor cocktail (Halt; Pierce, Rockford, IL). Total cell lysates were mixed with cobalt chelated discs (SwellGel; Pierce) for 2 hours and then decanted. Bound resin was washed four times (50 mM NaH2PO4, 300 mM NaCl, 40 mM imidazole, 10% glycerol, pH 8.0) and then eluted (50 mM NaH2PO4, 300 mM NaCl, 300 mM imidazole, and 10% glycerol [pH 8.0]).