Nucleoside transport in mammalian cell membranes

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Abstract
Organomercurials form stable stoichiometric complexes with thiolated nucleosides. The complexes inhibited uptake of ribonucleosides and cytosine arabinoside (CAR) in various types of normal and transformed cells. The inhibition was competitive and reversible (K i =3–6 μm). The interaction between complexes and transport system displayed a 1∶1 stoichiometry. Chemical factors which contributed to the inhibitory power were evaluated with a series of S-alkylated derivatives and S−Hg−R complexes of mercaptonucleosides. The inhibitory potency was not determined exclusively by the hydrophobic nature of either the S-alkylated or the S−Hg−R moieties. Chemical modification of cells with penetrating and nonpenetrating organomercurials lead to stimulation of nucleoside uptake and to an increase in its susceptibility to inhibition by S−Hg−R complexes or S-alkylated derivatives of mercaptopurine ribosides. The kinetic and chemical data obtained with nucleoside analogs and with chemical modifiers suggested complex features of nucleoside transport systems. Four distinct classes of sites were implied: (i) a substrate binding site susceptible directly to competitive inhibition by organomercurial-mercaptonucleoside complexes, (ii) an additional site susceptible either to S-arylalkylated or S-mercuriated derivatives of 6-mercaptopurine ribosides, (iii) SH-containing modifier sites whichstimulate uridine uptake upon binding of organomercurials, and (iv) SH-containing modifier sites whichinhibit the function upon binding of organomercurials. From the observation that only SH sites related to stimulation were susceptible to modification by macromolecular-SH modifier probes, some conclusions can be drawn regarding the disposition of the various sites in the cell membrane in general and among membrane components in particular.