The 6-TG inhibited Mpn growth with MIC value of 0.20 μg ml-1, which is equivalent to tetracycline (MIC = 0.1 μg ml-1). However, 6-MP, a 6-TG analog did not inhibit Mpn growth. Neither theophylline, 7-(2, 3-dihydroxypropyl) theophylline, allopurinol, nor caffeine inhibited Mpn growth. 6-TG strongly inhibited uptake and incorporation of nucleotides derived from Hx and Gua into DNA and RNA, indicating that the observed inhibition by 6-TG was both at the level of transport and metabolism. It is noteworthy that the uptake/metabolism of Hx and Gua was inhibited by all the analogs used. Thiopurines, especially click here mercaptopurines, are the first line drugs for the treatment of acute

leukemia since the 1950s. They are also used in the treatment of inflammatory bowel disease [43]. The 6-TG and 6-MP exert their cytotoxicity through incorporation into DNA as deoxy-6-thioguanosine. These thiopurines are metabolized to deoxy-6-thioguanosine triphosphate via the purine salvage pathway initiated by HPRT (Figure 4). Thiopurine methyl transferase is a key enzyme in converting mercaptopurine to its cytotoxic metabolites, which can either inhibit purine nucleotide biosynthesis

or incorporate into DNA or RNA, causing DNA damage and cell death [37]. Mpn does not possess the essential enzymes, inosine monophosphate dehydrogenase and thiopurine methyl transferase, to convert mercaptopurine to the cytotoxic thioguanine

nucleotides, the respective methyl thiopurine nucleotides. This may explain why 6-MP did not inhibit Mpn growth. To further investigate the PERK modulator inhibitor mechanism by which 6-TG inhibited Mpn growth, Mpn HPRT was expressed, purified, and characterized. Both Hx and Gua are good substrates for the enzyme and the Vmax 5-Fluoracil values for these substrates are in the same order of magnitude as the human enzyme [44]. In humans, the plasma concentrations of Hx and Gua are approximately 172 μM and 97 μM [45], which is close to the Km and S0.5 values of Mpn HPRT with Hx and Gua. These results Epothilone B (EPO906, Patupilone) suggest that Mpn HPRT is capable of efficiently salvaging both Hx and Gua. In addition, Mpn HPRT showed positive cooperativity with Gua, indicating that at higher Gua concentration the enzyme utilizes Gua better. 6-TG and 6-MP are structural analogs. The observed significant differences in their inhibitory effects with Mpn and human HPRT suggest that there are structural differences in binding of these two compounds to the respective HPRTs in their active sites. These differences could be used in future design of Mycoplasma specific inhibitors. HPRT has been suggested as a target for anti-parasite drug development and new compounds have been developed [46]. Halogenated pyrimidine analogs such as 5FdU inhibited Mpn and Ureaplasma growth, as reported in our earlier studies [30, 35].