Hedstrom L, Cheung K S, Wang C C
Department of Pharmaceutical Chemistry, School of Pharmacy, University of California, San Francisco 94143.
Biochem Pharmacol. 1990 Jan 1;39(1):151-60. doi: 10.1016/0006-2952(90)90659-9.
Tritrichomonas foetus relies primarily on the salvage of hypoxanthine to supply purine nucleotides. Mycophenolic acid disrupts T. foetus growth by specifically inhibiting inosine-5'-monophosphate (IMP) dehydrogenase, thereby blocking the biosynthesis of guanine nucleotides from hypoxanthine. We have cloned a T. foetus strain (mpar) that was 50-fold more resistant to mycophenolic acid than wild type (IC50 = 1 mM for mpar vs 20 microM for wild type). None of the usual mechanisms of drug resistance could be identified. IMP dehydrogenase isolated from T. foetus mpar was indistinguishable from the wild type enzyme. No difference in mycophenolic acid uptake or metabolism was detected between the wild type and mpar strains. Mycophenolic acid (100 microM) completely blocked the conversion of adenine and hypoxanthine to guanine nucleotides in T. foetus mpar, although no inhibition of T. foetus mpar growth was observed at this concentration. These observations indicate that the major purine salvage pathways must be altered in T. foetus mpar so that guanine nucleotide biosynthesis no longer requires IMP dehydrogenase. T. foetus mpar incorporated xanthine more efficiently into the nucleotide pool relative to hypoxanthine and guanine than wild type. Xanthine incorporation via XMP provided an IMP dehydrogenase independent route to guanine nucleotides that would enable the parasite to become mycophenolic acid resistant. No difference could be detected between wild type and mpar hypoxanthine-guanine-xanthine phosphoribosyltransferases, the key enzyme in purine base incorporation into nucleotides. Two alterations were identified in the purine salvage network of mpar: it was deficient in hypoxanthine transport and had diminished adenine deaminase activity. The apparent net result of these two changes was to lower the intracellular concentration of hypoxanthine in mpar. Hypoxanthine and adenine inhibited the incorporation of xanthine into the nucleotide pool in wild type T. foetus, but not in mpar. The mpar strain, therefore, can salvage xanthine more efficiently from a mixture of purines and thus bypass the drug block at IMP dehydrogenase.
胎儿三毛滴虫主要依靠次黄嘌呤的补救途径来供应嘌呤核苷酸。霉酚酸通过特异性抑制肌苷-5'-单磷酸（IMP）脱氢酶来破坏胎儿三毛滴虫的生长，从而阻断次黄嘌呤鸟嘌呤核苷酸的生物合成。我们克隆了一株对霉酚酸的抗性比野生型高50倍的胎儿三毛滴虫菌株（mpar）（mpar的IC50 = 1 mM，而野生型为20 microM）。未发现常见的耐药机制。从胎儿三毛滴虫mpar中分离出的IMP脱氢酶与野生型酶没有区别。在野生型和mpar菌株之间未检测到霉酚酸摄取或代谢的差异。尽管在此浓度下未观察到对胎儿三毛滴虫mpar生长的抑制，但霉酚酸（100 microM）完全阻断了胎儿三毛滴虫mpar中腺嘌呤和次黄嘌呤向鸟嘌呤核苷酸的转化。这些观察结果表明，胎儿三毛滴虫mpar中的主要嘌呤补救途径一定发生了改变，使得鸟嘌呤核苷酸的生物合成不再需要IMP脱氢酶。相对于次黄嘌呤和鸟嘌呤，胎儿三毛滴虫mpar将黄嘌呤更有效地掺入核苷酸池中。通过XMP掺入黄嘌呤提供了一条不依赖IMP脱氢酶的鸟嘌呤核苷酸途径，这将使寄生虫对霉酚酸产生抗性。在野生型和mpar次黄嘌呤-鸟嘌呤-黄嘌呤磷酸核糖转移酶（嘌呤碱基掺入核苷酸的关键酶）之间未检测到差异。在mpar的嘌呤补救网络中发现了两个改变：它缺乏次黄嘌呤转运且腺嘌呤脱氨酶活性降低。这两个变化的明显净结果是降低了mpar中次黄嘌呤的细胞内浓度。次黄嘌呤和腺嘌呤抑制野生型胎儿三毛滴虫中黄嘌呤掺入核苷酸池，但对mpar没有抑制作用。因此，mpar菌株可以更有效地从嘌呤混合物中补救黄嘌呤，从而绕过IMP脱氢酶处的药物阻断。
