Properties of the thymidine transport system of Chinese hamster ovary cells as probed by nitrobenzylthioinosine

Expression of high levels of nitrobenzylthioinosine-sensitive nucleoside transport in cultured human choriocarcinoma (BeWo) cells

We have examined binding of [3H]nitrobenzylthioinosine (NBMPR) and influx of [3H]thymidine in adherent cultures of human choriocarcinoma (BeWo) cells and, for comparison, cervical-carcinoma (HeLa) cells. Specific association of NBMPR with BeWo cells at 22 degrees C required 1.5 h to reach an equilibrium between free and bound ligand, whereas association with HeLa cells required 20-30 min. Scatchard analysis of NBMPR binding to low-density cultures of BeWo cells revealed a total of 27 x 10(6) sites per cell, consisting of two distinct populations that differed in their affinities for NBMPR. One population bound NBMPR with 'high' affinity (Bmax.1 15.0 pmol/10(6) cells; Kd1 0.6 nM) and the other, larger, population bound NBMPR with 'low' affinity (Bmax.2 29.0 pmol/10(6) cells; Kd2 14.5 nM). By contrast, HeLa cells possessed only 4.1 x 10(5) sites per cell, and these sites all bound NBMPR with the same affinity (Bmax. 0.7 pmol/10(6) cells; Kd 0.5 nM). Interaction of NBMPR with both populations of sites in BeWo cells could be blocked by nitrobenzylthioguanosine (NBTGR), dilazep or dipyridamole. Concentration-effect relationships for dilazep inhibition of binding of 1 nM- and 25 nM-NBMPR to BeWo cells were monophasic, with virtually complete inhibition achieved at 0.1 microM and 1 microM respectively. Plasma-membrane preparations from BeWo cells also had high numbers of NBMPR-binding sites, and u.v. irradiation of site-bound [3H]NBMPR in such preparations labelled polypeptides that migrated in electrophoretograms as a broad band with a peak M(r) of 60,000. The concentration-effect relationship for NBMPR inhibition of thymidine transport by BeWo cells was biphasic, with an IC50 for inhibition of the 'NBMPR-sensitive' component of 1.6 nM and a substantial (15-20%) component of flux that was not inhibited by 10 microM-NBMPR and was thus 'NBMPR-insensitive'. Vmax. values for thymidine transport by BeWo cells were 20-30-fold larger than the corresponding values for transport by HeLa cells. Elimination of the Na+ gradient had no effect on initial rates of thymidine fluxes measured in either the presence or the absence of NBMPR. Our results demonstrate that BeWo cells have an unusually large capacity for NBMPR-sensitive nucleoside transport, apparently resulting from high levels of expression of 'erythrocyte-like' transport elements, identified by their high-affinity interaction with NBMPR. The relationship of the low-affinity binding sites to NBMPR-sensitive transporter elements is uncertain.

Membrane transport of 1-beta-D-arabinofuranosylcytosine and accumulation of 1-beta-D-arabinofuranosylcytosine 5'-triphosphate in P388 murine leukemic cells resistant to vincristine

The membrane transport of ara-C and intracellular ara-CTP accumulation were investigated in P388 murine leukemic cells resistant to vincristine (P388/VCR) and its parent cell line. The transport of ara-C in P388/VCR cell line was a 1.4-fold increase at 30 sec compared to that in P388 parent cell line (P less than 0.01). The increase of the transport of ara-C in P388/VCR cell line, however, was not completely abolished by the nucleoside transport inhibitor, nitrobenzylthioinosine (NBTI) to the level in parent cell line. Scatchard analyses revealed that the resistant cells had significantly less NBTI binding sites than the parent cells had. These results suggested that the changes in ara-C transport in P388/VCR cells were due, in part, to increase of NBTI-insensitive transport sites in the membrane. The measurement of the intracellular ara-CTP concentration by high-performance liquid chromatography revealed that the intracellular ara-CTP level in P388/VCR cells was also significantly higher than that in parent cells (1.4-fold, P less than 0.01). As the transport of ara-C is rate limiting at a concentration of 1 microM in the both cell lines, we concluded that the accumulation of ara-CTP in P388/VCR cells might have partially resulted from the enhancement of the ara-C transport.

Inhibition of nucleoside transport by nitrobenzylthioformycin analogs

The formycin analogs of nitrobenzylthioinosine and nitrobenzylthioguanosine were synthesized and evaluated as nucleoside transport inhibitors. These analogs have a potential therapeutic advantage over their parent compounds in that their C-nucleosidic linkages prevent them from being degraded to the immunosuppressive agents, 6-mercaptopurine and 6-thioguanine. 7-[(4-Nitrobenzyl)-thio]-3-(beta-D-ribofuranosyl)pyrazolo[4,3- d]pyrimidine (NBTF) and 5-amino-7-[(4-nitrobenzyl)thio]-3-(beta-D- ribofuranosyl)pyrazolo[4,3-d]pyrimidine (NBTGF) were inhibitors of nucleoside transport in human erythrocytes and HL-60 leukemia cells. The IC50 value for nitrobenzylthioinosine, NBTF and NBTGF with 10% erythrocyte suspensions were 18, 18 and 40 nM respectively. Specific binding studies with [3H]NBTF yielded a Kd of 3.4 nM with erythrocytes, approximately 10-fold higher than values reported for nitrobenzylthioinosine. NBTF and nitrobenzylthioinosine bound to HL-60 cells with Kd values of 8.1 and 0.81 nM respectively. The octanol/water partition coefficients of nitrobenzylthioinosine, NBTF and NBTGF were 3.5, 3.2, and 2.8 respectively. NBTF could be expected to be equipotent with nitrobenzylthioinosine in whole blood where inhibitor concentrations of 10(-7) to 10(-6) M are required in order to saturate erythrocytic binding sites; hence, it may exhibit the advantages inherent in a C-nucleoside.

Enhancement of 5-fluorouracil's anticancer activity by dipyridamole

Although the interaction between FUra and DP in HCT 116 cells is fairly complex, data from other investigators indicate that in cell lines in which inhibition of TS is growth limiting at relatively low concentrations of fluoropyrimidines, DP appears to augment the cytotoxicity of FUra and FdUrd by blocking the salvage of dThd (Miller et al., 1987; Schwartz et al., 1987). The previous in vitro data regarding the ability of DP to modulate the toxicity of fluoropyrimidines was obtained in exponentially growing cells. An additional observation that warrants consideration is a report that the inhibition of nucleoside incorporation by DP changed as a function of time in culture (Zhen et al., 1986). Hepatoma 3924A cells in lag and log phase were highly sensitive to DP with IC50 values for dThd incorporation of 0.2 and 0.32 microM, respectively. In contrast, stationary phase cells were insensitive to DP (IC50 = 38.9 microM). Amphotericin B, an antifungal agent which perturbs cell membranes, restored the sensitivity to DP in stationary cells. Several investigators have presented information on the effect of DP on fluoropyrimidines in normal tissues. Lee and Park (1987) examined the effect of DP on FUra and MTX toxicity in a soft-agar cloning assay against two human cancer cell lines and on pooled normal human bone marrow (CFU-C). DP (1 microM) potentiated the action of both MTX (0.1 microM) and FUra (5 microM) on Hep-2 (epidermoid carcinoma), MCF-7 (breast carcinoma) and CFU-C in medium supplemented with either non-dialyzed or dialyzed serum. Woodcock et al. (1987) incubated gallbladder mucosa, obtained from patients undergoing elective surgery for cholelithiasis, with control medium or varying concentrations of DP for 1 hr, and then exposed the mucosal cells to 2.5 microCi [3H]-FdUrd (2.5 microM). After 1 hr, the uptake of FdUrd into the tissue was inhibited to 49% and 42% of control by 0.1 microM and 1 microM, respectively.

Species differences in sensitivity of nucleoside transport in erythrocytes and cultured cells to inhibition by nitrobenzylthioinosine, dipyridamole, dilazep and lidoflazine

Differences in sensitivity of uridine transport in erythrocytes and cultured cells to inhibition by dipyridamole, dilazep and lidoflazine were largely species-specific; uridine transport in human cells, and probably in pig and rabbit cells, was 2-3- and 10-times more sensitive to inhibition by dipyridamole (IC50 approx. 50 nM) and about 10- and 20-times more sensitive to dilazep inhibition (IC50 approx. 5 nM) than transport in mouse and rat cells, respectively. Uridine transport in human erythrocytes and HeLa cells was strongly inhibited by lidoflazine (IC50 10-140 nM), whereas that in both mouse and rat cells was highly resistant (IC50 greater than 10 microM). Superimposed on species-specific differences were some cell type specific differences in sensitivity of nucleoside transport to these inhibitors. Uridine transport in Walker 256 rat carcinoma cells was more resistant to dipyridamole and dilazep than that of other rat cells. Transport in human Hep-2 cells was more resistant to lidoflazine (IC50 2000 nM) than that of human erythrocytes and HeLa cells, whereas it showed similar sensitivity to dilazep and dipyridamole. Uridine transport in Chinese hamster cells was also more resistant to dilazep than that of baby hamster kidney cells. In addition HeLa cells and clones thereof expressed uridine transporters (about 50% each) with difference of about 1000-fold in sensitivity to inhibition by dilazep (IC50 approx. 5 nM and 5 microM, respectively).

Membrane transport and the antineoplastic action of nucleoside analogues

This article summarizes recent studies characterizing nucleoside transport in mammalian cells and discusses evidence for a role of membrane transport in the pharmacologic action of nucleoside analogues. Some of these studies have also addressed the controversy concerning the multiplicity in transport routes. It seems clear that erythrocytes and, perhaps, some other mammalian cells possess a single, broadly specific system for transporting nucleosides. However, substantial evidence from valid studies discriminating between transport and intracellular metabolism suggests that at least some mammalian cells, including some tumor cells, possess more than a single system. Evidence now exists for a determining role of membrane transport of nucleoside analogues in their cytotoxicity and, in the case of one pyrimidine nucleoside (AraC), in therapeutic responsiveness in leukemic patients. There are also numerous examples of transport-related resistance to nucleoside analogues. Included in this article are the results of studies from the authors' laboratory pertaining to the therapeutic activity of the purine nucleoside, FAraA, in murine tumor models. These studies provide evidence for a determining role of both membrane transport and intracellular phosphorylation in the selective antitumor action of this agent against murine leukemia. Substantially increased transport inward of FAraA occurs at pharmacologically achievable concentrations of this agent in tumor cells as compared to drug-limiting, normal proliferative epithelium of the small intestine. The basis for this differential appears to be the kinetic duality of FAraA and adenosine transport inward found in tumor cells, but not in proliferative intestinal epithelial cells. Tumor cells have highly saturable (low influx Km) and poorly saturable (high influx Km) systems for adenosine transport, both of which are shared by FAraA. In contrast, proliferative epithelial cells have only a poorly saturable system for these substrates. If a similar kinetic duality of nucleoside transport is found in other tumor cells certain implications arise concerning the significance of the duality to neoplastic transformation.

Nitrobenzylthioinosinate binding sites in HeLa cells: relationship with ecto-5'-nucleotidase

Nitrobenzylthioinosine (NBTI), a substrate for the ecto-5'-nucleotidase of HeLa cells, was used to probe the relationship between ecto-5'-nucleotidase dephosphorylation site and the dephosphorylated NBTI binding site. It may be assumed that the dephosphorylation site of the enzyme is separate and functions independently of the NBTI binding site. Evidences supporting these conclusions were based on the following observations: i. NBTI-P dephosphorylation progressed with similar rates in the presence or absence of NBTI in the incubation medium; ii. adenosine-5'-monophosphate inhibited the dephosphorylation of NBTI-P but did not affect the binding of NBTI. The inhibition was competitive and dependent on concentration; iii. the effect of the NBTI nonisotopic medium on the binding of free G-3H-NBTI and G-3H-NBTI derived from G-3H-NBTI-P was different, and an instantaneous isotopic dilution was observed with G-3H-NBTI, but not with G-3H-NBTI-P, as substrate.

Effects of Ca2+-channel antagonists on nucleoside and nucleobase transport in human erythrocytes and cultured mammalian cells

Lidoflazine strongly inhibited the equilibrium exchange of uridine in human erythrocytes (Ki approximately 16 nM). Uridine zero-trans influx was similarly inhibited by lidoflazine in cultured HeLa cells (IC50 approximately to 80 nM), whereas P388 mouse leukemia and Novikoff rat hepatoma cells were three orders of magnitude more resistant (IC50 greater than 50 microM). Uridine transport was also inhibited by nifedipine, verapamil, diltiazem, prenylamine and trifluoperazine, but only at similarly high concentrations in both human erythrocytes and the cell lines. IC50 values ranged from about 10 microM for nifedipine and about 20 microM for verapamil to more than 100 microM for diltiazem, prenylamine and trifluoperazine. The concentrations required for inhibition of nucleoside transport are several orders higher than those blocking Ca2+ channels. Lidoflazine competitively inhibited the binding of nitrobenzylthioinosine to high-affinity sites in human erythrocytes, but did not inhibit the dissociation of nitrobenzylthioinosine from these sites on the transporter as is observed with dipyridamole and dilazep.

Incomplete nucleoside transport deficiency with increased hypoxanthine transport capability in mutant T-lymphoblastoid cells

From a mutagenized population of wild-type mouse (S49) T-lymphoma cells, a clone, 80-5D2, was isolated in a single step by virtue of its ability to survive in 80 nM 5-fluorouridine. Unlike previously isolated nucleoside transport-deficient cell lines (A. Cohen, B. Ullman, and D. W. Martin, Jr., J. Biol. Chem. 254:112-116, 1979), 80-5D2 cells were only slightly less sensitive to growth inhibition by a variety of cytotoxic nucleosides and were capable of proliferating in hypoxanthine-amethopterin-thymidine-containing medium. The molecular basis for the phenotype of 80-5D2 cells was incomplete deficiency in the ability of the mutant cells to translocate nucleosides across the plasma membrane. Interestingly, mutant cells were more capable than wild-type cells of transporting the nucleobase hypoxanthine. Residual transport of adenosine into 80-5D2 cells was just as sensitive to inhibition by nucleosides and more sensitive to inhibition by hypoxanthine than that in wild-type cells, indicating that the phenomena of ligand binding and translocation can be uncoupled genetically. The 80-5D2 cells lacked cell surface binding sites for the potent inhibitor of nucleoside transport p-nitrobenzylthioinosine (NBMPR) and, consequently, were largely resistant to the physiological effects of NBMPR. However, the altered transporter retained its sensitivity to dipyridamole, another inhibitor of nucleoside transport. The biochemical phenotype of the 80-5D2 cell line supports the hypothesis that the determinants that comprise the nucleoside carrier site, the hypoxanthine carrier site, the NBMPR binding site, and the dipyridamole binding site of the nucleoside transport function of mouse S49 cells are genetically distinguishable.

Uptake and metabolism of nucleosides by embryos of the sea urchin Strongylocentrotus purpuratus

Uptake and metabolism of thymidine and adenosine have been studied in embryos of the sea urchin Strongylocentrotus purpuratus. Uptake of these nucleosides is found to be mutually competitive, with the Km for uptake of thymidine similar to its Ki for inhibition of adenosine uptake and vice versa. The metabolic studies show that adenosine is rapidly and completely phosphorylated upon entry, even at high exogenous concentrations which saturate the uptake mechanism. In contrast, at concentrations which saturate nucleoside uptake, thymidine becomes appreciably catabolized (up to 60%) to thymine and beta-amino-isobutyric acid in addition to its phosphorylation to thymine nucleotides. Negligible amounts of endogenous thymidine appear to remain unmetabolized following uptake in these embryos. The data provide strong in vivo evidence for separate metabolic pathways for thymidine and adenosine which have not previously been described in this organism. The observation of mutual competition during uptake, together with different routes of metabolism for these nucleosides, would suggest that the rate-limiting step in the uptake process is transport rather than metabolism. The specificity of this transport system for its nucleoside substrate has been examined in some detail in the present report. All naturally occurring nucleosides but only a limited number of nucleoside analogs are recognized by this membrane carrier. Neither purine nor pyrimidine bases are substrates for this transport system. Previous work by this laboratory has demonstrated the strict Na+-dependence of this carrier, its high affinity for nucleoside substrate, and its activation at fertilization. These observations and the substrate specificity studies of the present work together describe a unique transport system for nucleosides in sea urchin embryos which is quite different from those previously described in mammalian cells.

Photoaffinity labelling of a nitrobenzylthioinosine-binding polypeptide from cultured Novikoff hepatoma cells

Site-specific binding of nitrobenzylthioinosine (NBMPR) to plasma membranes of some animal cells results in the inhibition of the facilitated diffusion of nucleosides. The present study showed that nucleoside transport in Novikoff UA rat hepatoma cells is insensitive to site-saturating concentrations of NBMPR. Equilibrium binding experiments demonstrated the presence of high-affinity sites for NBMPR in a membrane-enriched fraction from these cells. In the presence of uridine or dipyridamole, specific binding of NBMPR at these sites was inhibited. When Novikoff UA membranes were covalently labelled with [3H]NBMPR by using photoaffinity techniques, specifically bound radioactivity was incorporated exclusively into a polypeptide(s) with an apparent Mr of 72,000-80,000, determined by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. Covalent labelling of this polypeptide was abolished in the presence of excess nitrobenzylthioguanosine (NBTGR) and reduced in the presence of adenosine, uridine or dipyridamole. The apparent Mr of the NBMPR-binding polypeptide in Novikoff UA cells is significantly higher than that reported for corresponding polypeptides in other cell types (Mr 45,000-66,000). When membrane-enriched preparations from S49 mouse lymphoma cells were photolabelled and mixed with labelled NovikoffUA membrane-enriched preparations, gel electrophoresis resolved the NBMPR-binding polypeptides from the two preparations.

Incomplete nucleoside transport deficiency with increased hypoxanthine transport capability in mutant T-lymphoblastoid cells

From a mutagenized population of wild-type mouse (S49) T-lymphoma cells, a clone, 80-5D2, was isolated in a single step by virtue of its ability to survive in 80 nM 5-fluorouridine. Unlike previously isolated nucleoside transport-deficient cell lines (A. Cohen, B. Ullman, and D. W. Martin, Jr., J. Biol. Chem. 254:112-116, 1979), 80-5D2 cells were only slightly less sensitive to growth inhibition by a variety of cytotoxic nucleosides and were capable of proliferating in hypoxanthine-amethopterin-thymidine-containing medium. The molecular basis for the phenotype of 80-5D2 cells was incomplete deficiency in the ability of the mutant cells to translocate nucleosides across the plasma membrane. Interestingly, mutant cells were more capable than wild-type cells of transporting the nucleobase hypoxanthine. Residual transport of adenosine into 80-5D2 cells was just as sensitive to inhibition by nucleosides and more sensitive to inhibition by hypoxanthine than that in wild-type cells, indicating that the phenomena of ligand binding and translocation can be uncoupled genetically. The 80-5D2 cells lacked cell surface binding sites for the potent inhibitor of nucleoside transport p-nitrobenzylthioinosine (NBMPR) and, consequently, were largely resistant to the physiological effects of NBMPR. However, the altered transporter retained its sensitivity to dipyridamole, another inhibitor of nucleoside transport. The biochemical phenotype of the 80-5D2 cell line supports the hypothesis that the determinants that comprise the nucleoside carrier site, the hypoxanthine carrier site, the NBMPR binding site, and the dipyridamole binding site of the nucleoside transport function of mouse S49 cells are genetically distinguishable.

An assay for inhibitors of nucleoside transport based upon the use of 5-[125I]iodo-2'-deoxyuridine as permeant

5-[125I]Iodo-2'-deoxyuridine (IdUrd) has been shown to serve as a permeant for the nucleoside transport system of human erythrocytes and to be matabolically inert in these cells. Linear initial velocities were obtained at 20 degrees C for 125IdUrd transport, yielding a Km of 73 +/- 18 microM (n = 6). Low-affinity inhibitors of 125IdUrd transport, such as adenosine (Ki = 32 +/- 2 microM, n = 2), could be characterized by Michaelis-Menten kinetics. However, high-affinity inhibitors, such as 6-[(4-nitrobenzyl)thio]-9-beta-D-ribofuranosylpurine, caused nonlinear initial velocities when added to the cells simultaneously with 125IdUrd. Conditions were defined (viz., 20-min pretreatment of cells with test compound followed by 5.0-min incubation with 1.0 microM 125IdUrd, all at 20 degrees C) whereby high-affinity inhibitors of IdUrd transport can be identified and evaluated according to their 50% inhibitory concentrations. The use of 125IdUrd as permeant greatly expedites the testing of compounds as inhibitors of nucleoside transport by allowing the cell pellets generated in these assays to be monitored directly in a gamma spectrometer, thereby circumventing the solubilization and decolorization of cell pellets required by assays that use 3H- or 14C-labeled nucleoside permeants.

Mutant mouse cells with nitrobenzylthioinosine-insensitive nucleoside transport functions

From a mutagenized population of wildtype S49 T lymphoma cells, clones were generated that were resistant to the physiological effects of the potent inhibitor of nucleoside transport, 4-nitrobenzyl-6-thioinosine (NBMPR). NBMPR protected wildtype cells from the cytotoxic effects of a spectrum of nucleosides, whereas two mutant clones, KAB1 and KAB5, were still sensitive to nucleoside-mediated cytotoxicity in the presence of NBMPR. In addition, NBMPR prevented wildtype cells from surviving in hypoxanthine-amethopterin-thymidine containing medium, whereas KAB1 and KAB5 cells grew normally. Rapid sampling transport studies indicated that mutant cells, unlike wildtype parental cells, had acquired a substantial NBMPR-insensitive nucleoside transport component. Binding studies with [3H]NBMPR indicated that KAB5 cells were 70-75% deficient in the number of NBMPR binding sites, whereas KAB1 cells possessed a wildtype complement of NBMPR binding sites. The characterization of the KAB1 and KAB5 cell lines suggested that the NBMPR binding site in wildtype S49 cells is genetically distinguishable from the nucleoside carrier site.

Differential sensitivity of RSVts (temperature-sensitive Rous-sarcoma virus)-infected rat kidney cells to nucleoside antibiotics at permissive and non-permissive temperatures

Among a variety of anti-tumour agents tested, oxanosine and 5-azacytidine were found to be significantly more effective in inhibiting growth of rat kidney cells infected with a temperature-sensitive mutant of Rous sarcoma virus at a permissive temperature (33 degrees C) than at a non-permissive temperature (39 degrees C). These two nucleoside antibiotics were antagonistic to each other in cytotoxicity. They seem to share the same carrier-mediated membrane-transport system, because dipyridamole, a potent inhibitor of nucleoside transport, protected cells from the cytotoxicity of both drugs. Thymidine transport, which is twice as fast in cells at 33 degrees C as at 39 degrees C, was competitively inhibited by both drugs. Thus the differential toxicity of oxanosine and 5-azacytidine at the two temperatures is thought to be due to their increased transport via the thymidine-transport system, which is somehow under the influence of the active src-gene product.

Nucleoside transport in Walker 256 rat carcinosarcoma and S49 mouse lymphoma cells. Differences in sensitivity to nitrobenzylthioinosine and thiol reagents

The characteristics of nucleoside transport were examined in Walker 256 rat carcinosarcoma and S49 mouse lymphoma cells. In Walker 256 cells the initial rates of uridine, thymidine and adenosine uptake were insensitive to the nucleoside transport inhibitor nitrobenzylthioinosine (NBMPR) (1 microM), but were partially inhibited by dipyridamole (10 microM), another inhibitor of nucleoside transport. In contrast, the transport of these nucleosides in S49 cells was completely blocked by both inhibitors. Nucleoside transport in Walker 256 and S49 cells also differed in its sensitivity to the thiol reagent p-chloromercuribenzenesulphonate (pCMBS). Uridine transport in Walker 256 cells was inhibited by pCMBS with an IC50 (concentration producing 50% inhibition) of less than 25 microM, and inhibition was readily reversed by beta-mercaptoethanol. In S49 cells uridine transport was only inhibited at much higher concentrations of pCMBS (IC50 approximately equal to 300 microM). In other respects nucleoside transport in Walker 256 and S49 cells were quite similar. The Km and Vmax. values for uridine transport were nearly identical, and the transporters of both cell lines appeared to accept a broad range of nucleosides as substrates. Uridine transport in Walker 256 cells was non-concentrative and did not require an energy source. These studies demonstrate that nucleoside uptake in Walker 256 cells is mediated by a facilitated-diffusion mechanism which differs markedly from that of S49 cells in its sensitivity to the transport inhibitor NBMPR and the thiol reagent pCMBS.

Evidence for the asymmetrical binding of p-chloromercuriphenyl sulphonate to the human erythrocyte nucleoside transporter

Nucleosides cross the human erythrocyte membrane by a facilitated-diffusion process which is selectively inhibited by nanomolar concentrations of nitrobenzylthioinosine (NBMPR). The chemical asymmetry of the transporter was investigated by studying the effects of p-chloromercuriphenyl sulphonate (PCMBS) on uridine transport and high-affinity NBMPR binding in inside-out and right-side-out membrane vesicles, unsealed erythrocyte ghosts and intact cells. PCMBS was an effective inhibitor of the transporter (50% inhibition at 30 microM), but only when the organomercurial had access to the cytoplasmic membrane surface. PCMBS inhibition of NBMPR binding to ghosts was reversed by incubation with dithiothreitol. Both uridine and NBMPR were able to protect the transporter against PCMBS inhibition.

Nitrobenzylthioinosine-sensitive and -resistant nucleoside transport in normal and transformed rat cells

Cultured Novikoff rat hepatoma and Walker 256 carcinoma cells have previously been reported to express only nitrobenzylthioinosine (NBTI)-resistant uridine transport and to lack high affinity NBTI-binding sites, whereas the latter are common on all other types of cultured mammalian cells from different species [1-7) X 10(5) sites/cell) which have been investigated with the exception of a transport-deficient cell variant which lacks high-affinity NBTI-binding sites. The present study shows that lack of NBTI sensitivity of transport and of NBTI-binding sites in Novikoff and Walker 256 cells are not related to the species or tissue origin of these cells. Uridine transport in a variant (NRM) of Novikoff hepatoma cells, in HTC rat hepatoma cells, normal rat kidney (NRK) cells, rat erythrocytes and rat hepatocytes was inhibited 15-60% by 10-500 nM NBTI and the cells expressed high-affinity NBTI-binding sites (Kd = 0.1-0.6 nM). The apparent turnover numbers for the NBTI-sensitive nucleoside carriers fell into two classes, with those for transformed cells about 10-times higher than those for the normal rat cells.

Concentrative transport of purine nucleosides in brush border vesicles of the rat kidney

Nephrotoxicity has been reported in humans and in animals during treatment with nucleosides. We described recently a Na+ gradient-dependent concentrative transport of adenosine in brush border vesicles isolated from the cortex of the rat kidney. The results of the present study suggest that that transport is shared by other purine nucleosides. The transport of adenosine is inhibited by several purines at micromolar concentrations. The transports of deoxyadenosine, of inosine and of guanosine are concentrative in the presence of a Na+ gradient and they are inhibited by adenosine. We conclude that there exists a concentrative system for reabsorption of various purine nucleosides in the proximal tubule. The transport of nucleosides is non-concentrative in all other cell types studied so far. The concentrative capacity of the transport of nucleosides in the kidney might explain the particular sensitivity of that organ during treatment with nucleosides.

Evidence that ribavirin is transported in Aedes albopictus cells by the nucleoside transport system

Aedes albopictus cells (CAE-3-6) deficient in the transport of nucleosides showed a markedly impaired ability to transport the antiviral compound, ribavirin. Furthermore, in order to inhibit either cell growth or viral replication, the transport negative CAE-3-6 cells required significantly higher concentrations of ribavirin than were needed for a similar degree of inhibition in the parental C7 cells. Although ribavirin did lower the level of GTP in CAE-3-6 cells the effect was slower and less striking than in the parental cells. These results indicate that ribavirin is transported across the plasma membrane of Aedes albopictus cells by the same transport system which functions to transport naturally occurring nucleosides.

Dephosphorylation of nitrobenzylthioinosine 5'-monophosphate by ecto 5'-nucleotidase of HeLa cells

HeLa cells as well as human and mouse erythrocytes possess membrane sites which bind the inhibitor of nucleoside transport, nitrobenzylthioinosine (NBMPR), reversibly but tightly (KD, 10(-9)-10(-10) M). Site-specific binding of the ligand correlates with inhibition of nucleoside transport. The present study showed that the 5'-phosphate of NBMPR, NBMPR-P, was not transport inhibitory. Upon exposure to [35S]NBMPR-P or [G-3H]NBMPR-P, HeLa cells retained the isotopic labels virtually exclusively in the form of NBMPR. The dephosphorylation of [G-3H]NBMPR-P by HeLa cells, assayed by the production of extracellular [G-3H]NBMPR, was competitively inhibited by AMP, but was not affected by the presence of 5 microM NBMPR, a concentration sufficient to completely occupy the transport inhibitory sites. Thus, the sites at which dephosphorylation of NBMPR occurs in HeLa cells are separate from and function independently of the high affinity sites which bind NBMPR.

Hypoxanthine transport in mammalian cells: cell type-specific differences in sensitivity to inhibition by dipyridamole and uridine

We have measured by rapid kinetic techniques the zero-trans influx of hypoxanthine in various cell lines and its sensitivity to inhibition by uridine, dipyridamole, nitrobenzylthioinosine and nitrobenzylthiopurine. The results and those reported earlier divided the cells into two distinct groups. In mouse P388, L1210 and L929 cells uridine and hypoxanthine had little effect on the transport of each other, supporting the view that nucleosides and hypoxanthine are transported by different carriers. In these cells, hypoxanthine transport was also uniquely resistant to inhibition by dipyridamole (IC50 (50% inhibition dose) greater than 30 microM). In Novikoff and HTC rat hepatoma, Chinese hamster ovary and Ehrlich ascites tumor cells, on the other hand, hypoxanthine and uridine inhibited the transport of each other about 50% at a concentration corresponding to the Michaelis-Menten constant of their transport, and hypoxanthine transport was strongly inhibited by dipyridamole (IC50 = 100 to 400 nM). Although these results are compatible with the view that nucleosides and hypoxanthine are transported by a common carrier in these cells, this conclusion is not supported by the finding that uridine transport is strongly inhibited in some of these cell lines, as in the first group of cells, by nitrobenzylthioinosine, whereas hypoxanthine transport is highly resistant in all cell lines tested. In contrast, the transport of both substrates is highly resistant to inhibition by nitrobenzylthiopurine. The Michaelis-Menten constants for uridine transport are about the same in all cell lines. The Michaelis-Menten constants for hypoxanthine transport are similar to those for uridine transport in some cell lines, but are much higher in others.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of sulfhydryl reagents on nucleoside transport in cultured mammalian cells

Incubation of Novikoff rat hepatoma cells; mouse L929, P388 and L1210 cells; and Chinese hamster ovary cells with sulfhydryl reagents, such as p-hydroxymercuribenzoate or p-hydroxymercuribenzenesulfonate, reduced the zero-trans influx of uridine in a concentration-dependent manner. The sensitivity of uridine transport to inhibition varied somewhat for the cell lines, Chinese hamster ovary cells being the most sensitive. Maximum inhibition by p-hydroxymercuribenzoate occurred in 10-20 min of incubation at 37 degrees C, and was associated with a decrease in maximum transport velocity without significant change in substrate affinity of the carrier. The development of inhibition of uridine influx correlated with binding of [14C]p-hydroxymercuribenzoate to the cells. Inhibition of transport also roughly correlated with a decreased binding of 6-nitrobenzylthioinosine to high-affinity binding sites on the cells (presumably representing the nucleoside transporter) without affecting binding affinity. Treatment of cells with p-hydroxymercuribenzenesulfonate reduced uridine influx and efflux to a similar extent. Inhibition of uridine transport and binding of [14C]p-hydroxymercuribenzoate were readily reversed by incubation of the cells with dithiothreitol. The results indicate that sulfhydryl groups are essential for the functioning of the nucleoside transporter, perhaps for the binding of substrate. Blockage of the sulfhydryl groups results in a reversible inactivation of the carrier. Treatment of the cells with the sulfhydryl reagents also caused a concentration-dependent increase in cell volume, which was readily reversed by incubation of the cells with dithiothreitol but seemed unrelated to the inhibition of nucleoside transport.

Nucleoside transport in cultured mammalian cells. Multiple forms with different sensitivity to inhibition by nitrobenzylthioinosine or hypoxanthine

The zero-trans influx of 500 microM uridine by CHO, P388, L1210 and L929 cells was inhibited by nitrobenzylthioinosine ( NBTI ) in a biphasic manner; 60-70% of total uridine influx by CHO cells and about 90% of that in P388, L1210 and L929 cells was inhibited by nmolar concentrations of NBTI (ID50 = 3-10 nM) and is designated NBTI -sensitive transport. The residual transport activity, designated NBTI -resistant transport, was inhibited by NBTI only at concentrations above 1 microM (ID50 = 10-50 microM). S49 cells exhibited only NBTI -sensitive uridine transport, whereas Novikoff cells exhibited only NBTI -resistant uridine transport. In all instances NBTI -sensitive transport correlated with the presence of between 7 7 X 10(4) and 7 X 10(5) high-affinity NBTI binding sites/cell (Kd = 0.3-1 nM). Novikoff cells lacked such sites. The two types of nucleoside transport, NBTI -resistant and NBTI -sensitive, were indistinguishable in substrate affinity, temperature dependence, substrate specificity, inhibition by structurally unrelated substances, such as dipyridamole or papaverine, and inhibition by sulfhydryl reagents or hypoxanthine. We suggest, therefore, that a single nucleoside transporter can exist in an NBTI -sensitive and an NBTI -resistant form depending on its disposition in the plasma membrane. The sensitive form expresses a high-affinity NBTI binding site(s) which is probably made up of the substrate binding site plus a hydrophobic region which interacts with the lipophilic nitrobenzyl group of NBTI . The latter site seems to be unavailable in NBTI -resistant transporters. The proportion of NBTI -resistant and sensitive uridine transport was constant during proportion of NBTI -resistant and sensitive uridine transport was constant during progression of P388 cells through the cell cycle and independent of the growth stage of the cells in culture. There were additional differences in uridine transport between cell lines which, however, did not correlate with NBTI sensitivity and might be related to the species origin of the cells. Uridine transport in Novikoff cells was more sensitive to inhibition by dipyridamole and papaverine than that in all other cell lines tested, whereas uridine transport in CHO cells was the most sensitive to inactivation by sulfhydryl reagents.

Kinetics of nitrobenzylthioinosine binding to the human erythrocyte nucleoside transporter

The kinetics of [3H]nitrobenzylthioinosine binding to human erythrocyte membranes was studied. The pseudo-first-order association was linear and consistent with a simple bimolecular reaction mechanism between nitrobenzylthioinosine and the nucleoside-transport mechanism. Dissociation of the [3H]nitrobenzylthioinosine complex at 22 degrees C was also linear (apparent k-1 congruent to 0.20 min-1). Adenosine was a competitive inhibitor of equilibrium high-affinity [3H]nitrobenzylthioinosine-binding activity (apparent Ki 0.1 mM). Dissociation of the [3H]nitrobenzylthioinosine-membrane complex was faster in the presence of adenosine and uridine, and this effect was proportional to the nucleoside concentration. Nucleoside concentrations less than 1 mM had no significant effect on the dissociation rate constant. In contrast, dissociation was slower in the presence of high concentrations (micromolar) of dipyridamole. Low concentrations of dipyridamole (2-200 nM) and nitrobenzylthioinosine concentrations as high as 2.5 microM had no effect on the rate of [3H]nitrobenzylthioinosine dissociation. These results are discussed in terms of possible distinct inhibitor and permeation sites, and are suggested to be consistent with both a single-site model for the binding of nitrobenzylthioinosine and permeant to the same site, or an allosteric-site model in which permeant and inhibitor bind to different sites.

The kinetics of dissociation of the inhibitor of nucleoside transport, nitrobenzylthioinosine, from the high-affinity binding sites of cultured hamster cells

Nucleoside transport in various types of animal cells is inhibited by the binding of nitrobenzylthioinosine (NBMPR) to a set of high-affinity sites on the plasma membrane. This work examined the binding of [3H]NBMPR to the nucleoside transporters of cultured Nil 8 hamster fibroblasts and of cells of a virus-transformed clone (Nil SV) derived from Nil 8. Experiments conducted with intact Nil 8 and Nil SV cells and with membrane preparations indicated that the two lines differed significantly in the cellular content of binding sites and only slightly in the affinities of these sites for NBMPR. Nil 8 and Nil SV cells possessed (4.2-8.0) X 10(5) and (2.0-4.0) X 10(6) sites per cell respectively, whereas the dissociation constants of site-bound NBMPR obtained with intact cells and with membrane preparations were similar, ranging from 0.29 to 1.5 nM. Dilazep, a potent inhibitor of nucleoside transport that is structurally unrelated to NBMPR, appeared to compete with NBMPR for binding to the high-affinity sites when tested under equilibrium conditions with Ki values for inhibition of NBMPR binding to Nil 8 and Nil SV cells respectively of 15 +/- 4 and 32 +/- 4 nM. The dissociation of NBMPR from the binding site--NBMPR complex of Nil SV membrane preparations was a first-order decay process with a rate constant of 0.68 +/- 0.26 min-1. The rate of dissociation of NBMPR from the binding-site complex of membrane preparations and intact cells was decreased significantly in the presence of dilazep and increased in the presence of the permeant uridine. These results suggest that the apparent competitive-inhibition kinetics obtained for dilazep under equilibrium conditions should not be interpreted as binding of dilazep to the same site as NBMPR but rather as binding of the two inhibitors to closely associated sites on the nucleoside transporter. Similarly, uridine also appears to bind to a site separate from the NBMPR-binding site.

Kinetic and thermodynamic studies on nitrobenzylthioinosine binding to the nucleoside transporter of Chinese hamster ovary cells

The binding of [G-3H]nitrobenzylthioinosine to intact Chinese hamster ovary cells has been studied kinetically and thermodynamically. The association of nitrobenzylthioinosine with cells is a second-order process which proceeds at 24 degrees C with a rate constant of 2 X 10(7) M-1 X S-1. Dissociation of the complex was characterized as a simple first-order process with rate constant on the order of 7 X 10(-3)S-1. The quotient of these is comparable to the dissociation constant as measured in equilibrium binding studies, 2.2 X 10(-10) M. The temperature dependence of the rate of association indicated an Arrhenius activation energy of 8.4 kcal X mol-1, while that of the equilibrium constant for dissociation indicated a standard enthalpy change of 8.8 kcal X mol-1. The large increase in affinity of nitrobenzylthioinosine as compared to natural nucleosides is attributable to an entropy-driven interaction with the binding site. Thymidine, dipyridamole and papaverine each decrease the apparent dissociation constant for the nitrobenzylthioinosine-cell complex; the latter, inhibitors of nucleoside transport, decrease the rate of dissociation of the complex.

Adenine transport and binding in cultured mammalian cells deficient in adenine phosphoribosyltransferase

Rapid kinetic techniques were employed to measure the transport of adenine in adenine phosphoribosyltransferase-deficient L929 and Chinese hamster ovary (CHO) cells in zero-trans entry and exit and equilibrium exchange procedures. The kinetic parameters of transport were computed by fitting appropriate integrated rate equations to time courses of transmembrane equilibration of radiolabeled adenine. Adenine transport conformed to the simple carrier model with directional symmetry and equal mobility of loaded and empty carrier. The Michaelis-Menten constants and maximum velocities for various strains of L929 cells fell between 2.3 and 3.5 mM and 90 and 150 pmol/microliters of cell water per s, respectively, values similar to those previously reported for CHO and Novikoff hepatoma cells. The corresponding values for hypoxanthine transport in L929 cells were 413 microM and 16 pmol/microliters of cell water per s. Adenine transport velocities were directly proportional to adenine concentrations between 0.03 and 50 microM in both CHO and Novikoff cells. The results indicate that adenine is transported in these cells by a single, low-affinity, high-capacity transporter. Adenine transport was inhibited by hypoxanthine in some cell strains, but not in others. Adenine also rapidly bound to L929 cells in a saturable manner (KD = 18 microM), presumably to the cell surface (about 3 X 10(7) sites per cell).

Nitrobenzylthioinosine-insensitive uridine transport in human lymphoblastoid and murine leukemia cells

Both human lymphoblastoid (RPMI 6410) and murine leukemia (L1210) cells were found to have a component of uridine transport which is insensitive to the nucleoside transport inhibitor nitrobenzylthioinosine (NBMPR). In both cell lines NBMPR-insensitive uridine transport is inhibited by other nucleosides and by the sulfhydryl reagent p-chloromercuribenzenesulfonate. In RPMI 6410 cells NBMPR-insensitive transport accounts for only 2% of the initial rate of uridine transport. In contrast, 20% of the initial rate of transport of L1210 cells is insensitive to NBMPR, and uridine uptake over longer periods (10 min) is completely insensitive to NBMPR.

Adenine transport and binding in cultured mammalian cells deficient in adenine phosphoribosyltransferase

Rapid kinetic techniques were employed to measure the transport of adenine in adenine phosphoribosyltransferase-deficient L929 and Chinese hamster ovary (CHO) cells in zero-trans entry and exit and equilibrium exchange procedures. The kinetic parameters of transport were computed by fitting appropriate integrated rate equations to time courses of transmembrane equilibration of radiolabeled adenine. Adenine transport conformed to the simple carrier model with directional symmetry and equal mobility of loaded and empty carrier. The Michaelis-Menten constants and maximum velocities for various strains of L929 cells fell between 2.3 and 3.5 mM and 90 and 150 pmol/microliters of cell water per s, respectively, values similar to those previously reported for CHO and Novikoff hepatoma cells. The corresponding values for hypoxanthine transport in L929 cells were 413 microM and 16 pmol/microliters of cell water per s. Adenine transport velocities were directly proportional to adenine concentrations between 0.03 and 50 microM in both CHO and Novikoff cells. The results indicate that adenine is transported in these cells by a single, low-affinity, high-capacity transporter. Adenine transport was inhibited by hypoxanthine in some cell strains, but not in others. Adenine also rapidly bound to L929 cells in a saturable manner (KD = 18 microM), presumably to the cell surface (about 3 X 10(7) sites per cell).

The hypoxanthine transporter of Novikoff rat hepatoma cells exhibits directional symmetry and equal mobility when empty or substrate-loaded

The kinetics of hypoxanthine transport were measured in hypoxanthine phosphoribosyltransferase-deficient Novikoff cells by rapid kinetic techniques applying both zero-trans and equilibrium exchange protocols. The data indicate operation of a simple carrier with directional symmetry and equal mobility when substrate loaded and empty. Zero-trans influx and efflux were about equivalent and so were zero-trans influx and equilibrium exchange flux. The apparent Michaelis-Menten constant and maximum velocity were about 500 microM and 100 pmol/s per microliter cell H2O, respectively. The time courses of accumulation of radioactively labeled hypoxanthine at a concentration above the Michaelis-Menten constant differed noticeably in zero-trans and equilibrium exchange mode, but computer simulations showed that the difference is predicted by the symmetrical carrier model and does not reflect trans-stimulation.

Erythrocyte nucleoside transport: asymmetrical binding of nitrobenzylthioinosine to nucleoside permeation sites

1. Nitrobenzylthioinosine is a potent and specific inhibitor of nucleoside translocation in animal cells. Kinetic and inhibitor binding studies were undertaken to clarify how this inhibitor interacts with the nucleoside transporter from human and nucleoside-permeable type sheep erythrocytes.2. [(3)H]nitrobenzylthioinosine inhibition of zero-trans [U-(14)C]uridine influx into nucleoside-permeable type sheep cells was consistent with simple competitive inhibition (apparent K(i) 1 nmol/l). Analysis of results using total inhibitor levels instead of cell-free inhibitor concentrations did not affect the inhibition pattern, but increased the apparent K(i) value by 5-fold.3. In contrast, [(3)H]nitrobenzylthioinosine was a non-competitive inhibitor of zero-trans [U-(14)C]uridine efflux (apparent Ki 1.5 nmol/l). Dipyridamole, another potent inhibitor of nucleoside translocation, also inhibited zero-trans [U-(14)C]uridine influx in a competitive manner (apparent K(i) 20-40 nmol/l).4. [(3)H]nitrobenzylthioinosine bound to high-affinity sites on cell membranes from human and nucleoside-permeable type sheep cells (apparent K(D) values approximately 1 nmol/l). Binding of inhibitor to these sites was competitively blocked by uridine, a well characterized substrate for the nucleoside transporter (apparent K(i) 1.25 and 0.9 mmol/l, respectively). These apparent K(i) values are close to the apparent K(m) for uridine equilibrium exchange in human erythrocytes.5. Similarly, deoxycytidine was found to be a competitive inhibitor of high-affinity [(3)H]nitrobenzylthioinosine binding activity (apparent K(i) 1.0 and 1.2 mmol/l for human and nucleoside-permeable type sheep cell membranes, respectively). This contrasts with a previous report that this nucleoside had no effect on inhibitor binding activity. Transport studies confirmed that deoxycytidine is a substrate for the erythrocyte nucleoside transporter. Apparent K(m) and V(max) values for [U-(14)C]-deoxycytidine zero-trans influx into human and nucleoside-permeable type sheep cells were comparable to those obtained for [U-(14)C]uridine.6. It is suggested from these results that nitrobenzylthioinosine competes directly with nucleosides for the permeation site of the nucleoside transporter, but that inhibitor binds preferentially to the external membrane surface.

Absence of binding sites for the transport inhibitor nitrobenzylthioinosine on nucleoside transport-deficient mouse lymphoma cells

Cells of an adenosine-resistant clone (AE1) of S49 mouse lymphoma cells were compared with cells of the parental line with respect to (a) characteristics of nucleoside transport, (b) high affinity binding of the inhibitor of nucleoside transport, nitrobenzylthioinosine (NBMPR), and (c) the antiproliferative effects of the nucleoside antibiotics, tubercidin, arabinosyladenine and showdomycin. Rates of inward transport of uridine, thymidine, adenosine, 2'-deoxyadenosine, tubercidin, showdomycin, and arabinosyladenine in AE1 cells were less than 1% of those in cells of the parental S49 line. The inhibitor of nucleoside transport, NBMPR, reduced rates of inward nucleoside transport in S49 cells to levels comparable to those seen in the transport-defective mutant. S49 cells possessed high affinity sites that bound NBMPR (6.6 X 10(4) sites/cell, Kd = 0.2 nM), whereas site-specific binding of NBMPR to AE1 cells was not demonstrable, indicating that loss of nucleoside transport activity in AE1 cells was accompanied by loss of the high affinity NBMPR binding sites. Relative to S49 cells, AE1 cells were resistant to the antiproliferative effects of tubercidin and showdomycin, but differences between the two cell lines in sensitivity toward arabinosyladenine were minor, suggesting that nucleoside transport activity was required for cytotoxicity of tubercidin and showdomycin, but not for that of arabinosyladenine.

Binding of nitrobenzylthioinosine to high-affinity sites on the nucleoside-transport mechanism of HeLa cells

Nitrobenzylthioinosine (NBMPR) binds reversibly, but with high affinity (Kd 0.1--1.2 nM), to inhibitory sites on nucleoside-transport elements of the plasma membrane in a variety of animal cells. The present study explored relationships in HeLa cells between NBMPR binding and inhibition of uridine transport. The Km value for inward transport of uridine by HeLa cells in both suspension and monolayer culture was about 0.1 mM. The affinity of the transport-inhibitory sites for uridine (Kd 1.7 mM), inosine (Kd 0.4 mM) and other nucleoside permeants was low relative to that for NBMPR. The pyrimidine homologue of NBMPR, nitrobenzylthiouridine, also exhibited low affinity for the NBMPR-binding sites. Pretreatment of HeLa cells with p-chloromercuribenzene sulphonate (p-CMBS) or N-ethylmaleimide (NEM) decreased binding of NBMPR to its high-affinity sites and inhibited uridine transport, indicating the presence of thiol groups essential to both processes. NEM, a more penetrable reagent than p-CMBS, inhibited binding and transport at much lower concentrations than the latter compound. Pretreatment of cells with concentrations of p-CMBS that alone had no effect on either NBMPR binding or uridine transport increased the sensitivity of transport to NBMPR inhibition and changed the shape of the NBMPR concentration-effect curve, suggesting synergistic inhibiton of uridine-transport activity by these two agents.

Distinct mechanisms of hypoxanthine and inosine transport in membrane vesicles isolated from Chinese hamster ovary and Balb 3T3 cells

Both enzyme-mediated group translocation and facilitated diffusion have been proposed as mechanisms by which mammalian cells take up purine bases and nucleosides. We have investigated the mechanisms for hypoxanthine and inosine transport by using membrane vesicles from Chinese hamster ovary cells (CHO), Balb/c 3T3 and SV3T3 cells prepared by identical procedures. Uptake mechanisms were characterized by analyzing intravesicular contents, determining which substrates could exchange with the transport products, assaying for hypoxanthine phosphoribosyltransferase activity, and measuring the stimulation of uptake of hypoxanthine by phosphoribosyl pyrophosphate (PRib-PP). We found that the uptake of hypoxanthine in Balb 3T3 vesicles was stimulated 3--4-fold by PRib-PP. The intravesicular product was predominantly IMP. The hypoxanthine phosphoribosyltransferase activity copurified with the vesicle preparation. These results suggest the possible involvement of this enzyme in hypoxanthine uptake in 3T3 vesicles. In contrast to the 3T3 vesicles, CHO vesicles prepared under identical procedures did not retain hypoxanthine phosphoribosyltransferase activity and did not demonstrate PRib-PP-stimulated hypoxanthine uptake. The intravesicular product of hypoxanthine uptake in CHO vesicles was hypoxanthine. These results and data from our kinetic and exchange studies indicated that CHO vesicles transport hypoxanthine via facilitated diffusion. An analogous situation was observed for inosine uptake; CHO vesicles accumulated inosine via a facilitated diffusion mechanism, while in the same experiments SV3T3 vesicles exhibited a purine nucleoside phosphorylase-dependent translocation of the ribose moiety of inosine. Vesicles prepared from a CHO cell line temperature-sensitive for hypoxanthine uptake (Azarts) showed a temperature-sensitivity in Km for uptake parallel to that of the intact cells. This suggests that the defect in Azarts may be caused by a missense mutation in the gene coding for the hypoxanthine transport carrier.

Facilitated transport of inosine and uridine in cultured mammalian cells is independent of nucleoside phosphorylases

The zero-trans uptake of uniformly and base-labeled inosine and uridine was measured a 25 degrees C in suspensions of Novikoff rat hepatoma cells, Chinese hamster ovary cells, mouse L cells, mouse S49 lymphoma cells and a purine-nucleoside phosphorylase-deficient subline thereof (NSU-1), and in monolayer culture of mouse 3T3 and L cells. The initial velocities of uptake of both nucleosides were about the same in all cell lines investigated, regardless of the position of the label or of the substrate concentration between 3 and 300 microM or whether or not the cells possessed uridine or purine-nucleoside phosphorylase activity. The kinetic parameters for the facilitated transport of uridine and inosine were also similar in phosphorylase positive and negative cell lines (K = 120--260 microM and V = 6--40 pmol/microliters cell water per s) and the transport activities of the cells exceeded their total phosphorylase activities by at least 10-fold for uridine and 1--2-fold for inosine. Chromatographic fractionation of the intracellular contents and of the culture fluid showed that the free nucleosides appeared intracellularly prior to and more rapidly than their phosphorolysis products. During the initial 20--60 s of uptake of U-14C-labeled nucleosides the rates of intracellular appearance of ribose-1-P and base were about the same. After several minutes of incubation, on the other hand, the main intracellular component was ribose-1-P whereas the base attained a low intracellular steady-state concentration and accumulated in the medium due to exit transport. Other nucleosides, dipyridamole and nitrobenzylthioinosine, specifically inhibited the transport of uridine and inosine, and depressed the intracellular accumulation of ribose-1-P and the formation of base commensurate with that inhibition. The data indicate that the metabolism of inosine and uridine by the various cell lines can be entirely accounted for by the facilitated transport of unmodified nucleoside into the cell followed by intracellular phosphorolysis.

S-substituted derivatives of 6-mercaptopurine ribosides interact both with the transport and metabolic phosphorylation of uridine by virus-transformed hamster fibroblasts

The uptake of uridine by mammalian cells consists of transport of uridine across the plasma membrane followed by its metabolic conversion, mainly by phosphorylation. S-substituted aromatic derivatives of 6-mercaptopurine ribosides are potent inhibitors of the nucleoside uptake systems in human erythrocytes and in mammalian cells in culture and have been studied extensively. We present here a theoretical analysis which enables one to decide whether transport of metabolites, their metabolic trapping within the cell, or both, are susceptible to inhibition. This analysis was applied in the study of the effect of some inhibitors on uridine and cytosine-beta-D-arabinoside uptake by transformed Nil-8 cells. It was found that in Nil-SV cells, both transport and metabolic conversion are susceptible to inhibition by nitrobenzylmercaptoinosine and by dansylaminoethylmercaptoguanosine. Nitrobenzylmercaptoinosine displays inhibition constants of 20 and 7 nM for transport and phosphorylation, respectively, while for dansylaminoethylmercaptoguanosine the inhibition constants are 1.8 and 0.6 microM, respectively, for the same processes. Cytosine-beta-D-arabinoside is a synthetic nucleoside which is not metabolizable in Nil cells. Its uptake properties are determined by the transport mechanism alone. The transport of this nucleoside into Nil-SV cells in inhibited by nitrobenzylmercaptoinosine and the inhibition constant found is approx. 5 times greater than that for uridine.