Independent characterization of thymidine transport and subsequent metabolism in Hymenolepis diminuta--II. Purification and preliminary analysis of thymidine kinase

1. An affinity column for the purification of thymidine kinase (TK) from the cestode Hymenolepis diminuta is described. Using an epoxy-activated Sepharose 6B affinity column containing thymidine as a ligand, a 698-fold purification of thymidine kinase was obtained. 2. Thymidine kinase eluted from this affinity column was partially characterized as having an apparent Km value of 3.94 microM thymidine. This value is very similar to those observed in mammalian systems. 3. Thymidine kinase appears to be an extremely active and ubiquitous enzyme, whose primary function is to rapidly phosphorylate incoming thymidine and thus "trap" it for the cell's use, reducing efflux to a minimum. 4. The apparent Km for TK is two orders of magnitude lower than the Kt for thymidine transport. Thus, theories postulating that long-term (2 min) uptake kinetics for thymidine actually represent subsequent metabolism must look further along the thymidine phosphorylating pathway, beyond TK and its very active role.     

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.     

Kinetics of Thymidine Uptake,Its Phosphorylation,and Incorporation into DNA in the Sugar-Beet Cell Culture

Thymidine uptake, its phosphorylation, and incorporation into DNA were studied in a fast-growing sugar-beet (Beta vulgaris L.) cell suspension. A high rate and specificity of thymidine uptake were observed: total uptake reached a steady-state level for 3 min. The average kinetic constants for thymidine uptake were calculated for eight-day-old cells at 20°C as K _M = 25 M and V _max = 11.6 pmol/(min mg fr wt). The values of K _M for thymidine phosphorylation in vivo (53.2 M) and K _M for thymidine kinase (EC 2.7.1.21), which we purified earlier from broad bean seedlings, were of the same order of magnitude. The kinetics of thymidine phosphorylation in vivo displayed two distinct phases, which were determined by external thymidine concentration. Above 100 M thymidine, the rate of the process tended to rise, indicating the possible involvement of another mechanism for thymidine phosphorylation, most likely with the participation of nonspecific nucleoside phosphotransferase (NPT; EC 2.7.1.77). A further stage of thymidine salvage, its incorporation into DNA, occurred with a high affinity for thymidine phosphates; K _M = 2.8 M. The presence of other nucleosides (uridine or a high concentration of adenosine) in the medium markedly inhibited thymidine uptake. Nevertheless, these nucleosides did not diminish the percentage of thymidine phosphates of total thymidine uptake, which pointed to the specificity of thymidine phosphorylation and the insignificance of NPT activity. The analogue of thymidine, 5"-amino-2",5"-dideoxythymidine, known as thymidine kinase inhibitor, had no effect on thymidine uptake. The data presented provide evidence that the main route of the thymidine salvage in fast-growing sugar-beet suspension engages thymidine kinase, and NPT is activated only when nucleosides flood the cell.     

Thymidine transport in phytohaemagglutinin-stimulated pig lymphocytes.

Thymidine and uridine transporters in peripheral pig lymphocytes have structural features in common, but are not identical. Accelerated entry of [3H]thymidine begins 12h after the addition of phytohaemagglutinin. The increased thymidine uptake into the cells is characterized by an increase in Vmax. Without alteration of the apparent Km(0.6+/-0.08muM). Thymidine kinase activity is increased 12h after stimulation. Both the increased thymidine uptake and the increased thymidine kinase activity are inhibited in cultures incubated with puromycin: rates of degradation of the two systems are unchanged after phytohaemagglutinin addition, and indicate similar half-lives of about 2h. Thymidine kinase is rate-limiting for thymidine entry up to 18h after phytohaemagglutinin addition; increase in its synthesis is detectable about 6h before net incorporation of thymidine into DNA is significantly promoted.     

Some properties of the thiamine uptake system in isolated rat hepatocytes

A kinetic study of [14C]thiamine uptake over a concentration range from 0.1 microM to 4 mM was performed in isolated rat hepatocytes. The results showed that two processes contribute to the entry in rat hepatocytes: a low affinity process with a Kt of 34.1 microM and Vmax of 20.8 pmol/10(5) cells per 30 s and a high affinity process with a Kt of 1.26 microM and Vmax of 1.21 pmol/10(5) cells per 30 s. The uptake of thiamine by the high affinity process was concentrative and reduced in a betaine medium or K+ medium. Both ouabain and 2,4-dinitrophenol decreased the thiamine uptake by the high affinity process. These findings indicate that the transport of thiamine via a high affinity process is dependent on Na+ and biological energy. The uptake of thiamine was strongly inhibited by thiamine analogs such as dimethialium and chloroethylthiamine. Among quarternary ammonium compounds other than thiamine derivatives, choline and acetylcholine significantly inhibited thiamine uptake by rat liver cells, whereas betaine and carnitine did not. A kinetic study of thiamine uptake by rat hepatocytes preloaded with pyrithiamine, a potent inhibitor of thiamine pyrophosphokinase, revealed that the biphasic property of thiamine uptake disappeared and a single carrier system for thiamine with a Kt of 40.5 microM, which was similar to the Kt value of the low affinity process, was retained. These results strongly suggest that thiamine transport system in rat liver cells is closely connected with thiamine pyrophosphokinase, which accelerates the uptake rat of thiamine by pyrophosphorylation at physiological concentrations of thiamine.     

Sodium-dependent nucleoside transport in mouse intestinal epithelial cells. Two transport systems with differing substrate specificities

Nucleoside transport was examined in freshly isolated mouse intestinal epithelial cells. The uptake of formycin B, the C nucleoside analog of inosine, was concentrative and required extracellular sodium. The initial rate of sodium-dependent formycin B transport was saturable with a Km of 45 +/- 3 microM. The purine nucleosides adenosine, inosine, guanosine, and deoxyadenosine were all good inhibitors of sodium-dependent formycin B transport with 50% inhibition (IC50) observed at concentrations less than 30 microM. Of the pyrimidine nucleosides examined, only uridine (IC50, 41 +/- 9 microM) was a good inhibitor. Thymidine and cytidine were poor inhibitors with IC50 values greater than 300 microM. Direct measurements of [3H]thymidine transport revealed, however, that the uptake of this nucleoside was also mediated by a sodium-dependent mechanism. Thymidine transport was inhibited by low concentrations of cytidine, uridine, adenosine, and deoxyadenosine (IC50 values less than 25 microM), but not by formycin B, inosine, or guanosine (IC50 values greater than 600 microM). These data indicate that there are two sodium-dependent mechanisms for nucleoside transport in mouse intestinal epithelial cells, and that formycin B and thymidine may serve as model substrates to distinguish between these transporters. Neither of these sodium-dependent transport mechanisms was inhibited by nitrobenzylmercaptopurine riboside (10 microM), a potent inhibitor of one of the equilibrative (facilitated diffusion) nucleoside transporters found in many cells.     

A nucleoside transporter is functionally linked to ectonucleotidases in rat liver canalicular membrane.

Prevention of nucleoside loss in bile is physiologically desirable because hepatocytes are the main source of nucleosides for animal cells which lack de novo nucleoside biosynthesis. We have demonstrated a Na+ gradient-energized, concentrative nucleoside transport system in canalicular membrane vesicles (CMV) from rat liver by studying [3H]adenosine uptake using a rapid filtration technique. The Na(+)-dependent nucleoside transporter accepts purine, analogues of purine nucleosides and uridine; exhibits high affinity for adenosine (apparent Km, 14 microM); is not inhibited by nitrobenzylthioinosine or dipyridamole, and is present in CMV but not in rat liver sinusoidal membrane vesicles. Adenosine transport in right side-out CMV was substantially greater than with inside-out CMV. CMV also contain abundant ecto-ATPase and ecto-AMPase (5'-nucleotidase). These ectoenzymes were shown to degrade nucleotides into nucleosides which were conserved by the Na(+)-dependent nucleoside transport system.     

Uptake of pyrimidines and their derivatives into Candida glabrata and Candida albicans

The uptake of pyrimidines and their derivatives into Candida glabrata and Candida albicans was measured using a novel technique in which the cells were rapidly separated from their suspending medium by centrifugation through a layer of an inert oil. The uptake of [14C]cytosine was linear for 30 s for all concentrations of pyrimidine tested. In C. glabrata but not C. albicans cytosine transport was mediated by both a high affinity (Km 0.8 +/- 0.1 microM), low capacity [V 40 +/- 4 pmol (microliters cell water)-1 s-1] and a low affinity [Km 240 +/- 35 microM], high capacity system [V 770 +/- 170 pmol (microliters cell water)-1 s-1]. The cytosine permease in C. glabrata was specific for cytosine and 5-fluorocytosine. In C. albicans there was only one cytosine transport system [Km 2.4 +/- 0.3 microM; V 50 +/- 4 pmol (microliters cell water)-1 s-1]; this system also transported adenine, guanine and hypoxanthine. Differences in nucleoside transport were also observed for C. glabrata and C. albicans, with the uridine permease in C. glabrata transporting only uridine and 5-fluorouridine whereas cytidine and adenosine were also transported by the uridine permease in C. albicans. Studies on the effect of nucleoside analogues on uridine transport in C. glabrata demonstrated the importance of the sugar moiety in determining the specificity of transport, with a hydroxyl residue on C-2 being apparently essential for transport.     

Hexose transport in L6 rat myoblasts. I. Rate-limiting step, kinetic properties, and evidence for two systems

The hexose transport system of undifferentiated L6 rat myoblasts was investigated. 2-Deoxy-D-glucose (2-DOG) and 2-deoxy-2-fluoro-D-glucose (2FG) were used as analogues to investigate the rate-limiting step of hexose uptake into the cell. Virtually all of the 2-DOG or 2FG taken up into the cell was found to be in the phosphorylated form. No significant pool of intracellular free sugar could be detected. This demonstrates that hexose transport, not phosphorylation, is the rate-limiting step. The inhibitory effect of various glucose analogues on 2-DOG and 3-O-methyl-D-glucose (3-OMG) uptake revealed that these two sugars may be taken up into the cell by different carriers. In addition, kinetics analysis of the transport of both sugars also indicates that two hexose transport systems may be present in L6 cells. 2-DOG is transported by high and low affinity transport systems (Km 0.6 mM and 2.9 mM, respectively), whereas 3-OMG is transported by a low affinity system (Km 3.5 mM). Treatment of cells with ionophores or energy uncouplers results in inactivation of the high affinity system, but not the low affinity system.     

Thymidine uptake and utilization in Escherichia coli: a new gene controlling nucleoside transport.

A commonly used strain of Escherichia coli K-12 was shown to be deficient in the transport of a number of nucleosides, including thymidine. Thymidine incorporation was unaffected. Strain AB2497 exhibited a strikingly lower thymidine pulse-label incorporation at low (less than 1 mug/ml) thymidine concentrations than do many other strains. The deficiency appeared to be due to mutation in a single gene. This gene, which we designated nup (for nucleoside uptake), is located at 10 to 13 min on the E. coli linkage map. In nup+ strains, the transport of a given nucleoside was relatively insensitive to large excesses of other nucleosides but was competitively inhibited by the same nucleoside. Mutants deficient inthymidine kinase are deficient in thymidine uptake but normal in deoxyadenosine uptake. A two-step model for nucleoside transport is presented in which the first step, utilizing the nup gene product, is a nonspecific translocation of nucleoside to the interior of the cell. In the second step, the individual nucleosides are modified by cellular enzymes (e.g., nucleosides kinases) facilitate accumulation.     

Sugar transport in Coprinus cinereus.

Two transport systems for glucose were detected: a high affinity system with a Km of 27 muM, and a low affinity system with a Km of 3.3 mM. The high affinity system transported glucose, 2-deoxy-D-glucose (Km = 26 muM), 3-O-methylglucose (Km = 19 muM), D-glucosamine (Km = 652 muM), D-fructose (Km = 2.3 mM) and L-sorbose (Km = 2.2 mM). All sugars were accumulated against concentration gradients. The high affinity system was strongly or completely inhibited by N-ethylmaleimide, quercetin, 2,4-dinitrophenol and sodium azide. The system had a distinct pH optimum (7.4) and optimum temperature (45 degrees C). The low affinity system transported glucose, 2-deoxy-D-glucose (Km = 7.5 mM), and 3-O-methylglucose (Km = 1.5 mM). Accumulation again occurred against a concentration gradient. The low affinity system was inhibited by N-ethylmaleimide, quercetin and 2,4-dinitrophenol, but not by sodium azide. The rate of uptake by the low affinity system was constant over a wide temperature range (30--50 degrees C) and was not much affected by pH; but as the pH of the medium was altered from 4.5 to 8.9 a co-ordinated increase in affinity for 2-deoxy-D-glucose (from 52.1 mM to 0.3 mM) and decrease in maximum velocity (by a factor of five) occurred. Both uptake systems were present insporelings germinated in media containing sodium acetate as sole carbon source. Only the low affinity system could initially be demonstrated in glucose-grown tissue, although the high affinity system was restored by starvation inglucose-free medium. The half-ti me for restoration of high affinity activity was 3.5 min and the process was unaffected by cycloheximide. Addition of glucose to an acetate-grown culture inactivated the high affinity system with a half-life of 5--7.5 s. Addition of cycloheximide to an acetate-grown culture caused decay of the high affinity system with a half-life of 80 min. Regulation is thus thought to depend on modulation of protein activity rather than synthesis, and the kinetics of glucose, 2-deoxy-D-glucose and 3-O-methylglucose uptake would be consistent with there being a single carrier showing negative co-operativity. Analysis of transport defective mutants revealed defects in both transport systems although the mutants used were alleles of a single gene. It is concluded that this gene (the ftr cistron) is the structural gene for an allosteric molecule which serves both transport systems.     

Evolvement of systems of serotonin uptake in rat hypothalamus during its development]

Kinetics of 3H serotonin accumulation into slices from hypothalamus have been compared in adult, puppy and foetus rat. In 15 days-old, as in adult rat, there are two components of 5 HT accumulation corresponding to the low and high affinity transport systems. For this latter, Km and Vmax values are much higher in adult than in 15 days old rat (in adult, Km=1,3 X 10(-7) and Vmax=0,33 X 10(-10); in 15 days old rat, Km=0,5 X 10(-7) and Vmax=0,125 X 10(-10)). On the opposite, in the 7 days old rat and in the 21 days old foetus, it is only possible to arbitrarely define one uptake system corresponding to the following apparent values: in the 7 days old rat, Km= 5 X 10(-7) and Vmax=2 X 10(-10), in the foetus, Km=0,2 X 10(-7) and Vmax=0,15 X 10(-10). These results showed important developmental differences in affinity of 3H serotonin to hypothalamus. The low and high affinity uptake systems existing in adult are only individualized in the 15 days old little rat.     

Purification of thymidine kinase by affinity chromatography with an enzyme inhibitor as the ligand

An affinity column for the purification of thymidine kinase is described. The ligand in this column is a glycoprotein isolated from rat kidney. This glycoprotein inhibits phosphorylation of thymidine in cultured cells and in a cell-free assay system. With an affinity column containing the glycoprotein as a ligand, a 24-fold purification of thymidine kinase from an ammonium sulfate fraction of a crude tissue extract can be obtained. Thymidine kinase eluted from the affinity column migrates as one major band on polyacrylamide and as one diffuse major band on sodium dodecyl sulfate-polyacrylamide. The affinity column, with thymidine kinase bound to the inhibitor, can also be used as an assay system. When the glycoprotein is covalently attached to Sepharose, it retains its binding capacity for thymidine kinase but has apparently lost its ability to inhibit the enzyme. Thymidine kinase eluted from the affinity column is again sensitive to the glycoprotein. It seems to be a carbohydrate moiety of the glycoprotein that is responsible for the inhibition.     

Transport of L-tyrosine by B16/F10 malignant melanocytes: characterization of the process

The main characteristics of L-tyrosine (L-Tyr) uptake by B16/F10 malignant melanocytes are reported. This amino acid can be taken up by two systems, both of them being saturable. The first one would be system L. This system can be studied in cells preloaded with amino acids that are a good substrate for system L, such as L-methionine or L-tryptophan. The kinetic parameters for L-Tyr uptake by this transport system are Vm = 6.5 pmol L-Tyr/10(3) cells.min and Km around 130 microM. The second system, probably the system ASC, shows lower capacity but higher affinity than the former. This system can be detected only in cells previously depleted of amino acids, showing approximate kinetic values of Vm 0.05 pmol L-Tyr/10(3) cells.min and Km around 5 microM. It is shown that the increase in cell density yields a decrease in the rate of L-Tyr uptake by system L, but this increase does not affect the high affinity system, alpha-MSH does not affect significantly the L-Tyr uptake by both systems. 2-Amino bicyclo-(2,2,1)-heptane-2-carboxylic acid produces a remarkable inhibition of the rate of L-Tyr uptake, but alpha-methylaminoisobutyric acid does not affect the rate of transport of this amino acid. The absence of sodium produces a slight but reliable decrease in the rate of L-Tyr uptake, supporting the involvement of two different transport systems. The ionophores monensin and nigericin enhance the transport by system L, but this effect is suppressed by the presence of ouabain. This finding indicates that the (Na+ -K+)-ATPase is essential for the stimulating action of ionophores.(ABSTRACT TRUNCATED AT 250 WORDS)     

Mechanism of copper transport from plasma to hepatocytes

The effects of plasma components on the kinetics of copper transport by rat hepatocytes were examined in an attempt to determine how copper is mobilized from plasma for uptake by the liver. Specific protein-facilitated transport was indicated by saturation kinetics, competition by related substrates, and similar kinetic parameters for uptake and efflux. For copper uptake, Km = 11 +/- 0.6 microM and Vmax = 2.7 +/- 0.6 nmol Cu/(min X mg protein). Zinc is a competitive inhibitor of copper uptake, and copper competes for zinc uptake. Copper efflux from preloaded cells is biphasic. The kinetic parameters for the initial rapid phase are similar to the parameters for uptake. Copper transport by hepatocytes is strictly passive. A variety of metabolic inhibitors have no effect on uptake and initial rates are solely dependent on extracellular-intracellular concentration gradients. Albumin markedly inhibits copper uptake by a substrate removal mechanism, and histidine facilitates albumin-inhibited copper uptake. The active species that delivers copper to hepatocytes under conditions of excess albumin and excess histidine is the His2Cu complex. Experiments with [3H]His2 64Cu showed that the transported species is free ionic copper. The kinetic parameters of copper transport by hepatocytes isolated from the brindled mouse model of Menkes' disease are normal. However, these cells show a decreased capacity to accumulate copper on prolonged incubation. An intracellular metabolic defect seems to be involved.     

Role of microtubules in insulin and glucagon stimulation of amino acid transport in isolated rat hepatocytes

The effects of the microtubule inhibitor, colchicine, on insulin or glucagon stimulation of alpha-amino[1-14C]-isobutyric acid (AIB) transport were investigated in isolated hepatocytes from normal fed rats. Under all conditions tested, AIB uptake appeared to occur through two components of transport: a low affinity (Km approximately 50 mM) component and a high affinity (Km approximately 1 mM) component. Within 2 h of incubation, insulin and glucagon, at maximal concentrations, increase AIB (0.1 mM) uptake by 2- to 3-fold and 4- to 6-fold, respectively. Colchicine, at the low concentration of 5 X 10(-7) M, slightly reduces basal AIB transport, decreases by 80% the simulatory effect of insulin, and diminishes by 40% the stimulatory effect of either glucagon or dibutyryl cAMP. Kinetic analysis of AIB influx indicates that the drug inhibits the increase in Vmax of a high affinity (Km approximately 1 mM) component of transport stimulated by insulin or glucagon, without affecting the kinetic parameters of a low affinity component of transport (Km approximately 50 mM). Various short term hormonal effects of insulin and glucagon (changes in glucose, urea, and lactate production) were found not to be modified by the drug. Vinblastine elicits similar changes as colchicine on AIB uptake. Lumicolchicine, a colchicine analogue that does not bind to tubulin, has no effect. The concentration of colchicine (10(-7) M) required for half-maximal inhibition of hormone-stimulated AIB transport is in the appropriate range for specific microtubule disruption. These data suggest that microtubules are involved in the regulation of the insulin or glucagon stimulation of AIB transport in isolated rat hepatocytes.     

Deoxycytidine transport and pyrimidine deoxynucleotide metabolism in phytohaemagglutinin-stimulated pig lymphocytes.

Increased entry of deoxy[3H]cytidine begins at about 12h after addition of phytohaemagglutinin to peripheral pig lymphocyte cultures, and is accompanied by a parallel stimulation of deoxycytidine kinase up to the beginning of DNA synthesis at 24h. The increased deoxycytidine uptake is characterized by an increase in Vmax. without alteration of the apparent Km (0.7 +/- 0.11 muM). Although the entries of both nucleosides are promoted at the same time, the stimulation of deoxycytidine uptake is less than that of thymidine, and the two nucleosides are transported by separate systems. In addition to deoxycytidien kinase, the synthesis of deoxycytidylate deaminase and thymidylate synthetase are stimulated after addition of phytohaemagglutinin, but to a lesser extent than that of thymidine kinase. The importance of the latter enzyme in forming dTMP, and of thymidylate kinase in providing dTTP, is discussed.     

SELECTIVE LOCALIZATION OF A HIGH AFFINITY CHOLINE UPTAKE SYSTEM AND ITS ROLE IN ACh FORMATTON IN CHOLINERGIC NERVE TERMINALS

The avian iris-ciliary nerve preparation exhibits two distinct choline uptake systems. One component, a sodium dependent, high affinity system Km-2 am and Vmax - 0.5 pmolpin per preparation is confined to nerve terminals. The other component is localized in muscle cells. It is sodium independent and low affinity system (Km - 200 am and Vmax - 16 pmol/min per muscle). The high affinity uptake of choline and the synthesis of ACh in the nerve terminals are coupled. Vmax Ach formation -0.5 pmol/min. is the same as Vmax for choline transport; however. with the external choline concentration equal to that of avian plasma only -50% of choline taken up is converted to ACh. In contrast to the nerve terminals, the cell bodies of the same neurons are deficient in the high affinity uptake-ACh synthesis coupled system. This indicates a nerve terminal membrane specialization related to neuro-transmitter synthesis.     

Cholic acid uptake and isolated rat hepatocytes.

Cholic acid uptake was studied in isolated rat hepatocytes using a centrifugal filtration technique to allow rapid sampling. Hepatocytes were found to adsorb as well as to transport cholic acid. The adsorption was characterized by a capacity of 24 nmol X mg cell protein-1 and an association constant of 0.59 X 103 M-1. Cholic acid uptake was linear with respect to concentration at or below 10 degree C, suggesting a unsaturable uptake process which was considered to represent simple diffusion and is quantitated by a diffusion coefficient of 1.76 pmol cholic acid X min-1 X mg protein-1 X muM-1. Above 10 degrees C the uptake curve was biphasic. After subtracting the unsaturable component from uptake rates at higher temperatures, a curve showing saturable kinetics resulted. The apparent Km and V values at 37 degrees C were calculated to be 31muM and 0.8 nmol X min-1 X mg protein-1 respectively. This saturable uptake process was temperature-dependent with an activation energy of 13 kcal X mol-1 (5.44 X 104 J X mol-1) and was inhibited by oligomycin and KCN. Countertransport was demonstrated with cholic, taurocholic and chenodeoxycholic acids. The results suggest that cholic acid is transported by an energy-dependent carrier-mediated process in addition to simple diffusion by hepatocytes, and that the postulated carrier has affinity for other bile acids.     

Residual nitrobenzylthioinosine-resistant nucleoside transport in a transport mutant (AE1) of S49 murine T-lymphoma cells.

The uptake of various nucleosides by S49 mouse T-lymphoma cells and that by a single-step nucleoside transport-defective mutant thereof (AE1) were compared. Residual nucleoside entry into AE1 cells occurred via two routes, nonmediated permeation and saturable, non-concentrative transport with broad substrate specificity and a Michaelis-Menten constant approximating that for thymidine transport in wild-type cells. However, in contrast to nucleoside transport in wild-type cells, residual nucleoside transport in AE1 cells was resistant to inhibition by nitrobenzylthioinosine. In its properties the latter resembled nitrobenzylthioinosine-resistant nucleoside transport observed in other types of mammalian cells. It amounted to less than 1% of the total nucleoside transport activity of wild-type S49 cells. The results indicate that nitrobenzylthioinosine-resistant and -sensitive nucleoside transports are genetically distinguishable. In wild-type cells, the salvage of thymidine, when present at concentrations higher than 1 to 10 microM, was limited by phosphorylation, because of the saturation of thymidine kinase. In AE1 cells, entry into the cells mainly limited thymidine salvage, but at high thymidine concentrations the combined entry via residual transport and nonmediated permeation was sufficiently rapid to support intracellular thymidine phosphorylation at rates comparable to those observed in wild-type cells.