Properties of a synthetic plasma membrane marker: fluorescent-mercury-dextran

Macromolecular conjugates of transport inhibitors: new tools for probing topography of anion transport proteins

Macromolecular-conjugated, water-soluble, membrane-impermeant compounds were designed and assessed as topological probes for chloride-transporting agencies. The novel compounds were derivatives of either disulfonic stilbene (DS) and benzylaminoethylsulfonate (BS), "classical" inhibitors of erythrocyte chloride-bicarbonate exchange, or of phenylanthranilates (PA), high-affinity blockers of epithelial chloride channels. Covalent reactive derivatives of various DS, BS, and PA were synthesized and coupled either directly to polyethylene glycol or via spacer arms of different lengths to dextrans. The macromolecular conjugates were demonstrably inhibitory to red blood cell anion exchange when the ligands were appropriately coupled: inhibitory efficacy strongly depended on the chemical structure of the coupled ligand and the spacer length between the inhibitory moiety and the macromolecule. Mechanistic studies indicated that impermeant DS and PA derivatives acted exofacially on sites, which although different in their affinity for chloride, shared geographical proximity. BS derivatives were unique in that they affected transport from either surface. The results suggest asymmetric aqueous access routes leading to the functional domain of the anion transporter from either membrane surface.

Modification of cation permeability of rabbit descending colon by sulphydryl reagents

1. Addition of the organic mercurials mersalyl, p-chloromercuribenzoate, and p-chloromercuribenzene sulphonate to the Ringer solution (140 mM-Na) bathing the luminal side of isolated epithelia of rabbit descending colon increases short-circuit current (Isc) and tissue conductance (Gt) when the spontaneous Isc is below 2-3 muequiv/cm2 hr. 2. The stimulation of Isc by mersalyl is due to an increase in Na absorption, simultaneously K secretion is induced, whereas Cl absorption is not affected. 3. Mersalyl inhibits Isc at Na concentrations below 50 mM. The Na concentration at which Isc is half-maximal (KNa) is shifted by mersalyl from 25 to 133 mM. The overshoot in Isc to a peak volume of 5 muequiv/cm2 hr observed when Na-depleted tissues are suddenly exposed to Na is markedly depressed by mersalyl. 4. Mersalyl inhibits non-competitively the blocking effect of amiloride on Isc. Both the stimulation of Isc and the inhibition of the amiloride effect by mersalyl have the same time course (half-time of the effects 30-40 min) and similar concentration-response curve (half-maximal effects with 2.0-2.6 x 10(-4) M), indicating a common mechanism. 5. The mersalyl effects on Isc and on the amiloride action are only partially reversed by dimercaptopropanol. p-Chloromercuribenzoate conjugated with dextran (mol. wt. 10,000) elicited the same effects as mersalyl. 6. The stoichiometry of the mersalyl-amiloride interaction, estimated by use of the Hill plot, is 1:1; a Hill coefficient of 1 was also obtained for the stimulating effect of mersalyl on Isc. 7. It is concluded that one sulphydryl group per luminal Na entry site controls both its Na conductance and cation selectivity. Titration of these sulphydryl groups by organic mercurials appear to fix the conductance of the luminal Na entry mechanism in a submaximal position and prevent its modulation by amiloride or variations in intra- and/or extracellular Na concentrations.

Diazontized (125I) diiodosulafanilic acid as a label for cell surface membranes. Studies on erythrocytes

1. Diazotized 2,6-diiodosulfanilic acid (DDISA) appears to have properties suitable to serve as an artificial, non-penetrating label of cell surface membranes. Therefore, the conditions for selective labeling of cell surface membranes as compared to intracellular proteins as well as a method for its chemical determination were explored in the present study. 2. DDISA reacts with alpha-naphthol at neutral pH to produce a compound (1-hydroxy-4-(2,6-diiodo-4-sulfo-1-phenylazo-(naphthylene)), DSPN) with a characteristic spectrum in the visible range (Amax 430 nm). The absorbance of the reaction product, DSPN, is linearly proportional to the concentration of DDISA and can be used as a method for the colorimetric determination of DDISA. Reaction of DDISA with a molar excess of alpha-naphthol was also used as a method for inactivating unreacted DDISA to terminate labeling prior to cell fractionation. 3. [125I]DDISA reacts avidly with a variety of basic, neutral and acidic proteins as well as with cell membranes to form an acid-stable covalent azo linkage. 4. Effectiveness of labeling of the surface membrane of intact erythrocytes after incubation with [125I]DDISA was assessed by th ratio of 125I incorporated into membrane proteins compared to intracellular proteins. When intact erythrocytes were exposed to [125I]DDISA, the optimal labeling of membranes occurred at 37 degrees C after 20 min of incubation time and at a concentration of 10(-4) M [125I]DDISA in the incubation media. Under these conditions the ratio of the specific activity (cpm 125I/mg protein) of the membrane fraction to the specific activity of the soluble protein fraction (membrane/supernatant ratio) was greater than 500. When incubations were conducted at 4 degrees C this ratio was less than 50. However, when osmotically lysed erythrocytes were incubated with [125I]DDISA the majority of the label reacted with the soluble protein fraction resulting in a membrane/supernatant ratio of 0.14. 5. The results thus suggest that [125I]DDISA used under the appropriate incubation conditions, including the inactivation and removal of [125I]DDISA by washing with alpha-naphthol, can serve as a highly selective membrane label with minimal incorporation into intracellular soluble proteins. The general applicability of this method for other cell types remains to be explored.

Nucleoside transport in mammalian cell membranes. III. Kinetic and chemical modification studies of cytosine-arabinoside and uridine transport in hamster cells in culture

Transport of the nucleoside analog cytosine-arabinoside (CAR) in transformed hamster cells in culture has been studied in conditions of minimal metabolic conversion. Uptake (zero-trans in) properties at 20 degrees C over a limited range of CAR concentrations were characterized by a Km of 350 micrometer and a maximal velocity (V) of 780 micrometer.min-1 (V/Km = 2.28 min-1). Equilibrium exhcange at 20 degrees C over a wider range of concentrations was best described by a saturable component with a Km of 500 micrometer and a v of 1230 micrometer.min-1 (V/Km = 2.26 min-1) and either a saturable component of high Km or a nonsaturable component of k = 0.3 min-1. For the saturable component, the v/Km values were similar in both procedures. CAR transport was inhibited by various metabolizable nucleosides. Uptake of some of these nucleosides was inhibited by CAR. CAR transport and uridine uptake were inhibited in a reversible but partially competitive fashion by high affinity probes like S-(p-nitrobenzyl-6-mercaptoinosine (NBMI) (Ki less than 0.5 nM) and in an irreversible fashion by SH reagents such as N-ethylmaleiimide (NEM). The organomercurial p-hydroxymercuribenzene sulfonate (pMBS) markedly stimulated transport of these nucleosides, but also markedly potentiated the inhibitory effects of either NBMI or NEM. The effects are interpreted either in terms of models which invoke allosteric properties or in terms of two transport systems which display distinct chemical susceptibilities to externally added probes.

Nucleoside transport in mammalian cell membranes. IV. Organomercurials and organomercurial-mercaptonucleoside complexes as probes for nucleoside transport systems in hamster cells

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 (Ki = 3--6 micrometer). 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-aklylated 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 which stimulate uridine uptake upon binding of organomercurials, and (iv) SH-containing modifier sites which inhibit 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.

The mechanism of interaction between high-affinity probes and the uridine transport system of mammalian cells

The carrier of uridine transport in hamster cells in culture is highly susceptible to the inhibitory effect of probes like S-benzylated derivatives of mercaptopurine nucleosides. The interaction between the probes and the carrier is competitive and reversible and it takes place at a site different from the substrate binding site. The Ki for the most potent derivative p-nitrobenzyl-6-mercaptoinosine is 0.15 n Molar at 20 degrees C. The effect of the probes is interpreted in terms of conformational change induced on the carrier upon binding of the probe. The carrier assumes distinct conformations depending on whether it is probe-free (form A) or probe bound (form B). Kinetic as well as chemical evidence supports the predictions of the allosteric carrier model. A single component of kinetics is observed either in the absence of inhibitor (Km form A) or at high concentrations of inhibitor (Km form B). A two component kinetics is observed at intermediate concentrations of inhibitor (some carriers in form B and others in form A). The two forms have distinct Km values for uridine: form A50 muMolar and form B 250 muMolar. Two forms have also different susceptibilities to the action of organomercurials: form A is insensitive whereas form B is highly inhibited by the chemical modified of SH groups. The existence of putative allosteric sites in carriers is discussed in terms of modifier sites capable of modulating transport activities as a result of specific membrane-ligand interactions.

Effects of dextran-linked chloromercuribenzoic acid on insulin release from microdissected pancreatic islets

Insulin release in response to dextran-linked p-chloromercuribenzoic acid was studied in microdissected pancreatic islets of non-inbred ob/ob-mice. No contamination of the dextran-linked mercurial with free chloromercuribenzoic acid was detected before or after the incubation with islets. In comparison with free mercurial, of the same thiol-blocking activity, the dextran-linked compound had a weak insulin-releasing action with a different dose vs. response relationship. The dextran-linked mercurial had no demonstrable effect on the islet content of cyclic AMP. The results support the hypothesis that free organic mercurials mainly stimulate insulin release by blocking thiol ground that are embedded within the beta-cell plasma membranes beneath their surfaces.

Membrane SH-groups related to adrenaline action in rat adipocytes: a comparative study using sulfhydryl reagents of different molecular size

The orientation of SH-groups within the fat cell membrane involved in adrenaline (and NaF) action was studied by comparing the effects of uncoupled p-CMB with those of a large derivative of this reagent -- p-CMB-dextran --. Preincubation of intact adipocytes with uncoupled p-CMB caused a dose-dependent inhibition of adrenaline (and NaF) stimulated adenylate cyclase activity as determined in ghosts prepared subsequently. Preincubation with p-CMB-dextran, however, influenced neither the lipolytic response to adrenaline nor the catecholamine (and NaF) activated adenylate cyclase activity. When p-CMB-dextran was present during ghost preparation, a dose-dependent inhibition of adrenaline (and NaF) stimulated adenylate cyclase activity was observed. These results suggest that the binding sites of adrenaline as well as SH-groups essential for activity of adenylate cyclase are not localized near enough to the exterior surface to be accessible for p-CMB-dextran. The polarity in the sensitivity of fat cell adenylate cyclase could not be observed when p-CMB-dextran was added directly to intact or fragmented ghosts. The abbreviations used are: Cyclic AMP, cyclic adenosine 3',5'-monophosphate; p-CMB, p-chloromercuribenzoate.

Amplification of the bradykinin response in rat uterus by pCMB-Dextran T 10

pCMB-Dextran T 10,a large, non-penetrating SH-blocking reagent caused a reversible amplification of bradykinin sensituvity in the isolated rat uterus. High concentrations of this compound (1 x 10-3 M) can evoke spontaneous uterus contraction. pCMB-Dextran T 10 did not alter the biological activity of bradykinin. It is assumed that superficial membrane SH-groups are involved in the uterus contraction mechanism.

p-CMB-dextran T 10. A useful tool in evaluating the functional importance of superficial SH-groups in rat adipocytes

A large SH-blocking reagent p-CMB-dextran T 10 (MW about 10,000) was applied to isolated rat adipocytes. This compound possesses the same reactivity towards SH-groups as uncoupled p-CMB. P-CMB-Dextran T 10 did not influence the basal glucose uptake in contrast to uncoupled p-CMB. The results are discussed with respect to the usefulness of p-CMB-dextran T 10 in evaluating the role of superficial SH-groups in various membrane functions.