Photoaffinity labeling of the adenosine transport system in adipocyte plasma membranes

Inhibition of hexose transport by adenosine derivatives in human erythrocytes

The human erythrocyte membrane carriers for hexoses and nucleosides have several structural features in common. In order to assess functional similarities, the effects of adenosine derivatives on hexose transport and cytochalasin B binding sites were studied. Adenosine inhibited zero-trans uptake of 3-O-methylglucose half-maximally at 5 mM, while more hydrophobic adenosine deaminase-resistant derivatives were ten- to 20-fold more potent transport inhibitors. However, degradation of adenosine accounted for very little of this difference in potency. Hexose transport was rapidly inhibited by N6-(L-2-phenylisopropyl)adenosine at 5 degrees C in a dose-dependent fashion (EC50 = 240 microM), to lower the transport Vmax without affecting the Km. A direct interaction with the carrier protein was further indicated by the finding that N6-(L-2-phenylisopropyl)adenosine competitively inhibited [3H]cytochalasin B binding to erythrocytes (Ki = 143 microM) and decreased [3H]cytochalasin B photolabeling of hexose carriers in erythrocyte ghosts. The cross-reactivity of adenosine and several of its derivatives with the hexose carrier suggests further homologies between the carriers for hexoses and nucleosides, possibly related to their ability to transport hydrophilic molecules through the lipid core of the plasma membrane.

Solubilization and partial purification of S-adenosyl-L-homocysteine binding proteins from the rat brain

Rat brain proteins presenting high-affinity binding of S-adenosyl-L-homocysteine were solubilized and purified. Extraction of binding protein was carried out in the presence of Triton X-100 and 1 M NaCl; this solubilized fraction exhibits similar kinetic properties than the membrane proteins. Purification was performed using affinity chromatography on S-adenosyl-L-homocysteine carboxyhexyl Sepharose 48 conjugate. The analysis of the affinity gel eluate by SDS-PAGE showed high purification ratios for two proteins exhibiting 54 and 68 kDa. Three activity peaks were separated when solubilized membrane proteins were submitted to isoelectric focusing; the activity peaks corresponded to proteins of pH1 6.0, 6.5, and 7.2. SDS-PAGE separation of proteins contained in each peak showed protein aggregation; a 54-kDa subunit was present in each aggregate. Solubilized membrane proteins were labeled by photoaffinity labeling with tritiated S-adenosyl-L-homocysteine; the 54- and 68-kDa proteins were found among the specifically labeled proteins. Finally, according to the previous data from the literature, the purified S-adenosyl-L-homocysteine binding proteins do not seem to be the same as adenosine receptors or phosphatidylethanolamine-N-methyltransferase.

Covalent labelling of ligand binding sites of human placental S-adenosylhomocysteine hydrolase with 8-azido derivatives of adenosine and cyclic AMP

S-Adenosylhomocysteine hydrolase (AdoHcyase) has previously been identified as a cytoplasmic adenosine and cyclic AMP binding protein. In order to examine the relationship between the adenosine and cyclic AMP binding sites on this enzyme we have explored the use of 8-azido analogues of adenosine and cyclic AMP as photoaffinity reagents for covalently labelling AdoHcyase purified from human placenta. 8-Azidoadenosine (8-N3-Ado), like adenosine, inactivated AdoHcyase, and the rate of inactivation was greatly increased by periodate oxidation. In addition, 8-N3-Ado was found to participate in the first step in the catalytic mechanism for AdoHcyase, resulting in conversion of enzyme-bound NAD+ to NADH, although it was not a substrate for the full enzyme-catalysed reaction. Radioactively labelled 8-N3-Ado, its periodate-oxidized derivative and 8-azidoadenosine 3', 5'-phosphate (8-N3-cAMP) bound specifically to adenosine binding sites on AdoHcyase and, after irradiation, became covalently linked to the enzyme. Photoaffinity-labelled enzyme could be precipitated by monoclonal antibody to human AdoHcyase. Two observations suggested that cyclic AMP and adenosine bind to the same sites on AdoHcyase. First cyclic AMP and adenosine each blocked binding of both radioactively labelled 8-N3-Ado and 8-N3-cAMP, and second, digestion with V8 proteinase generated identical patterns of peptides from AdoHcyase that had been photolabelled with [32P]8-N3-cAMP and [3H]8-N3-Ado. Binding sites for cyclic AMP on AdoHcyase were found to differ functionally and structurally from cyclic AMP binding sites on the R1 regulatory subunit of cyclic AMP-dependent protein kinase.

Interaction of [3H]arylazido aminopropionyl ATP ([3H]ANAPP3) with P2-purinergic receptors in the smooth muscle of the isolated guinea-pig vas deferens

Following its photolysis in the presence of the isolated guinea-pig vas deferens, the ATP photoaffinity label ANAPP3 produces a specific antagonism of adenine nucleotide-induced contractile responses which are mediated by P2-purinergic receptors. To characterize the site of covalent photoincorporation of ANAPP3, intact vasa deferentia were treated with [3H]ANAPP3 and samples of homogenate, cytosol and a crude membrane fraction were analyzed using sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Photolysis of [3H]ANAPP3 (10(-5) M; 3.0 mu Ci/ml) resulted in the incorporation of radioactivity into cellular components with apparent molecular weights of 54-66 and 43-57 kilodaltons. The photoincorporation of [3H]ANAPP3 was associated with the crude membrane fraction and not the cytosol, was reduced in the presence of ATP in an ATP-concentration-dependent manner, was lessened following pretreatment of the tissues with photolyzed nonradiolabeled ANAPP3, and was unaffected by the nucleoside transport inhibitor, dipyridamole. In tension studies on the same tissues the presence of ATP resulted in a concentration-dependent reduction in the initial contractile response to [3H]ANAPP3 the response to 3H was antagonized in tissues which had been pretreated with nonradiolabeled ANAPP3, and dipyridamole had no effect on the contractile response to [3H]ANAPP3. According to several criteria these findings indicate that the antagonism by photolyzed ANAPP3 of adenine nucleotide-induced responses is a direct result of the covalent insertion at or near the recognition site of cell-surface P2-purinergic receptors.

Effect of N6-methyladenosine on fat-cell glucose metabolism. Evidence for two modes of action

N6-Methyladenosine strongly stimulates [1-14C]glucose oxidation in rat adipocytes [J.E. Souness and V. Chagoya de Sanchez, Fedn Eur. Biochem. Soc. Lett. 125, 249 (1981)]. The effect of the adenosine analogue is largely independent of its action as an R-site agonist. Removal of endogenous adenosine was a prerequisite for the manifestation of the effect of N6-methyladenosine. Nucleoside uptake inhibitors, dipyridamole and nitrobenzylthioinosine, did not block the action of N6-methyladenosine on [1-14C]glucose oxidation. The effect of the adenosine analogue was not greatly influenced by N6-phenylisopropyladenosine, nicotinic acid or theophylline. Although N6-methyladenosine stimulated 3-O-methylglucose uptake into fat cells, it is uncertain whether this was its only effect on glucose metabolism, in view of the comparable enhancement of hexose transport elicited by N6-phenylisopropyladenosine, a much weaker stimulator of glucose oxidation. That hexose transport is not the sole site of action of N6-methyladenosine was supported by the finding that, under conditions which have been proposed to make glucose transport rate limiting, the adenosine analogue only weakly enhanced [1-14C]glucose oxidation. The conversion of glucose carbon 1 to 14CO2 was enhanced by N6-methyladenosine to a greater degree than that of carbon 6, suggesting an increase in pentose phosphate shunt activity. Mechanisms by which this could arise are discussed. Although similarities exist between the effects of insulin and N6-methyladenosine on adipocyte glucose metabolism, the mechanisms by which they stimulate glucose oxidation appear to be distinct, in view of the additivity of their actions on [1-14C]glucose conversion to 14CO2. The results indicate that N6-methyladenosine affects fat-cell glucose metabolism via two different mechanisms: (1) a weak R-site-dependent mode of action related to stimulation of glucose transport and inhibition of lipolysis, and (2) a strong R-site-independent effect of unknown mechanism.