Photocatalyzed labeling of adipocyte plasma membranes with an aryl azide derivative of glucose

Synthesis of photoaffinity probes [2′-iodo-4′-(3′′-trifluoromethyldiazirinyl)phenoxy]-d-glucopyranoside and [(4′-benzoyl)phenoxy]-d-glucopyranoside for the identification of sugar-binding and phlorizin-binding sites in the sodium/d-glucose cotransporter protein

In this paper we describe the synthesis and photochemical and biochemical properties of two new photoaffinity probes designed for studies on the structure-function relationship of the sodium D-glucose cotransporter (SGLT1). The two probes are [2(')-iodo-4(')-(3(")-trifluoromethyldiazirinyl)phenoxy]-D-glucopyranoside (TIPDG), a mimic for the phenyl glucopyranoside arbutin which is transported by SGLT1 with a very high affinity, and [(4(')-benzoyl)phenoxy]-D-glucopyranoside (BzG), a model compound for phlorizin, the most potent competitive inhibitor of sugar translocation by SGLT1. Both photoaffinity probes TIPDG (lambda(max)=358 nm) and BzG (lambda(max)=293 nm) can be activated at 350-360 nm, avoiding protein-damaging wavelengths. In inhibitor studies on sodium-dependent D-glucose uptake into rabbit intestinal brush border membrane vesicles TIPDG and BzG showed a fully competitive inhibition with regard to the sugar with respective K(i) values of 22+/-5 microM for TIPDG and 12+/-2 microM for BzG. These K(i) values are comparable to those of their parent compounds arbutin (25+/-6 microM) and phlorizin (8+/-1 microM). To further test the potential of TIPDG and BzG as photoaffinity probes, truncated loop 13 protein, supposed to be part of the substrate recognition site of SGLT1, was exposed to TIPDG and BzG in solution. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry analysis demonstrated that TIPDG and BzG successfully labeled the protein. These preliminary results suggest that both photoaffinity probes are promising tools for the study of the structure-function relationship of SGLT1 and other SGLT1 family transporter proteins.

Mutagenic activity of 6-azido deoxyhexoses and azido alcohols in Salmonella typhimurium and its inhibition by a structure-similar carbon source in the medium

6-Azido-6-deoxy (AZd) derivatives of D-glucose, D-mannose, D-altrose, D-allose, L-idose, D-galactose, D-galactonic acid and D-galactitol, 3-azido-1,2-propanediol (azidoglycerol), 3,1-diazido-2-propanol (diazidoglycerol) and (at much higher doses) 2-azidoethanol were mutagenic in Salmonella typhimurium strains TA100 and TA1535. The mutagenic response was similar to that induced by sodium azide, i.e., the azido compounds failed to induce mutations in strain TA98, and mutagenesis was independent of plasmid pKM101, and independent of external activation. The specific mutagenicity (his+ rev/mmole) of AZd-glucose and AZd-galactose was decreased with increasing concentrations of D-glucose or D-galactose in the minimal agar medium and enhanced 100-fold or more when 0.2% citrate rather than 0.2% glucose served as the carbon source in the medium. Similarly, the mutagenic efficiency of azidoglycerol was inhibited by glycerol but not by D-glucose or D-galactose; however, the mutagenicity of sodium azide was not influenced by any of these carbon sources in the medium. The inhibition of the mutagenic action of azido hexoses and azido alcohols by non-azido structural analogs is assumed to reside in competition in transmembrane transport or for the metabolic pathways.

Selective labeling of the erythrocyte hexose carrier with a maleimide derivative of glucosamine: relationship of an exofacial sulfhydryl to carrier conformation and structure

Sulfhydryl-reactive derivatives of glucosamine were synthesized as potentially transportable affinity labels of the human erythrocyte hexose carrier. N-Maleoylglycyl derivatives of either 6- or 2-amino-2-deoxy-D-glucopyranose were the most potent inhibitors of 3-O-methylglucose uptake, with concentrations of half-maximal irreversible inhibition of about 1 mM. Surprisingly, these derivatives were very poorly transported into erythrocytes. They reacted rather with an exofacial sulfhydryl on the carrier following a reversible binding step, the latter possibly to the exofacial substrate binding site. However, their reactivity was determined primarily by access to the exofacial sulfhydryl, which, as predicted by the one-site model of transport, required a carrier conformation with the exofacial substrate binding site exposed. Once reacted, the carrier was "locked" in a conformation unable to reorient inwardly and bind cytochalasin B. In intact erythrocytes the N-maleoylglycyl derivative of 2-[3H]glucosamine labeled predominantly an Mr 45,000-66,000 protein on gel electrophoresis in a quantitative and cytochalasin B inhibitable fashion. By use of changes in carrier conformation induced by competitive transport inhibitors in a "double" differential labeling method, virtually complete selectivity of labeling of the carrier protein was achieved, the latter permitting localization of the reactive exofacial sulfhydryl to an Mr 18,000-20,000 tryptic fragment of the carrier.

Photolabeling of the adipocyte hexose carrier with an aryl azide derivative of maltose

A nitrophenyl azide derivative of maltose, N-(4-azido-2-nitrophenyl)-maltosylamine (NAP-maltosylamine), was synthesized as a potential photoaffinity label for the hexose carrier of the rat adipocyte. This derivative inhibited 3-O-methylglucose uptake with a Ki of 1.3 mM in the dark, while that of maltose was 10.0 mM. Carbon-14-labeled maltose and NAP-maltosylamine entered adipocytes via the hexose carrier, the latter in a concentrative fashion. Photolysis of NAP-[14C]maltosylamine in the presence of an adipocyte low density microsomal membrane fraction labeled several electrophoretic bands. Among these are a 45 kDa band which showed features expected of the hexose carrier: its labeling was decreased 40% by D- but not L-glucose and pretreatment of intact adipocytes with insulin decreased labeling of the 45 kDa band by 10-40%, as predicted by the translocation theory of insulin-stimulated transport activation. These studies show the suitability of using carbon-1-modified sugar photoaffinity labels as probes for the hexose carrier and possibly of its regulation in rat adipocytes.

Reconstitution of the glucose transport activity of rat adipocytes

Rat epididymal fat cell membrane proteins were extracted from adipocyte ghosts with octylglucoside and incorporated by detergent dialysis into unilamellar phosphatidylcholine vesicles approx. 200 nm in diameter. The rate of glucose transport into the vesicles under zero-trans conditions was substrate dependent, saturable and inhibited by phloretin and cytochalasin B. Their maximum specific transport activity was 35.6 mumol/min per mg protein, and their half saturation constant for glucose was 15 mM. Glucose transport into the reconstituted vesicles was inhibited by only those sugars which competitively inhibited glucose transport into intact adipocytes. A major protein component of the vesicles was a 100 kDa protein which we had previously found to react with the affinity label maltosyl isothiocyanate (Malchoff, D.M., Olansky, L., Pohl, S. and Langdon, R.G. (1981) Fed. Proc. 40, 1893). Removal of adipocyte ghost membrane extrinsic proteins with dimethylmaleic anhydride followed by extraction of the resulting membrane pellet with octylglucoside yielded a solution which contained two major proteins, of Mr 100 000 and 85 000, with very small quantities of lower Mr proteins. Vesicles into which these proteins were incorporated had average specific transport activities of 624 mumol/min per mg protein and half saturation constants of 22 mM. Our results strongly indicate that the native glucose transporter of the rat adipocyte, like that of the human erythrocyte (Shelton, R.L. and Langdon, R.G. (1983) Biochim. Biophys. Acta 733, 25-33), is a 100 kDa protein.

Phloretinyl-3'-benzylazide: a high affinity probe for the sugar transporter in human erythrocytes. I. Hexose transport inhibition and photolabeling of mutarotase

A new phloretin derivative, phloretinyl-3'-benzylazide (PBAz), has been synthesized and compared with phloretin for its ability to inhibit the hexose transporter in human erythrocyte membranes in subdued light. Transport measurements were made using the light scattering (Orskov optical) method and a Millipore filtration technique with isotopically labeled sugars. Initial rates of sugar flux were measured under four different conditions to test for inhibition asymmetry. In each experimental condition, PBAz is from 6-20-times more potent than phloretin, making it one of the most effective reversible inhibitors known. Although both agents penetrate the cell membrane, they apparently fail to reach inhibitory levels at the inner surface over the time course of our nonequilibrated experiments, because of extensive binding to hemoglobin. The mechanism by which PBAz and its parent phloretin inhibit transport is pure competition with hexose for the carrier which faces the exterior of the membrane. If given time to equilibrate with the cells, the inhibition by both agents converts to a mixed type, i.e., both competitive and noncompetitive. The noncompetitive component could be due to inhibition of those transporter units oriented internally. Alternatively pre-equilibration with the inhibitors may cause them to attain high levels in the lipid membrane and produce nonspecific effects. PBAz and its precursor amine, phloretinyl-3'-benzylamine (PBA), compete with glucose for the sugar binding site on mutarotase at least as well as phloretin. When exposed to long wavelength ultraviolet radiation, PBAz is converted to a reactive intermediate which becomes covalently bound to the enzyme. Both irreversible ligand attachment and mutarotase inhibition are related to dose of the azide and irradiation time, but inactivation is from 5 to 6-times greater than label incorporation. We conclude that PBAz is a potentially useful photoaffinity labeling agent capable of covalently interacting with the transporter site facing the exterior of the red cell.

Maltosyl isothiocyanate: an affinity label for the glucose transporter of the human erythrocyte membrane. 1. Inhibition of glucose transport

Maltosyl isothiocyanate (MITC) has been synthesized from maltose with an overall yield of 88%. It has been found to be a potent irreversible inhibitor of zero trans influx of glucose with human erythrocytes. Kinetic analysis of glucose transport after treatment of erythrocytes with MITC revealed that VT was diminished while KT was unchanged. Transportable sugars and competitive inhibitors of monosaccharide transport protected against MITC inhibition, while carbohydrates which do not interact with the transporter gave no protection. Covalent inhibitors of anion transport were without effect on glucose transport. MITC fulfilled the kinetic requirements for an affinity label of the glucose transporter of human erythrocytes [Groman, E. V., Schultz, R. M., & Engel, L. L. (1977) Methods Enzymol. 46, 54].

Maltosyl isothiocyanate: an affinity label for the glucose transporter of the human erythrocyte membrane. 2. Identification of the transporter

Maltosyl isothiocyanate (MITC), a potent irreversible inhibitor of glucose transport in human erythrocytes [Mullins, R. E., & Langdon, R. G. (1980) Biochemistry (preceding paper in this issue)], has been found to react almost exclusively with band 3 of the human erythrocyte membrane. The incorporation of [14C]MITC into band 3 was found to be antagonized by transportable sugars or competitive inhibitors of transport. On the basis of [14C]MITC incorporation into band 3 and MITC inhibition of transport, it is estimated that there are 3 x 10(5) glucose transporters present in the erythrocyte membrane. It was found that [14C]MITC-labeled band 3 could be converted into 14C-labeled band 4.5 during the Triton X-100 extraction procedure described by Kasahara & Hinkle [Kasahara, M., & Hinkel, P. C. (1977) J. Biol. Chem. 252, 7384]. On the basis of the evidence presented here and in the preceding paper, it is suggested that in the native erythrocyte membrane a component of band 3 is the glucose transport protein and that during purification with nonionic detergents the transport protein may be enzymatically degraded with some retention of activity.

The effect of amino acid analogues and heat shock on gene expression in chicken embryo fibroblasts

The addition of certain amino acid analogues (canavanine, hydroxynorvaline, o-methylthreonine) or a mild heat shock at 45 degrees C caused chicken embryo fibroblasts to increase rapidly the synthesis of three proteins (molecular weights 22,000, 76,000 and 95,000 daltons) to levels which dominate the cells biosynthetic capacity and exceed the level of synthesis of the major cell structural proteins. Actinomycin D blocked the increased synthesis of p22, p76 and p95 in both analogue and heat shock-treated cells, while cycloheximide addition during the "induction" period blocked formation of these proteins only in analoguetreated cells. The elevated levels of synthesis for this set of proteins began to decrease shortly after restoration of the normal amino acid or normal temperature, and the normal pattern of cell protein synthesis was found 8 hr later. Induction of a similar set of proteins was detected in mouse L cells and baby hamster kidney cells after treatment with amino acid analogues or heat shock. Several laboratories have reported synthesis of proteins with similar molecular weights in cells subjected to conditions that alter glucose metabolism, and we speculate that these proteins may be associated with a hexose transport system.

Synthesis and properties of a photoaffinity probe for the glucagon receptor in hepatocyte plasma membranes

The reaction of glucagon with 4-fluoro-3-nitrophenylazide has been shown to afford the photosensitive derivative, Nepsilon-4-azido-2-nitrophenyl-glucagon. The structure and properties of this derivative were established by amino acid analysis, absorption and fluorescence spectroscopy, deamination, Edman degradation and photolysis. This photoaffinity derivative of glucagon has been used to label specifically glucagon binding sites on hepatocyte plasma membranes.

Retention of insulin-stimulated D-glucose transport activity by adipocyte plasma membranes following extraction of extrinsic proteins

Plasma membrane vesicles prepared from adipocytes incubated with insulin exhibited accelerated D-glucose transport activity characteristic to insulin action on intact fat cells. Both control and insulin-stimulated D-glucose transport activities were inhibited by cytochalasin B and thiol reagents. Extraction of plasma membranes with dimethylmaleic anhydride eluted 80% of the protein from plasma membrane vesicles. The two major glycoprotein bands (94,000 and 78,000 daltons) and small amounts of a 56,000-dalton band were retained in dodecyl sulfate gels of the extracted membranes. Both control and insulin-activated D-glucose transport activities were retained by plasma membrane vesicles extracted with dimethylmaleic anhydride. Cytochalasin B binding activity was also retained by extracted membrane vesicles and D-glucose uptake into extracted vesicles derived from untreated or insulin-treated fat cells was inhibited by cytochalasin B. These results suggest that the modification of the adipocyte hexose transport system elicited by insulin action is not altered by a major purification step which involves quantitative extraction of extrinsic membrane proteins.

The interaction of hepatocyte plasma membranes with an azide derivative of procaine

A photoreactive derivative of procaine, p-azidobenzoyldiethylaminoethanol hydrochloride, has been synthesized and used as a site-directed probe to label hepatocyte plasma membranes. The procaine derivative was shown to have membrane binding and Ca2+ displacement characteristics quite similar to that of procaine. Photolysis of the derivative in the presence of hepatocyte plasma membranes resulted in the covalent incorporation of the probe into both the protein and lipid fractions. Analysis of the labeled membranes by sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicated that one membrane protein was significantly labeled with a molecular weight of 21 400 in addition to membrane lipids. Both binding and labeling could be inhibited in the presence of an excess of procaine. The labeled membrane components may be involved in the binding of Ca2+ to the membrane system.