Genetic analysis of nucleoside transport in Leishmania donovani

Genetic dissection of nucleoside transport in Leishmania donovani indicates that the insect vector form of these parasites possesses two biochemically distinct nucleoside transport systems. The first transports inosine, guanosine, and formycin B, and the second transports pyrimidine nucleosides and the adenosine analogs, formycin A and tubercidin. Adenosine is transported by both systems. A mutant, FBD5, isolated by virtue of its resistance to growth inhibition by 5 microM formycin B, cannot efficiently transport inosine, guanosine, or formycin B. This cell line is also cross-resistant to growth inhibition by a spectrum of cytotoxic analogs of inosine and guanosine. A second parasite mutant, TUBA5, isolated for its resistance to 20 microM tubercidin, cannot take up from the culture medium radiolabeled tubercidin, formycin A, uridine, cytidine, or thymidine. Both the FBD5 and the TUBA5 cell lines have about a 50% reduced capacity to take up adenosine, indicating that adenosine is transported by both systems. A tubercidin-resistant clonal derivative of FBD5, FBD5-TUB, has acquired the combined biochemical phenotype of each single mutant. The wild-type and mutant cell lines transport purine bases and uracil with equal efficiency. Mutational analysis of the relative growth sensitivities to cytotoxic nucleoside analogs and the selective capacities to take up exogenous radiolabeled nucleosides from the culture medium have enabled us to define genetically the multiplicity and substrate specificities of the nucleoside transport systems in L. donovani promastigotes.

Biochemical effects of dipyridamole on purine overproduction and excretion by mutant murine T-lymphoblasts

A mutant murine T-cell line which overproduces purines and excretes massive quantities of inosine into the culture medium has served as a cell culture model for overproduction hyperuricemia (Ullman, B., Wormsted, M. A., Cohen, M. B., and Martin, D. W., Jr. (1982) Proc. Natl. Acad. Sci. U. S. A. 79, 5127-5131). Incubation of these cells with micromolar concentrations of dipyridamole, a potent inhibitor of nucleoside transport, prevents the excretion of inosine and depresses the rate of purine biosynthesis to that of wild type cells. These concentrations of dipyridamole have no effect on cellular growth rate or on the intracellular nucleoside triphosphate or phosphoribosylpyrophosphate pools. We suggest that dipyridamole might also be useful in ameliorating purine overproduction associated with hyperuricemia and gout.

Genetic studies on the role of the nucleoside transport function in nucleoside efflux, the inosine cycle, and purine biosynthesis

A mutant clone (AU-100) which is 90% deficient in adenylosuccinate synthetase activity was characterized from wild-type murine S49 T-lymphoma cells. This AU-100 cell line and its hypoxanthine-guanine phosphoribosyltransferase-deficient derivative, AUTG-50B, overproduce purines severalfold and excrete massive amounts of inosine into the culture medium (Ullman et al., Proc. Natl. Acad. Sci. U.S.A. 79:5127-5131, 1982). We introduced a mutation into both of these cell lines which make them incapable of taking up nucleosides from the culture medium. The genetic deficiency in nucleoside transport prevents the adenylosuccinate synthetase-deficient AU-100 cells from excreting inosine. Because of an extremely efficient intracellular inosine salvage system, the nucleoside transport-deficient AU-100 cells also no longer overproduce purines. AUTG-50B cells which have been made genetically deficient in nucleoside transport still overproduce purines but excrete hypoxanthine rather than inosine. These studies demonstrate genetically that nucleoside transport and nucleoside efflux share a common component and that nucleoside transport has an important regulatory function which profoundly affects the rates of purine biosynthesis and purine salvage.

Perforation of a suppurative solitary renal cyst

A fatal case of rupture of a large infected renal cyst is presented. Antibiotic therapy is indicated after aspiration of a suppurative renal cyst, and recurrence, particularly if large, is an indication for prompt surgical intervention.

Selective area tuberculosis surveys in Singapore 1978

A random population sample survey in Singapore 1975 revealed that a relatively high proportion of active pulmonary tuberculosis patients remained undiagnosed, mainly in the older age groups. Previous studies disclosed the cause to the lack of symptoms of concern in these patients. A new programme was devised to discover hidden elderly patients by mass chest X-ray screening of population aged 45 years and older living in areas with known high incidence of tuberculosis. The yield of cases was less than expected mainly due to the dislocation and dispersion of population from recent extensive urban re-development in Singapore. Nevertheless the results were comparable to the most productive case finding programme carried out in this department.