Mediated Transport of Nucleosides in Human Erythrocytes

Specificity of nucleoside transport in Neurospora crassa

The specificity of nucleoside uptake in germinating conidia of Neurospora crassa was investigated by examining the kinetics of [2-14C]uridine and [8-14C]-adenosine uptake in the wild-type, ad-8, and ud-1 pyr-1 strains. The results obtained strongly indicate that nucleoside transport in N. crassa is mediated solely by a general transport system which accepts both purine and pyrimidine nucleosides. Studies directed at characterizing the specificity of the transport system indicate that general structural features of the nucleoside which enhance its efficiency in binding to the transport system include: (i) a purine or pyrimidine as the heterocyclic ring, (ii) an unfunctionalized ribose or 2'-deoxyribose as the sugar unit, (iii) a beta-configuration about the anomeric carbon, (iv) the absence of substituents at C8 in the purine series and at C5 and C6 in the pyrimidine series, (v) the presence of a C5-C6 double bond in the pyrimidine series, and (vi) the absence of a charge on the heterocyclic ring.

Defective transport of thymidine by cultured cells resistant to 5-bromodeoxyuridine

A line of HeLa cells resistant to 5-bromo-2'-deoxyuridine (BUdR) was established by continuous culture in growth medium containing BUdR; during the selection period, BUdR concentrations, initially 15 micrometer, were gradually increased to 100 micrometer. Cells of a clone (HeLa/B5) established from this line were also resistant to 5-fluoro-2'-deoxyuridine (FUdR), but not to the free base, 5-fluorouracil. Although extracts of HeLa/B5 cells exhibited levels of thymidine kinase activity comparable to those of parental cells, rates of uptake of BUdR, FUdR, and thymidine into intact cells were much reduced. The kinetics of uptake of uridine and adenosine, nucleosides which appear to be transported independently of thymidine in HeLa cells, were similar for HeLa/B5 and the parental line (HeLa/O). Relative to thymidine uptake by HeLa/O cells, that by HeLa/B5 cells was distinctly less sensitive to nitrobenzylthioinosine (NBMPR), a specific inhibitor of nucleoside transport in various types of animal cells. Despite this difference in NBMPR sensitivity, both cell lines possessed the same number of high affinity NBMPR binding sites per mg cell protein. The altered kinetics of thymidine uptake and the NBMPR insensitivity of that function in HeLA/B5 cells suggest that resistance to BUdR is due to an altered thymidine transport mechanism.

Fractionation of human erythrocyte membranes. Presence of the nucleoside transport complex in an insoluble residue

(1) Human erythrocyte membranes, when dialysed against water at pH 9.5, were partly solubilized, losing 80% of the membrane proteins and 65% of the membrane lipids. Sodium dodecyl sulphate gel electrophoresis of the particulate material revealed selective removal of proteins from the membrane. (2) The lipid-rich particulate material remaining retained the ability to bind specifically the nucleoside transport inhibitor, nitrobenzylthioinosine, previously shown to bind selectively to the nucleoside transport mechanism of whole erythrocytes and erythrocyte ghosts.

Transport and countertransport of thymidine in ATP depleted and thymidine kinase-deficient Novikoff rat hepatoma and mouse L cells: evidence of a high Km facilitated diffusion system with wide nucleoside specificity

Incubation of cultured Novikoff rat hepatoma and mouse L cells in a glucose-free basal medium containing 5 mM KCN and 5 mM iodoacetate for about 10 minutes resulted in a complete depletion of the cells of ATP. ATP-depleted wild type cells or thymidine kinase-deficient sublines of Novikoff or L cells took up thymidine rapidly from the medium without concentrating it intracellularly, and exhibited countertransport of thymidine. Thus uptake was by facilitated diffusion. This transport system differs from the substrate-specific, low-Km (0.5 muM] thymidine transport system previously described for various types of cultured cells in that it exhibits an at least 100-fold higher Km and transports equally well various ribo- and deoxyribonucleosides. The results suggest that the rate-limiting step in thymidine incorporation into the nucleotide pool by wild type cells is phosphorylation rather than transport, or that the cells possess two transport systems, a facilitated diffusion system with low substrate specificity and a second system which involves substrate phosphorylation by thymidine kinase.

The uptake of pyrimidine nucleosides in Tetrahymena. I. Uridine

Uridine uptake was examined in Tetrahymena pyriformis GL-7 in defined medium under conditions where food vacuole formation is not a significant factor in solute acquisition by the cell. The results indicate the presence of a saturable mechanism which follows Michaelis-Menten kinetics. When corrected for diffusion the apparent Km for the carrier is 2.3 +/- 0.6 muM and the Vmax is 7.3 +/- 0.2 X 10(-7) nmoles/cell/min. It is evident from nucleotide pool analysis that most of the radioactivity of externally supplied [3H]uridine appears in UMP with the remainder in UTP. Uridine is apparently phosphorylated immediately upon entry into the cell and neither uridine-cytidine kinase activity nor RNA synthesis are rate-limiting in the uptake process. Uridine transport is competitively inhibited by a variety of ribo- and deoxyribonucleosides as well as several nucleoside analogs. Neither uracil nor ribose or deoxyribose are effective inhibitors of uridine transport indicating the carrier is specific for the nucleoside. There is little difference between the Ki values for ribo- as opposed to deoxyribonucleosides except in the case of deoxyguanosine which is much less effective as an inhibitor under the conditions of this study, than all the other nucleosides, including guanosine.

Nitrobenzylthionionosine binding sites in the erythrocyte membrane

Nitrobenzylthioinosine binds tightly, but reversibly, to sites in the human erythrocyte membrane; occupancy of these sites blocks the transport of uridine and of other nucleosides. This report described the inhibition of nitrobenzylthioinosine binding at these sites by substrates of the uridine transport mechanism and by compounds related to nitrobenzylthioinosine. For some of these compounds dissociation constants for binding at the nitrobenzylthioinosine sites were determined, assumming competition with nitrobenzylthioinosine. Deoxycytidine, a substrate for the uridine transport mechanism, did not inhibit binding of nitrobenzylthioinosine, suggesting that binding sites for the latter are distinct from nucleoside sites directly involved in transport.

Influence of membrane lipid fluidity on glucose and uridine facilitated diffusion in human erythrocytes

A central question which must be resolved before acceptable molecular descriptions of facilitated diffusion systems can be provided is the nature of the spatial and functional relationships between the transport proteins and the membrane lipids. In the work reported here, this question was addressed by investigating the dependence of the rates of glucose and uridine facilitated diffusion in human erythrocytes on membrane lipid fluidity. Two approaches were used to alter the lipid fluidity: treatment with ether, an anesthetic, and the exchange of a synthetic 3-ketosteroid, cholest-4-en-3-one, for membrane chloesterol. Both of these treatments result in a significant increase in membrane lipid fluidity, as judged by the increase in the rates of passive diffusion of uridine through cell membranes and of glucose through membrane lipid bilayer vesicles. Ether produces no change in the Km of either transport process, a slight decrease in the V for glucose transport, and no significant change in the V for uridine transport. Replacement of membrane cholesterol by cholest-4-en-3-one reduces the V for glucose transport slightly, without altering the Km, and reduces both the Km and V for uridine transport. The absence of the expected increase in the V of facilitated diffusion with increasing membrane lipid fluidity observed here with human erythrocytes is not consistent with models for the transport process which feature movement of transport proteins which are in direct contact with the bulk lipids of the membrane.