Inhibition by nucleosides of glucose-transport activity in human erythrocytes.

The interaction of nucleosides with the glucose carrier of human erythrocytes was examined by studying the effect of nucleosides on reversible cytochalasin B-binding activity and glucose transport. Adenosine, inosine and thymidine were more potent inhibitors of cytochalasin B binding to human erythrocyte membranes than was D-glucose [IC50 (concentration causing 50% inhibition) values of 10, 24, 28 and 38 mM respectively]. Moreover, low concentrations of thymidine and adenosine inhibited D-glucose-sensitive cytochalasin B binding in an apparently competitive manner. Thymidine, a nucleoside not metabolized by human erythrocytes, inhibited glucose influx by intact cells with an IC50 value of 9 mM when preincubated with the erythrocytes. In contrast, thymidine was an order of magnitude less potent as an inhibitor of glucose influx when added simultaneously with the radioactive glucose. Consistent with this finding was the demonstration that glucose influx by inside-out vesicles prepared from human erythrocytes was more susceptible to thymidine inhibition than glucose influx by right-side-out vesicles. These data, together with previous suggestions that cytochalasin B binds to the glucose carrier at the inner face of the membrane, indicate that nucleosides are capable of inhibiting glucose-transport activity by interacting at the cytoplasmic surface of the glucose transporter. Nucleosides may also exhibit a low-affinity interaction at the extracellular face of the glucose transporter.     

The stilbene derivatives,nucleosides, and nucleosides modified by stilbene derivatives

In this short review, including 187 references, the issues of biological activity of stilbene derivatives and nucleosides and the biological and medicinal potential of fusion of these two classes are discussed. The stilbenes, especially the stilbenoids, and nucleosides are both biologically active. Hybrids formed from binding of these compounds have not yet been broadly studied. However, those that have been investigated exhibit desirable medicinal properties. The review is divided in such parts: I. Derivative of stilbene (biomedical investigations, biological activities in cells, enzymes and hazard), parts II. naturally occurred nucleoside and its derivatives: uridine, thymidine and 5-methyluridine, cytidine, adenosine, guanosine and part III. hybrid molecules- drugs and hybrid molecules- nucleoside - stilbene and its derivative.     

Guanosine monophosphate synthetase from Escherichia coli B-96. Inhibition by nucleosides.

The mechanism of inhibition of GMP synthetase by purine and purine-analog nucleosides was investigated. It was found that in addition to allowing the nucleoside to bind to the enzyme (Udaka, S., and Moyed, H. S. (1963) J. Biol. Chem. 238, 2797)PPi was also a competitive inhibitor with respect to ATP. A rate equation was derived to describe this inhibitory model for two competitive inhibitors where the binding of one inhibitor is contingent upon the binding of the other. The inhibition constants for a large number of nucleosides were then determined. It was found that the initial enzyme-inhibitor complex (of all nucleoside inhibitors) was slowly (0.2 min-1) transformed into a secondary (nondissociating) complex. The two inhibitory complexes appeared to exist in equilibrium. While decoyenine, N6-allyladenosine, and adenosine had similar inhibition constants for the initial complex (0.7 to 1.0 muM), their apparent inhibition constants for the secondary complex were 0.004, 0.06, and 0.5 muM respectively. These differences in the apparent dissociation constants from the secondary complexes are due to different equilibria between the initial and the secondary complexes. The ratios of the secondary complex to the initial complex at equilibrium were 3,250, 290, and 11 for decovenine, N6-allyladenosine, and adenosine, respectively.     

Mediated transport of nucleosides by human erythrocytes specificity toward purine nucleosides as permeants

Transport of uridine and thymidine across the plasma membrane of human eruthrocytes is mediated by a facilitated diffusion mechanism with broad specificity toward the base portion and narrow specificity toward the sugar portion of pyrimidine nucleosides. Specificity of this mechanism was further investigated by measuring efflux of radioactivity when erythrocytes containing radioactive uridine were incubated in medium containing purine nucleosides. Adenosine, guanosine, inosine, and arabinosyladenine accelerated uridine efflux and were therefore considered substrates for the transport mechanism. 6-Thioinosine, 6-thioguanosine, and several S-substituted 6-thiopurine ribonucleosides inhibited efflux of radioactive uridine. Adenine nucleosides with sugar moieties other than ribose or arabinose inhibited or had no effect on uridine efflux.     

L-purine nucleosides as selective antimalarials.

L-nucleosides selectively enter malaria infected erythrocytes and have the unique ability to be metabolised by the malarial adenosine deaminase. This has allowed us to design novel L-nucleosides as potential anti-malarials.     

The synthesis of cyclopropano nucleosides.

A tricyclic, fused cyclopropano nucleoside 8 containing a ketal group was synthesized by the one-pot seven sequential reactions of a trimesylated allofuranosyl adenine derivative 6 with Mg(OMe)2. When KOH was used instead of Mg(OMe)2, an alpha, beta-unsaturated ketonucleoside 7 was obtained.     

Inactivation of S-adenosylhomocysteine hydrolase by nucleosides

The irreversible inactivation of S-adenosylhomocysteine hydrolase purified from hamster and bovine liver by adenosine analogs substituted in the 5' and 2 positions has been investigated in detail. 5'-Cyano-5'-deoxyadenosine inactivates as potently as 9-beta-D-arabinofuranosyladenine (Ara-A). Substitution of the Ara-A at the 2 position by halogens or deleting N at the 3 position decreases its potency. Although weak, 2',3'-dideoxyadenosine can also inactivate the enzyme. The irreversible inactivation of the hydrolase in rat hepatocytes incubated with 2-chloroadenosine or 3-deaza-Ara-A could be demonstrated, concomitant with increases in 35S-labeled S-adenosylhomocysteine and S-adenosylmethionine in the hepatocytes.     

Preservation of red blood cells with purines and nucleosides. II. Uptake and utilization of purines and nucleosides by stored red blood cells

The uptake of adenine, guanine, guanosine and inosine by stored red cells was investigated in whole blood and red cell resuspensions at initial concentrations of 0.25, 0.5 and 0.75 mM for adenine and 0.5 mM for the other additives using a rapid ion-exchange chromatographic microanalysis of purines and nucleosides in plasma and whole blood. Increasing adenine concentrations from 0.25 to 0.75 mM in blood elevated the adenine uptake from 0.3 up to 0.8 mmol/l red cells during 2 hours after collecting blood. The intra-/extracellular distribution ratio changed from 1 : 1.3 to 1: 1.7. Some 2 hours after withdrawing blood into CPD--solution with purines and nucleosides the uptake of adenine and guanine resulted in 40 per cent and 70 per cent respectively and of guanosine and inosine in 80 and 90 per cent respectively. The replacement of plasma by a resuspending solution gave the same uptake rates for purines and nucleosides. The nucleosides were rapidly split to purines and R-1-P and disappeared from blood during one week. Adenine and guanine were utilized to 80 to 90 per cent only after 3 weeks. During the same period the utilization of guanine was smaller by 40 per cent than that of adenine due to the different activity of the purine nucleoside phosphorylase for these substrates. The plasma of all analyzed blood samples contained hypoxanthine and inosine, but guanine and guanosine were detected only in those samples to which one of them was added. After 3 weeks of storage the highest concentration of hypoxanthine was found in CPD-AI blood with 600 microM in plasma and the highest concentration of synthesized inosine in CPD-AG blood with a concentration of 100 microM in plasma. Three ways of utilization of purines by stored red cells were discussed : the synthesis of nucleotide monophosphates, the formation of nucleosides, and the deamination. The portions of these ways change during storage. The most effective concentrations of adenine and guanosine in stored blood seems to be 0.25 and 0.5 mM respectively. The full utilization of the nucleoside requires the addition of inorganic phosphate.     

Phenotypic conversion of cultured mouse embryo cells by aza pyrimidine nucleosides.

Cells of the $\text{C3H}\text{10}\text{T}\text{1}\text{2}\text{CL8}$ line, which are nonmyoblastic in nature, form functional myotubes when treated with low concentrations of 5-azacytidine. Further characterization of the myotubes revealed that they arise from the fusion of mononucleated precursors and not as a result of endoreplication. They accumulate histochemically detectable myosin ATPase activity as well as acetylcholine receptors capable of binding radioactively labeled α-bungarotoxin. The deoxy analog, 5-aza-2′-deoxycytidine, induced myogenic conversion at one-tenth of the maximally effective concentration of 5-azacytidine. The ability of both analogs to induce myotube formation and to cause cytotoxicity was strongly influenced by cotreatment with certain pyrimidine nucleosides. These effects were consistent with a requirement for metabolism of both aza compounds to phosphorylated derivatives and with a mechanism of action based on their incorporation into DNA. Concentrations of the analogs causing myogenic conversion did not substantially alter rates of DNA, RNA, or protein synthesis as measured by precursor incorporation into intact cells. The induction of myotubes by 5-azacytidine in cells synchronized by two different methods required that treatment with the analog was carried out at a critical phase early in S phase. Thus the mechanism of drug action appears to be linked to specific DNA synthesis.