Inhibition of uridine phosphorylase from Giardia lamblia by pyrimidine analogs

Abstract Fifty-six pyrimidine analogs were tested as possible inhibitors of uridine phosphorylase from Giardia lamblia . Values of K i were determined for eight of these which demonstrated an inhibition greater than 60% under the standard conditions of uridine at 1 mM (approximately 1.5 times the K m ) and inhibitor at 1 mM. All were competitive with respect to uridine. The most effective inhibitors were uracil analogs substituted at the C-5 position with electron withdrawing groups (nitro groups or halogens). The inhibitory effect at the 5-position appeared to be further enhanced by substitution at the C-6 position with electron releasing groups. The order of effectiveness as inhibitors was 6-mrthyl-5-nitrouracil > 6-amino-5-nitrouracil > 5-benzylacyclouridine > 5-nitrouracil > 5-fluorouracil > 5-bromouracil > 6-benzyl-2-thiouracil > 1,3-dimethyluracil with K i values of 10, 12, 44, 56, 119, 230, 190 and > 1000 ωM, respectively. The compounds were also effective inhibitors of the thymidine phosphorylase activity of the enzyme. The effect of the more potent compounds on G. lamblia in in vitro culture are currently under investigation.