High affinity sodium-dependent nucleobase transport in cultured renal epithelial cells (LLC-PK1)

SLC23A3 is a renal hypoxanthine transporter

LLC-PK1 renal cells show Na⁺-dependent and Na⁺-independent hypoxanthine uptake. While the latter is inhibited by adenine, neither are inhibited by xanthine. In rats, intestinal Na⁺-dependent hypoxanthine transporter Slc23a4 is not expressed in the kidney, and its action is inhibited by xanthine. This study aimed to clone Slc23a4-paralog SLC23A3 from the human kidney and investigate its hypoxanthine transport activity. We observed Na⁺-dependent 10 nM [³H]-hypoxanthine uptake in SLC23A3 RNA-injected Xenopus oocytes. Moreover, 100 μM xanthine did not inhibit Na⁺-independent 300 nM [³H]-hypoxanthine uptake, whereas 100 μM adenine did. These results confirm that SLC23A3 is a hypoxanthine transporter in the human kidney.

Molecular Basis of Nucleobase Transport Systems in Mammals

Nucleobases are water-soluble compounds that need specific transporters to cross biological membranes. Cumulative evidence based on studies using animal tissues and cells indicates that the carrier-mediated transport systems for purine and pyrimidine nucleobases can be classified into the following two types: concentrative transport systems that mediate nucleobase transport depending on the sodium ion concentration gradient; and other systems that mediate facilitated diffusion depending on the concentration gradient of the substrate. Recently, several molecular transporters that are involved in both transport systems have been identified. The function and activity of these transporters could be of pharmacological significance considering the roles that they play not only in nucleotide synthesis and metabolism but also in the pharmacokinetics and delivery of a variety of nucleobase analogues used in anticancer and antiviral drug therapy. The present review provides an overview of the recent advances in our understanding of the molecular basis of nucleobase transport systems, focusing on the transporters that mediate purine nucleobases, and discusses the involvement of intracellular metabolism in purine nucleobase transport and chemotherapy using ganciclovir.

Pyrimidine metabolism in schistosomes: A comparison with other parasites and the search for potential chemotherapeutic targets

Schistosomes are responsible for the parasitic disease schistosomiasis, an acute and chronic parasitic ailment that affects >240 million people in 70 countries worldwide. It is the second most devastating parasitic disease after malaria. At least 200,000 deaths per year are associated with the disease. In the absence of the availability of vaccines, chemotherapy is the main stay for combating schistosomiasis. The antischistosomal arsenal is currently limited to a single drug, Praziquantel, which is quite effective with a single-day treatment and virtually no host-toxicity. Recently, however, the question of reduced activity of Praziquantel has been raised. Therefore, the search for alternative antischistosomal drugs merits the study of new approaches of chemotherapy. The rational design of a drug is usually based on biochemical and physiological differences between pathogens and host. Pyrimidine metabolism is an excellent target for such studies. Schistosomes, unlike most of the host tissues, require a very active pyrimidine metabolism for the synthesis of DNA and RNA. This is essential for the production of the enormous numbers of eggs deposited daily by the parasite to which the granulomas response precipitates the pathogenesis of schistosomiasis. Furthermore, there are sufficient differences between corresponding enzymes of pyrimidine metabolism from the host and the parasite that can be exploited to design specific inhibitors or "subversive substrates" for the parasitic enzymes. Specificities of pyrimidine transport also diverge significantly between parasites and their mammalian host. This review deals with studies on pyrimidine metabolism in schistosomes and highlights the unique characteristic of this metabolism that could constitute excellent potential targets for the design of safe and effective antischistosomal drugs. In addition, pyrimidine metabolism in schistosomes is compared with that in other parasites where studies on pyrimidine metabolism have been more elaborate, in the hope of providing leads on how to identify likely chemotherapeutic targets which have not been looked at in schistosomes.

Comparative in vitro evaluation of transportability and toxicity of capecitabine and its metabolites in cells derived from normal human kidney and renal cancers

The goal of this study was to understand roles of nucleoside and nucleobase transport processes in capecitabine pharmacology in cells derived from human renal proximal tubule cells (hRPTCs) and three human renal cell carcinoma (RCC) cell lines, A498, A704, and Caki-1. Human equilibrative nucleoside transporters 1 and 2 (hENT1 and hENT2) mediated activities and a sodium-independent nucleobase activity were present in hRPTCs. In hRPTCs, uptake of 5′-deoxy-5-fluorouridine (DFUR), a nucleoside metabolite of capecitabine, was pH dependent with highest uptake seen at pH 6.0. In RCC cell lines, hENT1 was the major nucleoside transporter. Nucleobase transport activity was variable among the three RCC cell lines, with Caki-1 showing the highest and A498 showing the lowest activities. Treatment of RCC cell lines with interferon alpha (IFN-α) increased thymidine phosphorylase levels and prior treatment of RCC cell lines with IFN-α followed by 5-FU or DFUR resulted in enhanced sensitivity of all cell lines to 5-FU and two of three cell lines to DFUR. We report for the first time a nucleobase transport activity in hRPTCs and RCC cell lines. In addition, our in vitro cytotoxicity results showed that RCC cell lines differed in their response to 5-FU and DFUR and prior treatment with IFN-α potentiated cytotoxic response to metabolites of capecitabine.

The role of nucleotides in the immune and gastrointestinal systems: potential clinical applications

Nucleotides are low molecular weight biological molecules key to biochemical processes. Sources include de novo synthesis, recovery via salvage mechanisms, and dietary intakes. Although endogenous production serves as the main nucleotide source, evidence suggests that exogenous sources are essential to immune competence, intestinal development, and recovery. Dietary nucleotides serve a marked role in rapidly proliferating cells where they are necessary for optimal function. Accordingly, dietary nucleotides are deemed conditionally essential in the presence of various physiological stresses, including growth and development, recovery from injury, infection, and certain disease states. Clinical studies that evaluated nutrition formulations of nucleotides in combination with other specific nutrient substances demonstrated improved clinical outcomes in patients characterized as critically ill, injured, immune suppressed, or with chronic gastrointestinal conditions. However, conclusions regarding specific benefits of nucleotides are limited. Scientific substantiation of nucleotide supplementation in infant formula has been reported to improve the maturation and development of the intestinal tract as well as immune function. All medical nutrition products except for one immune-modulating formulation are devoid of nucleotides. In an effort to build on this, the current review presents the data to support potential clinical applications for nucleotides in enteral nutrition that may contribute to improved outcomes in physiologically stressed patients.

Identification of agents that reduce renal hypoxia-reoxygenation injury using cell-based screening: purine nucleosides are alternative energy sources in LLC-PK1 cells during hypoxia

Acute tubular necrosis is a clinical problem that lacks specific therapy and is characterized by high mortality rate. The ischemic renal injury affects the proximal tubule cells causing dysfunction and cell death after severe hypoperfusion. We utilized a cell-based screening approach in a hypoxia-reoxygenation model of tubular injury to search for cytoprotective action using a library of pharmacologically active compounds. Oxygen-glucose deprivation (OGD) induced ATP depletion, suppressed aerobic and anaerobic metabolism, increased the permeability of the monolayer, caused poly(ADP-ribose) polymerase cleavage and caspase-dependent cell death. The only compound that proved cytoprotective either applied prior to the hypoxia induction or during the reoxygenation was adenosine. The protective effect of adenosine required the coordinated actions of adenosine deaminase and adenosine kinase, but did not requisite the purine receptors. Adenosine and inosine better preserved the cellular ATP content during ischemia than equimolar amount of glucose, and accelerated the restoration of the cellular ATP pool following the OGD. Our results suggest that radical changes occur in the cellular metabolism to respond to the energy demand during and following hypoxia, which include the use of nucleosides as an essential energy source. Thus purine nucleoside supplementation holds promise in the treatment of acute renal failure.

Nucleobase transport by human equilibrative nucleoside transporter 1 (hENT1)

The human equilibrative nucleoside transporters hENT1 and hENT2 (each with 456 residues) are 40% identical in amino acid sequence and contain 11 putative transmembrane helices. Both transport purine and pyrimidine nucleosides and are distinguished functionally by a difference in sensitivity to inhibition by nanomolar concentrations of nitrobenzylmercaptopurine ribonucleoside (NBMPR), hENT1 being NBMPR-sensitive. Previously, we used heterologous expression in Xenopus oocytes to demonstrate that recombinant hENT2 and its rat ortholog rENT2 also transport purine and pyrimidine bases, h/rENT2 representing the first identified mammalian nucleobase transporter proteins (Yao, S. Y., Ng, A. M., Vickers, M. F., Sundaram, M., Cass, C. E., Baldwin, S. A., and Young, J. D. (2002) J. Biol. Chem. 277, 24938-24948). The same study also revealed lower, but significant, transport of hypoxanthine by h/rENT1. In the present investigation, we have used the enhanced Xenopus oocyte expression vector pGEMHE to demonstrate that hENT1 additionally transports thymine and adenine and, to a lesser extent, uracil and guanine. Fluxes of hypoxanthine, thymine, and adenine by hENT1 were saturable and inhibited by NBMPR. Ratios of V(max) (pmol/oocyte · min(-1)):K(m) (mm), a measure of transport efficiency, were 86, 177, and 120 for hypoxantine, thymine, and adenine, respectively, compared with 265 for uridine. Hypoxanthine influx was competitively inhibited by uridine, indicating common or overlapping nucleobase and nucleoside permeant binding pockets, and the anticancer nucleobase drugs 5-fluorouracil and 6-mercaptopurine were also transported. Nucleobase transport activity was absent from an engineered cysteine-less version hENT1 (hENT1C-) in which all 10 endogenous cysteine residues were mutated to serine. Site-directed mutagenesis identified Cys-414 in transmembrane helix 10 of hENT1 as the residue conferring nucleobase transport activity to the wild-type transporter.

Identification and functional characterization of the first nucleobase transporter in mammals: implication in the species difference in the intestinal absorption mechanism of nucleobases and their analogs between higher primates and other mammals

Nucleobases are important compounds that constitute nucleosides and nucleic acids. Although it has long been suggested that specific transporters are involved in their intestinal absorption and uptake in other tissues, none of their molecular entities have been identified in mammals to date. Here we describe identification of rat Slc23a4 as the first sodium-dependent nucleobase transporter (rSNBT1). The mRNA of rSNBT1 was expressed highly and only in the small intestine. When transiently expressed in HEK293 cells, rSNBT1 could transport uracil most efficiently. The transport of uracil mediated by rSNBT1 was sodium-dependent and saturable with a Michaelis constant of 21.2 microM. Thymine, guanine, hypoxanthine, and xanthine were also transported, but adenine was not. It was also suggested by studies of the inhibitory effect on rSNBT1-mediated uracil transport that several nucleobase analogs such as 5-fluorouracil are recognized by rSNBT1, but cytosine and nucleosides are not or only poorly recognized. Furthermore, rSNBT1 fused with green fluorescent protein was mainly localized at the apical membrane, when stably expressed in polarized Madin-Darby canine kidney II cells. These characteristics of rSNBT1 were almost fully in agreement with those of the carrier-mediated transport system involved in intestinal uracil uptake. Therefore, it is likely that rSNBT1 is its molecular entity or at least in part responsible for that. It was also found that the gene orthologous to the rSNBT1 gene is genetically defective in humans. This may have a biological and evolutional meaning in the transport and metabolism of nucleobases. The present study provides novel insights into the specific transport and metabolism of nucleobases and their analogs for therapeutic use.

Characterization of nucleobase transport by mouse Sertoli cell line TM4

In the spermatogenesis, many nucleosides and nucleobases are needed for the salvage nucleotide biosynthesis. One of the roles of Sertoli cells is to provide such nutrients to spermatogenic cells located within the blood-testis barrier (BTB). We have already shown that nucleoside transporters are expressed and are functional in primary-cultured rat Sertoli cells and TM4 cells derived from mouse testis. Here, we examined the uptakes of purine ([3H]guanine) and pyrimidine ([3H]uracil) nucleobases using TM4 cells. Uptakes of both nucleobases were time- and concentration-dependent, and kinetic analysis indicated the involvement of high-affinity transport systems. Uptake of uracil was significantly reduced in the absence of Na+, although guanine uptake was mainly mediated by a sodium-independent transport system in TM4 cells. Guanine uptake was inhibited by other purine nucleobases, but not by pyrimidine nucleobases. Only pyrimidine nucleobases reduced uracil uptake. In addition, mycophenolic acid, an inosine monophosphate dehydrogenase inhibitor, up-regulated guanine uptake. These results suggested that there are distinct transport systems for purine and pyrimidine nucleobases in cells of mouse Sertoli cell line TM4.

A purine-selective nucleobase/nucleoside transporter in PK15NTD cells

Nucleoside and nucleobase transporters are important for salvage of purines and pyrimidines and for transport of their analog drugs into cells. However, the pathways for nucleobase translocation in mammalian cells are not well characterized. We identified an Na-independent purine-selective nucleobase/nucleoside transport system in the nucleoside transporter-deficient PK15NTD cells. This transport system has 1,000-fold higher affinity for nucleobases than nucleosides with K(m) values of 2.5 +/- 0.7 microM for [(3)H]adenine, 6.4 +/- 0.5 microM for [(3)H]guanine, 1.1 +/- 0.1 mM for [(3)H]guanosine, and 4.2 +/- 0.5 mM [(3)H]adenosine. The uptake of [(3)H]guanine (0.05 microM) was inhibited by other nucleobases and nucleobase analog drugs (at 0.5-1 mM in the order of potency): 6-mercaptopurine = thioguanine = guanine > adenine >>> thymine = fluorouracil = uracil. Cytosine and methylcytosine had no effect. Nucleoside analog drugs with modification at 2' and/or 5 positions (all at 1 mM) were more potent than adenosine in competing the uptake of [(3)H]guanine: 2-chloro-2'-deoxyadenosine > 2-chloroadenosine > 2'3'-dideoxyadenosine = 2'-deoxyadenosine > 5-deoxyadenosine > adenosine. 2-Chloro-2'-deoxyadenosine and 2-chloroadenosine inhibited [(3)H]guanine uptake with IC(50) values of 68 +/- 5 and 99 +/- 10 microM, respectively. The nucleobase/nucleoside transporter was resistant to nitrobenzylthioinosine {6-[(4-nitrobenzyl) thiol]-9-beta-D-ribofuranosylpurine}, dipyridamole, and dilazep, but was inhibited by papaverine, the organic cation transporter inhibitor decynium-22 (IC(50) of approximately 1 microM), and by acidic pH (pH = 5.5). In conclusion, we have identified a mammalian purine-selective nucleobase/nucleoside transporter with high affinity for purine nucleobases. This transporter is potentially important for transporting naturally occurring purines and purine analog drugs into cells.

Hypoxanthine decreases equilibrative type of adenosine transport in lymphocytes from Lesch-Nyhan patients

BACKGROUND

Lesch-Nyhan (LN) syndrome is associated with deficient hypoxanthine-guanine phosphoribosyltransferase (HPRT) activity, but the connection between the aberrant purine metabolism and the neurological symptoms remains largely unknown. The aim of this study was to determine adenosine transporter subtypes affected by HPRT deficiency and by the associated hypoxanthine excess.

MATERIALS AND METHODS

Nucleoside transporter types (depending on their sodium dependence and 10 microm nitrobenzylthioinosine, NBTI, sensitivity) involved in adenosine transport were compared between peripheral blood lymphocytes (PBL) obtained from nine LN patients, PBL(LN) (2-21 years) and from nine controls, PBL(C) (2-23 years) under basal conditions and after 25 microm hypoxanthine incubation.

RESULTS

We found four types of adenosine transporters in PBL: equilibrative and concentrative transporters that are either sensitive (ENT1 or cs) or insensitive (ENT2 or ci) to NBTI. Adenosine ENT1 uptake was the predominant transporter in both PBL(C) (55%) and PBL(LN) (46%). Under basal conditions no significant differences were found in adenosine transport between PBL(C) and PBL(LN). Incubation of PBL with 25 microm hypoxanthine markedly decreased total adenosine transport in both cell types. Hypoxanthine affected equilibrative transport (mainly ENT2 type) in PBL(LN) and PBL(C). Only in PBL(C) was concentrative transport affected by hypoxanthine. Expressions of human (h) ENT1 and hENT2 mRNA were not significantly modified by hypoxanthine incubation in PBL(C).

CONCLUSIONS

This study contributes to further knowledge of the defective adenosine transport found in PBL(LN). Increased hypoxanthine levels, similar to those reported in HPRT deficient patients, reduced adenosine uptake by 32% in PBL(LN) as compared to normal transport.

Novel Na+-independent and adenine-specific transport system for adenine in primary cultured rat cortical neurons

Endogenous adenine is an important modulator of cell survival and activity in the central nervous system. In the present study, we examined the transport mechanisms for adenine in primary cultured rat cortical neurons and astrocytes. [3H]Adenine was time-dependently taken up into neurons, but not into astrocytes. In kinetic analysis, the [3H]adenine uptake by neurons was observed to be saturable, and an Eadie-Hofstee plot showed that a single component was involved in the uptake, with kinetic parameters of Km=6.09 microM and Vmax=0.340 nmol/mg protein per min. In inhibition assaying by nucleobases and nucleosides, and inhibitors for equilibrative nucleoside transporters, organic ion transporters and peptide transporters, which were reported to transport nucleobases and their analogues, the [3H]adenine uptake by neurons was found to be significantly inhibited by excess concentrations of adenine, hypoxanthine and adenosine, and was greatly reduced only by the addition of adenine. Therefore, it was indicated that adenine in the extracellular fluid in the central nervous system is taken up into neurons, but not into astrocytes, and that neurons may present a novel Na+ -independent and adenine-specific transport system.

Characterization of novel Na+-dependent nucleobase transport systems at the blood-testis barrier

In the testis, nucleosides and nucleobases are important substrates of the salvage pathway for nucleotide biosynthesis, and one of the roles of Sertoli cells is to provide nutrients and metabolic precursors to spermatogenic cells located within the blood-testis barrier (BTB). We have already shown that concentrative and equilibrative nucleoside transporters are expressed and are functional in primary-cultured rat Sertoli cells as a BTB model, but little is known about nucleobase transport at the BTB or about the genes encoding specific nucleobase transporters in mammalian cells. In the present study, we examined the uptake of purine ([3H]guanine) and pyrimidine ([3H]uracil) nucleobases by primary-cultured rat Sertoli cells. The uptake of both nucleobases was time and concentration dependent. Kinetic analysis showed the involvement of three different transport systems in guanine uptake. In contrast, uracil uptake was mediated by a single Na+-dependent high-affinity transport system. Guanine uptake was inhibited by other purine nucleobases but not by pyrimidine nucleobases, whereas uracil uptake was inhibited only by pyrimidine nucleobases. In conclusion, it was suggested that there might be purine- or pyrimidine-selective nucleobase transporters in rat Sertoli cells.

Potential chemotherapeutic targets in the purine metabolism of parasites

Parasites are responsible for a wide variety of infectious diseases in human as well as in domestic and wild animals, causing an enormous health and economical blight. Current containment strategies are not entirely successful and parasitic infections are on the rise. In the absence of availability of antiparasitic vaccines, chemotherapy remains the mainstay for the treatment of most parasitic diseases. However, there is an urgent need for new drugs to prevent or combat some major parasitic infections because of lack of a single effective approach for controlling the parasites (e.g., trypanosomiasis) or because some serious parasitic infections developed resistance to presently available drugs (e.g., malaria). The rational design of a drug is usually based on biochemical and physiological differences between pathogens and host. Some of the most striking differences between parasites and their mammalian host are found in purine metabolism. Purine nucleotides can be synthesized by the de novo and/or the so-called "salvage" pathways. Unlike their mammalian host, most parasites studied lack the pathways for de novo purine biosynthesis and rely on the salvage pathways to meet their purine demands. Moreover, because of the great phylogenic separation between the host and the parasite, there are in some cases sufficient distinctions between corresponding enzymes of the purine salvage from the host and the parasite that can be exploited to design specific inhibitors or "subversive substrates" for the parasitic enzymes. Furthermore, the specificities of purine transport, the first step in purine salvage, diverge significantly between parasites and their mammalian host. This review highlights the unique transporters and enzymes responsible for the salvage of purines in parasites that could constitute excellent potential targets for the design of safe and effective antiparasitic drugs.

Functional differences in nucleoside and nucleobase transporters expressed on the rabbit corneal epithelial cell line (SIRC) and isolated rabbit cornea

The purpose of this study was to investigate the expression of nucleoside/nucleobase transporters on the Statens Seruminstitut rabbit corneal (SIRC) epithelial cell line and to evaluate SIRC as an in vitro screening tool for delineating the mechanism of corneal permeation of nucleoside analogs. SIRC cells (passages 410-425) were used to study uptake of [3H]thymidine, [3H]adenine, and [3H]ganciclovir. Transport of [3H]adenine and [3H]ganciclovir was studied across isolated rabbit cornea. Uptake and transport studies were performed for 2 minutes and 120 minutes, respectively, at 34 degrees C. Thymidine uptake by SIRC displayed saturable kinetics (K(m) = 595.9 +/- 80.4 microM, and V(max) = 289.5 +/- 17.2 pmol/min/mg protein). Uptake was inhibited by both purine and pyrimidine nucleosides but not by nucleobases. [3H]thymidine uptake was sodium and energy independent but was inhibited by nitrobenzylthioinosine at nanomolar concentrations. Adenine uptake by SIRC consisted of a saturable component (K(m) = 14.4 +/- 2.3 microM, V(max) = 0.4 +/- 0.04 nmol/min/mg protein) and a nonsaturable component. Uptake of adenine was inhibited by purine nucleobases but not by the nucleosides or pyrimidine nucleobases and was independent of sodium, energy, and nitrobenzylthioinosine. [3H]ganciclovir uptake involved a carrier-mediated component and was inhibited by the purine nucleobases but not by the nucleosides or pyrimidine nucleobases. However, transport of [3H]adenine across the isolated rabbit cornea was not inhibited by unlabeled adenine. Further, corneal permeability of ganciclovir across a 100-fold concentration range remained constant, indicating that ganciclovir permeates the cornea primarily by passive diffusion. Nucleoside and nucleobase transporters on rabbit cornea and corneal epithelial cell line, SIRC, are functionally different, undermining the utility of the SIRC cell line as an in vitro screening tool for elucidating the corneal permeation mechanism of nucleoside analogs.

Different substrate recognition motifs of human and trypanosome nucleobase transporters. Selective uptake of purine antimetabolites

The therapeutic index of antimetabolites such as purine analogues is in large part determined by the extent to which it is selectively accumulated by the target cell. In the current study we have compared the transport of purine nucleobase analogues by the H2 transporter of bloodstream form Trypanosoma brucei brucei and the equilibrative nucleobase transporter of human erythrocytes. The H2 transporter forms hydrogen bonds with hypoxanthine at positions N3, N7, N(1)H, and N(9)H of the purine ring, with apparent Delta G(0) of 7.7-12.6 kJ/mol. The transporter also appears to H-bond with the amine group of adenine. The human transporter forms hydrogen bonds that form to (6)NH(2) and N1 of adenine. An H-bond is also formed with N3 and the 6-keto and amine groups of guanine but not with the protonated N1, thus explaining the low affinity for hypoxanthine. N7 and N9 do not directly interact with the human transporter in the form of H-bonds, and it is proposed that pi-pi stacking interactions contribute significantly to permeant binding. The potential for selective uptake of antimetabolites by the parasite transporter was demonstrated.

Functional and molecular characterization of nucleobase transport by recombinant human and rat equilibrative nucleoside transporters 1 and 2. Chimeric constructs reveal a role for the ENT2 helix 5-6 region in nucleobase translocation

The human (h) and rat (r) equilibrative (Na(+)-independent) nucleoside transporters (ENTs) hENT1, rENT1, hENT2, and rENT2 belong to a family of integral membrane proteins with 11 transmembrane domains (TMs) and are distinguished functionally by differences in sensitivity to inhibition by nitrobenzylthioinosine and coronary vasoactive drugs. Structurally, the proteins have a large glycosylated loop between TMs 1 and 2 and a large cytoplasmic loop between TMs 6 and 7. In the present study, hENT1, rENT1, hENT2, and rENT2 were produced in Xenopus laevis oocytes and investigated for their ability to transport pyrimidine and purine nucleobases. hENT2 and rENT2 efficiently transported radiolabeled hypoxanthine, adenine, guanine, uracil, and thymine (apparent K(m) values 0.7-2.6 mm), and hENT2, but not rENT2, also transported cytosine. These findings were independently confirmed by hypoxanthine transport experiments with recombinant hENT2 produced in purine-cytosine permease (FCY2)-deficient Saccharomyces cerevisiae and provide the first direct demonstration that the ENT2 isoform is a dual mechanism for the cellular uptake of nucleosides and nucleobases, both of which are physiologically important salvage metabolites. In contrast, recombinant hENT1 and rENT1 mediated negligible oocyte fluxes of hypoxanthine relative to hENT2 and rENT2. Chimeric experiments between rENT1 and rENT2 using splice sites at rENT1 residues 99 (end of TM 2), 171 (between TMs 4 and 5), and 231 (end of TM 6) identified TMs 5-6 of rENT2 (amino acid residues 172-231) as a determinant of nucleobase transport activity, suggesting that this domain forms part(s) of the ENT2 substrate translocation channel.

Hypoxanthine transport in human tumour cell lines: relationship to the inhibition of hypoxanthine rescue by dipyridamole

Hypoxanthine (HPX) uptake was investigated in four human tumour cell lines previously characterised as being sensitive (ds: A549 and MCF7) or insensitive (di: COR-L23 and T-47D) to dipyridamole (DP)-induced inhibition of HPX rescue from antipurine antifolate-induced growth inhibition. The aim of the study was to determine the mechanism underlying the differential sensitivity of HPX rescue to DP. The time-course of HPX uptake in the two ds cell lines was different in comparison to the two di cell lines. The initial rate of HPX uptake in the di cell lines was more rapid than in the ds cell lines such that at 60 sec the amount of HPX taken up by the former was 2-6 times higher than that taken up by the later. The K(t) and T(max) for HPX transport in di COR-L23 cells were 870 microM and 4.75 microM/10(6) cells/min and 1390 microM and 1.78 microM/10(6) cells/min in ds A549 cells. HPX transport was not sodium-dependent in these cells. Equilibrative nucleoside transporter 2 (ENT2)-mediated thymidine transport was also higher in di cells. DP inhibited HPX uptake into ds cell lines by > or =48% and by < or =20% in the di cell lines. Competition studies with HPX and thymidine transport via ENT2 indicated an overlap between nucleoside and nucleobase transport transporters in the breast cancer cell lines (MCF7 and T-47D). These studies showed that more rapid and extensive HPX uptake, as well as reduced sensitivity to DP inhibition, is associated with the inability of DP to prevent HPX rescue from antipurine antifolate-induced growth inhibition in certain human tumour cell lines.

Nucleosides and nucleotides: natural bioactive substances in milk and colostrum

Nucleotides, nucleosides and nucleobases belong to the non-protein-nitrogen (NPN) fraction of milk. The largest amounts of ribonucleosides and ribonucleotides--ribose forms only were considered in this review--were measured directly after parturition in bovine milk and other ruminants as well as in the milk of humans. Generally, concentrations of most of the nucleos(t)ides tend to decrease gradually with advancing lactation period or nursing time. The species-specific pattern of these minor constituents in milk from different mammals is a remarkable property and confirms, at least, the specific physiological impact of these minor compounds in early life. The physiological capacity of these compounds in milk is given by the total potentially available nucleosides. The main dietary sources of nucleos(t)ides are nucleoproteins and nucleic acids which are converted in the course of intestinal digestion into nucleosides and nucleobases the preferred forms for absorption in the intestine. Thus, nucleosides and nucleobases are suggested to be the acting components of dietary and/or supplemented nucleic acid-related compounds in the gut. They are used by the body as exogenous trophochemical sources and can be important for optimal metabolic functions. Up to 15 % of the total daily need for a breast-fed infant was calculated to come from this dietary source. Concerning their biological role they not only act as metabolites but are also involved as bioactive substances in the regulation of body functions. Dietary nucleotides affect immune modulation, e.g. they enhance antibody responses of infants as shown by a study with more than 300 full-term healthy infants. Dietary nucleos(t)ides are found to contribute to iron absorption in the gut and to influence desaturation and elongation rates in fatty acid synthesis, in particular long-chain polyunsaturated fatty acids in early stages of life. The in vitro modulation of cell proliferation and apoptosis has been described by ribonucleosides, in particular by modified components using human cell culture models. Due to the bio- and trophochemical properties of dietary nucleos(t)ides, the European Commission has allowed the use of supplementation with specific ribonucleotides in the manufacture of infant and follow-on formula. From the technochemical point of view, the ribonucleoside pattern is influenced by thermal treatment of milk. In addition ribonucleosides are useful indicators for quantifying adulterations of milk and milk products.

Nucleobase transporters (review)

Purines and pyrimidines play a key role in nucleic acid and nucleotide metabolism of all cells. In addition, they can be used as nitrogen sources in plants and many microorganisms. Transport of nucleobases across biological membranes is mediated by specific transmembrane transport proteins. Nucleobase transporters have been identified genetically and/or physiologically in bacteria, fungi, protozoa, algae, plants and mammals. A limited number of bacterial and fungal transporter genes have been cloned and analysed in great detail at the molecular level. Very recently, nucleobase transporters have been identified in plants. In other systems, with less accessible genetics, such as vertebrates and protozoa, no nucleobase transporter genes have been identified, and the transporters have been characterized and classified by physiological and biochemical approaches instead. In this review, it is shown that nucleobase transporters and similar sequences of unknown function present in databases constitute three basic families, which will be designated NAT, PRT and PUP. The first includes members from archea, eubacteria, fungi, plants and metazoa, the second is restricted to prokaryotes and fungi, and the last one is only found in plants. Interestingly, mammalian ascorbate transporters are homologous to NAT sequences. The function of different nucleobase transporters is also described, as is how their expression is regulated and what is currently known about their structure-function relationships. Common features emerging from these studies are expected to prove critical in understanding what governs nucleobase transporter specificity and in selecting proper model microbial systems for cloning and studying plant, protozoan and mammalian nucleobase transporters of agricultural, pharmacological and medical importance.

Pharmacokinetics and pharmacodynamics of allopurinol in elderly and young subjects

AIMS

The prevalence of hyperuricaemia and gout increases with age as does the incidence of adverse effects to allopurinol, the major uric acid lowering drug. The present study was performed to compare the disposition and effects of allopurinol and its active metabolite oxipurinol in elderly and young subjects without major health problems.

METHODS

Ten elderly (age range 71-93 years) and nine young subjects (24-35 years) received an oral dose of 200 mg allopurinol in an open, single dose, cross sectional design. Four of these individuals were additionally dosed with 200 mg allopurinol intravenously. Plasma and urine concentrations of allopurinol, oxipurinol, hypoxanthine, xanthine, and uric acid were measured by h. p.l.c.

RESULTS

Total clearance of allopurinol was not different in elderly (15.7+/-3.8 ml min-1 kg-1, mean+/-s.e. mean) and young subjects (15.7+/-2.1), whereas total clearance of oxipurinol was significantly reduced in the aged (0.24+/-0.03) compared with young controls (0.37+/-0.05) as was the distribution volume of oxipurinol (0.60+/-0.09 and 0.84+/-0.07 l kg-1, respectively). Oxipurinol was eliminated primarily by the kidneys, allopurinol by metabolism. Fractional peroral bioavailability of allopurinol was 0.81+/-0.16 (n=4, two elderly and two young subjects). Although maximal plasma concentrations of oxipurinol were significantly higher in elderly (5. 63+/-0.83 microgram ml-1 ) than in young persons (3.75+/-0.25) as was the area under the oxipurinol plasma concentration-time curve, AUC (260+/-46 and 166+/-23 microgram ml-1 h, respectively), the pharmacodynamic effect of oxipurinol was smaller in elderly than young subjects (time-dependent decrease of plasma uric acid 83+/-30 microgram ml-1 h in elderly compared with 176+/-21 in young controls). Oxipurinol increased the renal clearance of xanthine, suggesting inhibition of tubular xanthine reabsorption by oxipurinol.

CONCLUSIONS

Although allopurinol elimination is not reduced in the aged, that of its active metabolite oxipurinol is because of an age-dependent decline in renal function. Xanthine oxidase inhibition by oxipurinol appears to be reduced in old age. In addition to its uricostatic action, oxipurinol has a xanthinuric effect which is also diminished in the elderly.

Nucleobase transport in opossum kidney epithelial cells and Xenopus laevis oocytes: the characterisation, structure-activity relationship of uracil analogues and oocyte expression studies of sodium-dependent and -independent hypoxanthine uptake

The characteristics of hypoxanthine transport were examined in opossum kidney (OK) epithelial cells and Xenopus laevis oocytes. In both cell types hypoxanthine influx was mediated by two distinct transport systems: a high-affinity Na+-dependent system and a Na+-independent transporter. Na+-dependent hypoxanthine transport in OK cells was saturable (Km 0.78+/-0.29 microM) and was inhibited by guanine, uracil, thymine and 5-fluorouracil (Ki values 0.5-7 microM), whereas adenine had no effect. Substitutions at the 2- and 4-position had a marked effect on the ability of uracil to inhibit Na+/hypoxanthine influx by OK cells revealing that an oxo group at both the 2- and 4-positions of uracil is required for interacting with the transporter. The properties of Na+-dependent hypoxanthine influx in oocytes were similar to those observed in OK cells. In particular, xanthine and oxypurinol inhibited hypoxanthine influx, a characteristic not observed previously for the Na+/nucleobase carrier in pig LLC-PK1 renal cells. Na+-independent hypoxanthine influx in OK cells and oocytes was of a lower affinity (Km 90-180 microM). Adenine and guanine inhibited Na+-independent hypoxanthine flux in OK cells, but had no effect in oocytes. Injection of LLC-PK1 mRNA into oocytes resulted in a 1.5-fold stimulation of Na+/hypoxanthine flux over water-injected oocytes. These results reveal further heterogeneity in Na+/nucleobase cotransporters.

Molecular characterization of two novel transporters from human and mouse kidney and from LLC-PK1 cells reveals a novel conserved family that is homologous to bacterial and Aspergillus nucleobase transporters

Nucleobase transport is important for the metabolism of nucleic acids and antiviral and antineoplastic drugs. This transport has been functionally described in several mammalian cells but has not been well characterized molecularly. We report the cloning of two novel transporters. YSPL2 encodes a 650-residue protein and has an ubiquitous 8 kb transcript. The human and pig homologs are 95% similar. YSPL3 encodes a 598-residue protein with a 3 kb transcript that is expressed only in kidney and liver. Human YSPL2 and YSPL3 are 60% similar at the amino acid level and both show 31% similarity to the first nucleobase permease gene described in vertebrates, YSPL1. These proteins appear to be members of a new family of possible nucleobase transporters with significant sequence similarities with bacterial and Aspergillus nucleobase transporters. Further functional studies will be needed to unveil the role of these transporters in nucleic acid metabolism in normal and in disease states.

Apoptosis induced by modified ribonucleosides in human cell culture systems

The in vitro modulation of apoptosis and cell proliferation by modified in comparison with non-modified ribonucleosides was investigated for the first time using peripheral blood lymphocytes, HL-60 cells and Caco-2 cells as human cell culture models. Modulating effects of several ribonucleosides were found in the range of 10(-7)-10(-3) mol/l. The following ribonucleosides induced significant apoptosis of HL-60 cells: adenosine, N6-dimethyladenosine, N6-(2-isopentenyl)-adenosine, N2-dimethylguanosine. A significant apoptotic effect on PBL was found with N6-dimethyladenosine and N6-(2-isopentenyl)-adenosine. N6-Dimethyladenosine, N6-(2-isopentenyl)-adenosine and guanosine had a pronounced inhibitory effect on Caco-2 cell apoptosis. Regarding the known function of ribonucleosides as pathobiochemical marker molecules for cancer, the possibility of a selective apoptotic effect against malignant cells is discussed.

Purine nucleobase transport in bloodstream forms of Trypanosoma brucei is mediated by two novel transporters

The mechanism and inhibitor sensitivity of hypoxanthine transport by bloodstream forms of Trypanosoma brucei brucei was investigated. The dose response curve for the inhibition of hypoxanthine transport (1 microM) by guanosine was biphasic; approximately 90% of transport activity was inhibited with a Ki value of 10.8 +/- 1.8 microM, but 10% of the activity remained insensitive to concentrations as high as 2 mM. These two components of hypoxanthine transport are defined as guanosine-sensitive (H2) and guanosine-insensitive (H3). Hypoxanthine influx by both components was saturable, but there was a marked difference in their Km values (123 +/- 15 nM and 4.7 +/- 0.9 microM for H2 and H3, respectively) although the Vmax values (1.1 +/- 0.2 and 1.1 +/- 0.1 pmol (10[7] cells)[-1] s[-1], n = 3) were similar. Hypoxanthine uptake via the H2 carrier was inhibited by purine bases and analogues as well as by some pyrimidine bases and one nucleoside (guanosine), whereas the H3 transporter was sensitive only to inhibition by purine nucleobases. H2-mediated hypoxanthine uptake was inhibited by ionophores, ion exchangers and the potential H+-ATPase inhibitors, N,N'-dicyclohexylcarbodiimide (DCCD) and N-ethylmaleimide (NEM). Measurements of the intracellular pH and membrane potential of bloodstream trypanosomes in the presence and absence of these agents established a linear correlation between protonmotive force and rate of [3H]hypoxanthine (30 nM) uptake. We conclude that hypoxanthine transport in bloodstream forms of T. b. brucei occurs by two transport systems with different affinities and substrate specificities, one of which, H2, appears to function as a H+-/hypoxanthine symporter.

Hypoxanthine uptake through a purine-selective nucleobase transporter in Trypanosoma brucei brucei procyclic cells is driven by protonmotive force

The mechanism of purine nucleobase transport in procyclic cells of the protozoan parasite Trypanosoma brucei brucei was investigated. Hypoxanthine uptake at 22 degrees C was rapid and saturable, exhibiting an apparent Km of 9.3 +/- 2.0 microM and a Vmax of 4.5 +/- 0.8 pmol x (10(7) cells)(-1) x s(-1). All the natural purine nucleobases tested (Ki 1.8-7.2 microM), as well as the purine analogues oxypurinol and allopurinol, inhibited hypoxanthine influx in a manner consistent with the presence of a single high-affinity carrier. Nucleosides and pyrimidine nucleobases had little or no effect on hypoxanthine influx. The uptake process was independent of extracellular sodium, but inhibited by ionophores inducing cytosolic acidification (carbonyl cyanide chlorophenylhydrazone, nigericin, valinomycin) or membrane depolarisation (gramicidin) as well as by the adenosine triphosphatase inhibitors N-ethylmaleimide and N,N'-dicyclohexylcarbodiimide. Using the fluorescent dyes bisoxonol and 2',7'-bis-(carboxyethyl)-5,6-carboxy-fluorescein to determine membrane potential and intracellular pH (pHi), the rate of hypoxanthine uptake was shown to be directly proportional to the protonmotive force. Similarly, under alkaline extracellular conditions hypoxanthine uptake was reversibly inhibited alongside a reduction in protonmotive force. In addition, hypoxanthine accelerated the rate of pH, recovery to pH 7 after base-loading with NH4Cl, indicative of a proton influx concurrent with hypoxanthine transport. Finally, after pretreatment of cells with N-ethylmaleimide, hypoxanthine induced a slow membrane depolarisation, demonstrating that hypoxanthine transport is electrogenic. These data show that hypoxanthine uptake in T. b. brucei procyclic cells is dependent on the protonmotive force, and are consistent with a nucleobase/H+-symporter model for this transporter.

Inhibition of calcium-induced insulin secretion from intact HIT-T15 or INS-1 beta cells by GTP depletion

Using intact rat islets, we previously observed that GTP depletion (achieved through the use of mycophenolic acid or other synthesis inhibitors) impedes nutrient- but not K+-induced insulin secretion. It was concluded that a proximal nutrient-dependent step in stimulus-secretion coupling (but not the process of Ca2+-induced exocytosis itself) is modulated by ambient GTP levels. To examine Ca2+-dependent steps further in intact beta cells, INS-1 cells (which synthesize GTP and ATP similarly to rat islets) and HIT-T15 cells (whose synthesis of purine nucleotides is different) were studied following cell culture for 1-18 hr in various concentrations of mycophenolic acid (MPA) or mizoribine (MZ). Both agents profoundly reduced GTP content (mean: -78%) and lowered the GTP/GDP ratio by an average of -73%; concomitantly, MPA or MZ reduced insulin secretion induced by 10 mM glucose, 30 or 40 mM KCl, or 100 microM tolbutamide, independent of any changes in cell viability, insulin content, ATP content, the ATP/ADP ratio, or cytosolic free Ca2+ concentrations. In INS-1 cells (which appear to have normal nucleobase transport and "salvage" pathway activities), guanine (but not adenine) restored GTP content, the GTP/GDP ratio, and Ca2+-induced secretion. In HIT cells, the phosphoribosylation of exogenous guanine or hypoxanthine is defective; however, provision of 500 microM guanosine (but not adenosine) reversed the effects of MPA. We conclude that, at least in certain situations, a requisite role for GTP in the distal step(s) of exocytosis can be demonstrated.

Regulation of nucleobase transport in LLC-PK1 renal epithelia by protein kinase C

The involvement of protein kinase C (PKC) in the regulation of Na(+)-dependent and -independent hypoxanthine transport was investigated by exposing confluent monolayers of LLC-PK1 renal epithelia cells to the PKC activator, phorbol 12-myristate 13-acetate (PMA). Chronic exposure (> 2 h) of LLC-PK1 monolayers to 16 nM PMA resulted in approximately 75% inhibition of Na(+)-dependent hypoxanthine influx occurring maximally at 8 h and persisting for 72 h. In contrast, PMA had little effect on Na(+)-independent hypoxanthine influx at 8 h, but longer exposure resulted in stimulation of influx (approximately 3-fold) that peaked at 24 h and thereafter declined to control levels at 72 h. The effects of PMA were dose-dependent and were associated with changes in Vmax of transport (2-4-fold) with no significant change in apparent K(m). 4 alpha-Phorbol, a phorbol ester that does not activate PKC, had no effect on hypoxanthine transport by LLC-PK1 cells. The diacylglycerol kinase inhibitor, R59022 (10 microM), partially inhibited (28%) Na(+)-dependent hypoxanthine influx. In addition, the PMA-induced effects on hypoxanthine transport were reversed by Ro-31-8220 (1 and 5 microM) and calphostin C (50 nM), potent and selective inhibitors of PKC. The increase in Na(+)-independent hypoxanthine influx following exposure to PMA was blocked by the protein synthesis inhibitor, cycloheximide (20 microM), and correlated with an increase in LLC-PK1 cell proliferation. The PMA-induced decrease in Na(+)-dependent hypoxanthine transport was independent of PMA effects on cell proliferation and not dependent on protein synthesis. These results are consistent with the proposal that the PMA-induced effects on hypoxanthine transport are due to PKC activation.

Membrane transport of nucleobases: interaction with inhibitors

1. The kinetic properties and the mechanism of nucleobase transport and transport inhibition are briefly reviewed. 2. Many purine derivatives even when bearing large substituents on N9 and C6 are inhibitors of nucleobase transport, some are also substrates. 3. Papaverine and other benzyl-isoquinolines are efficient inhibitors of facilitated transport of nucleobases. 4. Papaverine is a noncompetitive inhibitor of nucleobase transport in human erythrocytes. 5. Reduction of the aromatic isoquinoline to the tetrahydro form causes loss of inhibitory activity whereas replacement of methoxy groups by ethoxy groups leads to increased activity. 6. Papaverine also inhibits sodium dependent active nucleobase transport in pig kidney cells. 7. The nucleoside transport inhibitors dipyridamole and dilazep have no effect on facilitated diffusion transport of nucleobases, but inhibit in micromolar concentrations active sodium dependent nucleobase transport in pig kidney cells.

Mechanisms of nucleobase transport in rabbit choroid plexus. Evidence for a Na(+)-dependent nucleobase transporter with broad substrate selectivity

The overall goal of this study was to determine the mechanisms by which nucleobases are transported in the choroid plexus. Choroid plexus tissue slices were obtained from the lateral ventricles of rabbit brains and depleted of ATP with 2,4-dinitrophenol. In the presence of an initial inwardly directed Na+ gradient, hypoxanthine accumulated in the tissue slices against a concentration gradient. Na(+)-stimulated hypoxanthine uptake was saturable with a Km of 31.1 +/- 9.71 microM and a Vmax of 2.69 +/- 0.941 nmol/g/s (mean +/- S.E.). Na(+)-stimulated hypoxanthine uptake was inhibited by (100) microM naturally occurring purine and pyrimidine nucleobases (adenine, cytosine, guanine, hypoxanthine, thymine, uracil, and xanthine) as well as by the nucleoside analog, dideoxyadenosine. The stoichiometric coupling ratio between Na+ and hypoxanthine was 1.7:1. The data demonstrate the presence of a novel Na(+)-dependent nucleobase transporter in the choroid plexus, which is distinct from the previously described Na(+)-nucleoside transporter in choroid plexus and from Na(+)-dependent nucleobase transporters in other tissues in terms of its kinetics, substrate selectivity, and Na(+)-nucleobase stoichiometry. This transporter may play a role in the targeting of both salvageable nucleobases and therapeutic nucleoside analogs to the central nervous system.

In vivo adenovirus-mediated gene transfer of the Escherichia coli cytosine deaminase gene to human colon carcinoma-derived tumors induces chemosensitivity to 5-fluorocytosine

To evaluate the concept that in vivo transfer of the Escherichia coli cytosine deaminase gene will confer sensitivity of a solid tumor to the prodrug 5-fluorocytosine (5FC), we constructed an adenovirus vector (AdCMV.CD) carrying the cytosine deaminase gene driven by the cytomegalovirus (CMV) promoter, infected HT29 colon carcinoma cells in vitro and in vivo, and evaluated cell growth over time. AdCMV.CD produced a functional cytosine deaminase protein in HT29 cells in vitro as evidenced by the ability of lysates from the infected cells to convert [3H]5FC to its active metabolite 5-fluorouracil (5FU). The AdCMV.CD vector effectively suppressed HT29 cell growth in vitro in the presence of 5FC in a dose-dependent manner. Infection with AdCMV.CD, when as few as 10% of cells expressed the cytosine deaminase gene, was associated with a bystander effect when combined with 5FC in cell mixing studies. Further, this bystander effect was not dependent on cell-to-cell contact as demonstrated by suppression of [3H]thymidine incorporation in HT29 cells when supernatant from AdCMV.CD-infected cells treated with 5FC was transferred cells. Consistent with these in vitro observations, when AdCMV.CD was directly injected into established subcutaneous HT29 tumors in nude mice receiving 5FC, there was a four-fold reduction in tumor size at day 15 compared to controls, and a five-fold reduction at day 28. These observations suggest that adenovirus-mediated gene transfer of the E. coli cytosine deaminase gene and concomitant administration of 5FC may have potential as a strategy for local control of the growth of tumor cells susceptible to 5FU.

Genetic and molecular characterization of a gene encoding a wide specificity purine permease of Aspergillus nidulans reveals a novel family of transporters conserved in prokaryotes and eukaryotes

In Aspergillus nidulans, loss-of-function mutations in the uapA and azgA genes, encoding the major uric acid-xanthine and hypoxanthine-adenine-guanine permeases, respectively, result in impaired utilization of these purines as sole nitrogen sources. The residual growth of the mutant strains is due to the activity of a broad specificity purine permease. We have identified uapC, the gene coding for this third permease through the isolation of both gain-of-function and loss-of-function mutations. Uptake studies with wild-type and mutant strains confirmed the genetic analysis and showed that the UapC protein contributes 30% and 8-10% to uric acid and hypoxanthine transport rates, respectively. The uapC gene was cloned, its expression studied, its sequence and transcript map established, and the sequence of its putative product analyzed. uapC message accumulation is: (i) weakly induced by 2-thiouric acid; (ii) repressed by ammonium; (iii) dependent on functional uaY and areA regulatory gene products (mediating uric acid induction and nitrogen metabolite repression, respectively); (iv) increased by uapC gain-of-function mutations which specifically, but partially, suppress a leucine to valine mutation in the zinc finger of the protein coded by the areA gene. The putative uapC gene product is a highly hydrophobic protein of 580 amino acids (M(r) = 61,251) including 12-14 putative transmembrane segments. The UapC protein is highly similar (58% identity) to the UapA permease and significantly similar (23-34% identity) to a number of bacterial transporters. Comparisons of the sequences and hydropathy profiles of members of this novel family of transporters yield insights into their structure, functionally important residues, and possible evolutionary relationships.

Inhibition of purine nucleobase transport in human erythrocytes and cell lines by papaverine. Investigation of structure-activity relationship

Papaverine was found to be an effective inhibitor of hypoxanthine transport not only in human erythrocytes, but also in the human cell lines HL60 (myeloic) and U937 (monocytic). IC50 values for inhibition of hypoxanthine influx ranged from 6 to 20 microM. In erythrocytes papaverine was found to be a non-competitive inhibitor of hypoxanthine equilibrium-exchange transport with a Ki value of approximately 13 microM, which is in close agreement with the respective IC50 values estimated for zero-trans influx of hypoxanthine. In addition papaverine also had a slight inhibitory effect on unmediated nucleobase transport, most likely due to a perturbation of the membrane lipid environment. Several papaverine analogs were tested for their inhibitory effect on nucleobase transport. Only ethaverine was as effective as papaverine. Drotaverine and berberine were moderately inhibitory while laudanosine had no inhibitory effect at all. Isoquinoline acted as a very weak inhibitor.