Antiretroviral therapy suppresses replication of HIV allowing restoration and/or preservation of the immune system. Providing combination antiretroviral therapy during pregnancy can treat maternal HIV infection and/or reduce perinatal HIV transmission. However, providing treatment to pregnant women is challenging due to physiological changes that can alter antiretroviral pharmacokinetics. Suboptimal drug exposure can result in HIV RNA rebound, the selection of resistant virus or an increased risk of HIV-1 transmission to the infant. Increased drug exposure can produce unwarranted maternal adverse effects and/or fetal toxicity. Subsequently, dose adjustments may be necessary during pregnancy to achieve comparable antiretroviral exposure to non-pregnant adults. For several antiretrovirals, systemic exposure is decreased during the last trimester of pregnancy. By 6-12 weeks postpartum, concentrations return to those prior to pregnancy. Also, the extent of antiretroviral placental transfer to the fetus and degree of antiretroviral excretion into breast milk varies within, and between, antiretroviral drug classes. It is necessary to consider the pharmacological characteristics of each antiretroviral when optimizing combination therapy during pregnancy to treat maternal HIV infection and prevent perinatal HIV transmission.

Purine nucleoside phosphorylase (PNP) is an important enzyme in purine metabolism and cleaves purine nucleosides to their respective bases. Mycobacterial PNP is specific for 6-oxopurines and cannot account for the adenosine (Ado) cleavage activity that has been detected in M. tuberculosis and M. smegmatis cultures. In the current work, two Ado cleavage activities were identified from M. smegmatis cell extracts. The first activity was biochemically determined to be a phosphorylase that could reversibly catalyze adenosine ؉ phosphate 7 adenine ؉ alpha-D-ribose-1-phosphate. Our purification scheme led to a 30-fold purification of this activity, with the removal of more than 99.9% of total protein. While Ado was the preferred substrate, inosine and guanosine were also cleaved, with 43% and 32% of the Ado activity, respectively. Our data suggest that M. smegmatis expresses two PNPs: a previously described trimeric PNP that can cleave inosine and guanosine only and a second, novel PNP (Ado-PNP) that can cleave Ado, inosine, and guanosine. Ado-PNP had an apparent K m (K m app ) of 98 ؎ 6 M (with Ado) and a native molecular mass of 125 ؎ 7 kDa. The second Ado cleavage activity was identified as 5-methylthioadenosine phosphorylase (MTAP) based on its biochemical properties and mass spectrometry analysis. Our study marks the first report of the existence of MTAP in any bacterium. Since human cells do not readily convert Ado to Ade, an understanding of the substrate preferences of these enzymes could lead to the identification of Ado analogs that could be selectively activated to toxic products in mycobacteria.

5′-methylthioadenosine (MTA) is a natural purine that is metabolized by methylthioadenosine phosphorylase (MTAP, E.C 2.4.2.28) in Eukarya and Archaea but generally not in bacteria. In this work, Rv0535, which has been annotated as a probable MTAP in M. tuberculosis, was expressed in and purified from E. coli BL21 (DE3). The purified protein displayed properties of a phosphorylase and MTA was the preferred substrate. Adenosine and S-adenosyl-L-homocysteine were poor substrates and no activity was detected with 5′-methylthioinosine, the other natural purines or the natural pyrimidines. Kinetic analysis of M. tuberculosis MTAP showed that the Km value for MTA was 9.1 μM. Rv0535 was estimated as a 30 kDa protein on a denaturing SDS-PAGE gel, which agreed with the molecular mass predicted by its gene sequence. Using gel filtration chromatography, the native molecular mass of the enzyme was determined to be 60 ± 4 kDa, and thus indicates that M. tuberculosis MTAP is a dimer. Differences in active site between mycobacterial and human MTAPs were identified by homology modeling based on the crystal of the human enzyme. A complete structure activity relationship analysis could identify differences in substrate specificity between the two enzymes to aid in the development of purine-based, anti-tuberculosis drugs.