Radioiodination of a photoactivatable heterobifunctional reagent

The Drosophila acetylcholine receptor subunit D alpha5 is part of an alpha-bungarotoxin binding acetylcholine receptor

The central nervous system of Drosophila melanogaster contains an alpha-bungarotoxin-binding protein with the properties expected of a nicotinic acetylcholine receptor. This protein was purified 5800-fold from membranes prepared from Drosophila heads. The protein was solubilized with 1% Triton X-100 and 0.5 M sodium chloride and then purified using an alpha-cobratoxin column followed by a lentil lectin affinity column. The purified protein had a specific activity of 3.9 micromol of 125I-alpha-bungarotoxin binding sites/g of protein. The subunit composition of the purified receptor was determined by sodium dodecyl sulfate polyacrylamide gel electrophoresis. This subunit profile was identical with that revealed by in situ labeling of the membrane-bound protein using the photolyzable methyl-4-azidobenzoimidate derivative of 125I-alpha-bungarotoxin. The purified receptor reveals two different protein bands with molecular masses of 42 and 57 kDa. From sedimentation analysis of the purified protein complex in H2O and D2O and gel filtration, a mass of 270 kDa was calculated. The receptor has a s(20,w) of 9.4 and a Stoke's radius of 7.4 nm. The frictional coefficient was calculated to be 1.7 indicating a highly asymmetric protein complex compatible with a transmembrane protein forming an ion channel. The sequence of a peptide obtained after tryptic digestion of the 42-kDa protein allowed the specific identification of the Drosophila D alpha5 subunit by sequence comparison. A peptide-specific antibody raised against the D alpha5 subunit provides further evidence that this subunit is a component of an alpha-bungarotoxin binding nicotinic acetylcholine receptor from the central nervous system of Drosophila.

Partial characterization of chorionic gonadotropin-like binding sites from the bacteria Xanthomonas maltophilia

The gram-negative bacterium, Xanthomonas maltophilia, has low- and high-affinity luteinizing hormone/chorionic gonadotropin (LH/CG)-binding sites, similar to the LH/CG receptor found in mammals. Although the low-affinity site binds both LH and human CG (hCG), the high-affinity site is specific for hCG. In the current investigation, these two binding sites were independently isolated from X. maltophilia for further characterization. To isolate functional binding sites, we developed a solubilization method using the detergent zwittergent 3,14 and high glycerol concentrations that allowed for the maintenance of ligand-binding integrity. Gel filtration experiments established molecular weights of 170 and 11.5 kDa for the two binding sites, which were supported by data from photoaffinity labeling and ultracentrifugation experiments. Gel filtration data also suggested the presence of a third binding site of 5.4 kDa. The 170-kDa site had a binding affinity of Kd = 12 x 10(-6) and bound both LH and hCG. The small molecular weight site had an affinity of Kd = 9.4 x 10(-8) and was CG specific. Collectively, these data demonstrate the presence of multiple hormone binding sites in X. maltophilia that differ in molecular size, binding affinity, and ligand specificity.

Conotoxin MI inhibits the alpha-delta acetylcholine binding site of the Torpedo marmorata receptor

The muscle-type nicotinic receptor has two pharmacologically distinguishable acetylcholine binding sites at the alpha-gamma and alpha-delta subunit interfaces; alpha-conotoxins can bind them selectively. As reported, alpha-conotoxin MI has greater affinity for the site near the alpha-delta interface of the BC(3)H1 cell receptor but, in the case of the Torpedo californica receptor, displays greater affinity for that near the alpha-gamma interface. To further investigate ligand selectivity, we study the conotoxin MI-Torpedo marmorata receptor interaction. In this work, we show the binding of alpha-conotoxin MI to the T. marmorata receptor and the influence of the antagonist alpha-Bungarotoxin and the agonist carbamylcholine on such binding; in addition, and contrasting with the results for the Torpedo californica receptor, we identify the alpha-delta subunit interface as the high affinity binding site. This is the first work describing different characteristics of the interaction between alpha-conotoxin MI and receptors from different species of the same genus.

Synthesis and use of 3'-(azidoiodosalicyl) derivatives of 2', 5'-dideoxyadenosine as photoaffinity ligands for adenylyl cyclase

3'-[(4-Azidosalicyl)glycyl]-2',5'-dideoxyadenosine (1), 3'- [(4-azidosalicyl)-gamma-aminobutyryl]-2',5'-dideoxyadenosine (2), and the (125)I-labeled mono- and diiodinated analogs of 1 were synthesized and tested as photoaffinity probes for adenylyl cyclases. Kinetics for inhibition of purified type I enzyme by 1 was noncompetitive with respect to Mn(*)5'-ATP in the absence of light, implying a P-site mechanism of inhibition. In a UV-dependent manner both 1 and 2 and the iodinated derivative of 1 irreversibly inactivated membrane-bound and purified forms of recombinant type I bovine adenylyl cyclase expressed in ovarian cells of either the fall armyworm (Sf9) or Trichoplasia ni (High Five). Irreversible inactivation was independent of 5'-ATP and was prevented by 2', 5'-dideoxyadenosine. Adenylyl cyclase, whether purified from bovine brain or in membranes from High Five cells expressing type I enzyme, when subjected to UV irradiation in the presence of (125)I-labeled 1 resulted in radioactive incorporation into protein migrating at approximately 116 kDa. The cross-linking of 1 and its iodinated derivative with adenylyl cyclase suggests potential for such compounds to be useful in structural studies of adenylyl cyclases or of other proteins for which adenine nucleosides are substrates or allosteric regulators.

Synthesis of azidophospholipids and labeling of lysophosphatidylcholine acyltransferase from developing soybean cotyledons

A photoreactive substrate analog of lysophosphatidylcholine (LPC), 1-([(4-azidosalicyl)-12-amino)]dodecanoyl-sn-glycerol-3-phospho cholin e (azido-LPC) was synthesized. Fast atom bombardment mass spectrometry was employed to confirm the structures of azido-LPC and its intermediates. Azido-LPC was used to label putative acyl-CoA:LPC acyltransferase from microsomal membranes of developing soybean cotyledons. The synthesized substrate analog acts as a substrate for the target acyltransferases and phospholipases in the dark. When the microsomal membranes were incubated with the acyl acceptor analog and immediately photolyzed, LPC acyltransferase was irreversibly inhibited. Photoinactivation of the enzyme by the photoprobe decreased in the presence of LPC. Microsomal membranes were photolyzed with 125I-labeled azido-LPC and analyzed by SDS-PAGE followed by autoradiography. These revealed that the analog preferentially labeled 54- and 114-kDa polypeptides. Substrate protected the labeling of both the polypeptides. In our earlier report, the same polypeptides were also labeled with photoreactive acyl-CoA analogs, suggesting that these polypeptides could be putative LPC acyltransferase(s). These results demonstrated that the photoreactive phospholipid analog could be a powerful tool to label acyltransferases involved in lipid biosynthesis.

Synthesis and characterization of an aryl-azidoparoxetine. A novel photo-affinity probe for serotonin-transporter

Paroxetine is an effective antidepressant drug and potent serotonin (5-HT) uptake inhibitor. It selectively labels 5-HT transporter on platelets and neurons. We report here the synthesis of an aryl-azido derivative of paroxetine, which is a novel photoactive and irreversible ligand for the [3H]paroxetine binding site on the platelet 5-HT transporter. The compound inhibited [3H]paroxetine binding (IC50, 55 nM) and 5-HT uptake (IC50, 12 nM) at equilibrium conditions and inactivated 10-20% of [3H]paroxetine binding sites upon irradiation at 320 nm. SDS-PAGE of platelet protein extract labelled with the radioactive analogue of the synthesized probe revealed the presence of four radioactive bands of which the 71-kDa one was the most prominent.

Photoaffinity labeling of cottonseed microsomal N-acylphosphatidylethanolamine synthase protein with a substrate analogue, 12-[(4-azidosalicyl)amino]dodecanoic acid

N-Acylphosphatidylethanolamine (NAPE), an unusual acylated derivative of phosphatidylethanolamine (PE), is synthesized from free fatty acids and PE in cotton seedlings (Chapman and Moore (1993) Plant Physiol. 102(3), 761-769). Here we use a photoreactive dodecanoic acid analogue, [12-(4-azidosalicy)amino]dodecanoic acid (ASD), and its 125I-labeled derivative to identify a protein subunit which corresponds to this cottonseed NAPE synthase activity. Dodecylmaltoside (DDM)-solubilized microsomal NAPE synthase enzyme was irreversibly and progressively inactivated by adding increasing concentrations of ASD and illuminating with UV254 light. Protection from this photoinactivation was afforded by the natural substrate, palmitic acid. In low light, microsomal NAPE synthase utilized ASD as a substrate to synthesize NAPE; palmitic acid competed for this activity. NAPE synthase activity was measured directly in gel slices following nondenaturing PAGE of DDM-solubilized microsomal membrane proteins. Two-dimensional electrophoresis (nondenaturing PAGE, followed by SDS-PAGE) of photoaffinity-labeled, DDM-solubilized microsomal proteins revealed a 64 kDa polypeptide that was associated with the active NAPE synthase enzyme. Also, a 64 kDa protein was photoaffinity labeled in all NAPE synthase isozyme fractions isolated by preparative isoelectric focusing; photoaffinity labeling of this 64 kDa polypeptide was diminished in the presence of exogenously supplied palmitic acid. Collectively, our results demonstrate that ASD specifically interacts with NAPE synthase in a manner analogous to its fatty acid substrate and indicate that a 64 kDa polypeptide is a component of cottonseed microsomal NAPE synthase. ASD will be a useful molecular probe in future studies aimed at understanding the physiological role of this NAPE synthase enzyme in membranes of plant cells.

Photolabeling of soybean microsomal membrane proteins with photoreactive acyl-CoA analogs

Synthesis of 32P-labeled 12-azidooleoyl-CoA and 125I-labeled 12-[(azidosalicyl)amino]dodecanoyl-CoA (ASD-CoA) was achieved. The synthesized radioactive, photoreactive reagents were tested as photoaffinity labels for acyl-CoA:lysophosphatidylcholine acyltransferase from the microsomal membranes of developing soybean cotyledons. When a mixture of microsomal membranes and the azidooleoyl-CoA or ASD-CoA were incubated in the dark, the analogs were recognized as substrate and competitive inhibitor, respectively. The enzyme preferentially utilizes unsaturated acyl-CoAs rather than saturated acyl-CoAs. Incubation of microsomal membranes with acyl-CoA analogs and immediately followed by photolysis resulted in an irreversible inhibition of lysophosphatidylcholine acyltransferase activity. Analysis of photolyzed microsomal membranes by SDS/PAGE and autoradiography revealed that azidooleoyl-CoA preferentially labeled eight acyl-CoA binding proteins, but ASD-CoA labeled only three polypeptides with molecular masses of 110, 90 and 32 kDa that are commonly labeled by both the analogs. Oleoyl-CoA and dodecanoyl-CoA protect the enzyme against photoinactivation by azidooleoyl-CoA and ASD-CoA, respectively. The protection was profound in 110-kDa polypeptide indicating that this protein could be lysophosphatidylcholine acyltransferase. These results demonstrate that the photoaffinity of acyl-CoA analogs makes them potential probes to identify and characterize lipid biosynthetic enzymes.

Localization of adjacent binding domains for cellular proteins over the minute virus of mice P4 promoter by site-specific photoaffinity labelling

A photoaffinity labelling (PHL) procedure was used to localize the specific binding sites for A92L fibroblast nuclear proteins on the minute virus of mice (MVM) P4 promoter. We describe a chemical and biochemical method for the construction of precisely modified photoreactive (phr) DNA probes. In this method, a phenylazide group is attached to the DNA fragment by coupling SASD [sulfosuccinimidyl-2-(p-azidosalicyl-amido)ethyl-1,3'-dithiopro pio nate] to the primary amino group of the linker arm present at any predetermined position. These phr probes would identify, upon photocrosslinking, only those proteins which bind to the location of the phr group. Specifically, two phr probes representing the 139-172 bp region of the MVM P4 promoter were constructed in which the highly phr phenylazide group was attached with a linker at nucleotide 168, towards the side of the GC box proximal to the TATA box. The PHL studies with these photoprobes revealed that although the proteins of 95 and 120-kDa bind near nt 168 of the P4 promoter, the 120-kDa protein requires the region between the TATA box and the GC box for binding to the MVM P4 promoter.

Photolabelling of Salmonella typhimurium LT2 sialidase. Identification of a peptide with a predicted structural similarity to the active sites of influenza-virus sialidases

The sialidase from Salmonella typhimurium LT2 was characterized by using photoaffinity-labelling techniques. The well-known sialidase inhibitor 5-acetamido-2,6-anhydro-3,5-dideoxy-D-glycero-D-galacto-non- 2-enonic acid (Neu5Ac2en) was modified to contain an amino group at C-9, which permitted the incorporation of 4-azidosalicylic acid in amide linkage at this position. Labelling of the purified protein with the radioactive (125I) photoprobe was determined to be highly specific for a region within the active-site cavity. This conclusion was based on the observation that the competitive inhibitor Neu5Ac2en in the photolysis mixture prevented labelling of the protein. In contrast, compounds with structural and chemical features similar to the probe and Neu5Ac2en, but which were not competitive enzyme inhibitors, did not affect the photolabelling of the protein. The peptide interacting with the probe was identified by CNBr treatment of the labelled protein, followed by N-terminal sequence analysis. Inspection of the primary structure of the protein, predicted from the cloned structural gene for the sialidase [Hoyer, Hamilton, Steenbergen & Vimr (1992) Mol. Microbiol. 6, 873-884] revealed that the label was incorporated into a 9.6 kDa fragment situated within the terminal third of the molecule near the C-terminal end. Secondary-structural predictions using the Garnier-Robson algorithm [Garnier, Osguthorpe & Robson (1978) J. Mol. Biol. 120, 97-120] of the labelled peptide revealed a structural similarity to the active site of influenza-A- and Sendai-HN-virus sialidases with a repetitive series of alternating beta-sheets connected with loops.

Obviation of drug resistance and affinity purification of P-glycoprotein by isoquinolinesulfonamides

A newly synthesized isoquinolinesulfonamide named H-85; N-[2-[N-formyl-N-[[3-(4-chlorophenyl)-2-propenyl] amino] ethyl]-5-isoquinolinesulfonamide was found to reverse drug resistance in multidrug resistant P388 murine leukemic cells (P388/ADR). The energy-dependent extrusion of [3H]vinblastine from P388/ADR-cells was significantly suppressed by 10 microM H-85 but not so the efflux from the sensitive P388 cells. A 140-kDa protein overexpressed in P388/ADR cells was photoaffinity labeled with a vinblastine analog; N-(P-azid-[3-125I]salicyl-N'-(beta-aminoethyl) vindesine and H-85 selectively inhibited photolabeling of the 140-kDa protein. This 140-kDa protein was purified to apparent homogeneity by succeeding steps of phosphocellulose, DEAE-cellulose, and W-66 (a derivative of H-85)-coupled sepharose chromatography. The purified 140-kDa protein proved to be immunopositive with the P-glycoprotein-specific monoclonal antibody, C219.

Activated alpha 2-macroglobulin is a principal defensin-binding protein

Defensins are highly abundant and variably cationic peptides that possess antimicrobial, cytotoxic, and chemoattractant properties and equip mammalian phagocytes for participation in host defense and inflammatory processes. We studied the binding of the human defensin HNP-1 by proteins in plasma and serum and identified activated (F-form) alpha 2-macroglobulin (alpha 2M) as a principal binding protein for HNP-1. In contrast, native (S-form) alpha 2M bound little HNP-1. The binding of HNP-1 by F-form alpha 2M was resistant to salt and boiling in 2% sodium dodecyl sulfate but was ablated by dithiothreitol. Pretreatment of methylamine-activated serum or plasma by iodoacetamide substantially decreased the binding of HNP-1 to alpha 2M, suggesting that thiol groups in activated alpha 2M play a role in defensin binding, possibly by covalently trapping defensins via thiol-disulfide exchange. Western blots of conventionally collected sera showed endogenous defensins complexed with the F-form of alpha 2M, indicating that the generation of defensin-alpha 2M complexes was not limited to the in vitro model of methylamine-activated serum or plasma and radiolabeled HNP-1. Previous studies indicated that native alpha 2M can be converted to its F-form by many proteases, including those released by neutrophils and platelets, and that the F-form is recognized and internalized by specific receptors on macrophages and hepatocytes. Our findings suggest that the alpha 2M system may function as a scavenger of defensins and other peptide mediators in inflamed tissues and may constitute an important mechanism for the regulation and containment of inflammation.

Synthesis of 3-(4-azido-5-iodosalicylamido)-4-hydroxycoumarin: photoaffinity labeling of rat liver dicoumarol-sensitive NAD(P)H: quinone reductase

A photoaffinity analog of 4-hydroxycoumarin containing an arylazido derivative at the 3-position has been synthesized and characterized. This compound, 3-(4-azido-5-iodosalicylamido)-4-hydroxycoumarin, serves as a strong competitive inhibitor of the dicoumarol-sensitive NAD(P)H: quinone reductase (DT-diaphorase) from rat liver, having an apparent inhibition constant of 4.2 10(-7) M. Irradiation of the reductase with ultraviolet light in the presence 10 microM of the photoprobe resulted in the covalent labeling of 2% of the reductase molecules. The enzyme is protected from labeling to greater than 99% by the inclusion of 3 microM dicoumarol, consistent with the specific labeling of the 4-hydroxycoumarin binding site of this enzyme. Furthermore, the quinone reductase was shown to specifically labeled by the probe even when contained within crude fractions rat liver cytosol.

Regulation of the skeletal sarcoplasmic reticulum Ca2+ pump by phospholamban in reconstituted phospholipid vesicles

Phospholamban is the regulator of the Ca(2+)-ATPase in cardiac sarcoplasmic reticulum (SR). The mechanism of regulation appears to involve inhibition by dephosphorylated phospholamban, and phosphorylation may relieve this inhibition. Fast-twitch skeletal muscle SR does not contain phospholamban, and it is not known whether the Ca(2+)-ATPase isoform from this muscle may be also subject to regulation by phospholamban in a similar manner as the cardiac isoform. To determine this we reconstituted the skeletal isoform of the SR Ca(2+)-ATPase with phospholamban in phosphatidylcholine proteoliposomes. Inclusion of phospholamban was associated with significant inhibition of the initial rates of Ca2+ uptake at pCa 6.0, and phosphorylation of phospholamban by the catalytic subunit of cAMP-dependent protein kinase reversed the inhibitory effects on the Ca2+ pump. Similar effects of phospholamban were also observed using phosphatidylcholine:phosphatidylserine proteoliposomes, in which the Ca2+ pump was activated by the negatively charged phospholipids (24). Regulation of the Ca(2+)-ATPase appeared to involve binding with the hydrophilic portion of phospholamban, as evidenced by cross-linking experiments, using a synthetic peptide that corresponded to amino acids 1-25 of phospholamban. These findings suggest that the fast-twitch isoform of the SR Ca(2+)-ATPase may be also regulated by phospholamban, although this regulator is not expressed in fast-twitch skeletal muscles.

Melittin inhibition of the gastric (H+ + K+) ATPase and photoaffinity labeling with [125I]azidosalicylyl melittin

Melittin is a 26-amino acid amphipathic polypeptide toxin from bee venom which forms anion-selective ion channels in bilayers and biological membranes under the influence of membrane potential. Melittin has been shown to interact with a number of membrane proteins. We found that melittin inhibited K+-stimulated ATP hydrolysis by the (H+ + K+) ATPase in parietal cell apical membrane vesicles derived from histamine-stimulated rabbit gastric mucosa with a KIapp of 0.5 micron. Melittin also inhibited K+-stimulated p-nitrophenyl hydrolysis activity which is associated with the gastric (H+ + K+) ATPase in a dose-dependent manner with a KIapp of 0.95 micron. ATP-driven, K+-dependent H+ transport was inhibited over this same concentration range, even in the absence of a membrane potential. Melittin did not appear to increase the H+ leak from vesicle with preformed H+ gradients when the H+ pump was arrested by Mg2+ chelation, but all possible membrane perturbation effects were difficult to rule out. However, the data suggest that melittin exerts its inhibitory effect through interaction with the (H+ + K+) ATPase. In order to determine whether direct interactions between the (H+ + K+) ATPase and melittin occurred, a radioactive derivative of melittin, [125I]azidosalicylyl melittin, was prepared and photoreacted with sealed rabbit gastric membranes and highly purified hog gastric membrane containing the (H+ + K+) ATPase. In the purified hog preparation only a 95,000-Da band, the (H+ + K+) ATPase was labeled, while in the rabbit preparation a 95,000-Da band and one other membrane protein of 70,000 Da were labeled with this reagent. Label incorporation into the (H+ + K+) ATPase and the 70,000-Da band was greatly reduced by addition of excess unlabeled melittin, suggesting specificity of the interaction. Label incorporation occurred in the absence of ATP or added salts and was not reduced by SCH28080 (a K+ site inhibitor) suggesting that the melittin binding site was distinct from the luminal K+ site of action of SCH28080.

Azido derivatives of dicarboxylic acids for photoaffinity labeling of mitochondrial carriers

New photoaffinity probes, N-(4-azidosalicylic)-aminosuccinic acid, 3-(4-azidophenylazo)-4-hydroxyphenylmalonic acid, (4-azido-2-nitroanilino)-N-succinic acid, 4-azidophenacylthiosuccinic acid and 4-azidophenylsuccinic acid, were synthesized and characterized chemically. They differ in the distance between dicarboxylic and azido groups, hydrophobicity and acidic moiety. These between dicarboxylic and azido groups, hydrophobicity and acidic moiety. These reagents can be applied for photoaffinity labeling of mitochondrial anion carriers and enzymes interacting with dicarboxylic acids. Inhibition and labeling of the dicarboxylate carrier is presented.

Photoaffinity labeling of the multidrug-resistance-related P-glycoprotein with photoactive analogs of verapamil

Verapamil, a phenylalkylamine calcium channel blocker, has been shown to reverse multidrug resistance in tumor cells, possibly by increasing drug retention through interaction with an outward drug transporter of the resistant cells. In this study two photoactive radioactive analogs of verapamil, N-(p-azido[3,5-3H]benzoyl)aminomethyl verapamil and N-(p-azido[3-125I]salicyl)aminomethyl verapamil, were synthesized and used to identify the possible biochemical target(s) for verapamil in multidrug-resistant DC-3F/VCRd-5L Chinese hamster lung cells selected for resistance to vincristine. The results show that a specifically labeled 150- to 180-kDa membrane protein in resistant cells was immunoprecipitated with a monoclonal antibody specific for P-glycoprotein. Phenylalkylamine binding specificity was established by competitive blocking of specific photolabeling with the nonradioactive photoactive analogs as well as with verapamil. Photoaffinity labeling was also inhibited by 50 microM concentrations of the calcium channel blockers nimodipine, nifedipine, nicardipine, azidopine, bepridil, and diltiazem and partially by prenylamine. Bay K8644, a calcium channel agonist, also inhibited P-glycoprotein photolabeling. Moreover, P-glycoprotein labeling was inhibited in a dose-dependent manner by vinblastine with half-maximal inhibition at 0.2 microM compared to that by verapamil at 8 microM. Photolabeling was also partially inhibited by two of the drugs to which these cells are cross-resistant, doxorubicin and actinomycin D, at 100 microM, but not by colchicine. These data provide direct evidence that P-glycoprotein has broad drug recognition capacity and that it serves as a molecular target for calcium channel blocker action in reversing multidrug resistance.

The use of sulfosuccinimidyl-2-(p-azidosalicylamido)-1,3'-dithiopropionate as a crosslinking reagent to identify cell surface receptors

Conditions for solubilizing and iodinating the heterobifunctional thiol-cleavable photoreactive crosslinking reagent sulfosuccinimidyl-2-(p-azidosalicylamido)-1,3'-dithiopropionate which leave the ester moiety, disulfide bond, and azido group reactive are described. Iodination was performed in a mixture of dimethyl sulfoxide and bicarbonate, pH 9.0 (1:20, v/v), as solubilizing agent and Iodogen as oxidant. The lectin phytohemagglutinin was derivatized with the iodinated crosslinker and the interaction between phytohemagglutinin and mononuclear cells was chosen as the model system to monitor the efficiency of sulfosuccinimidyl-2-(p-azidosalicylamido)-1,3'-dithiopropionate as a crosslinking reagent. Transfer of 125I to the biologically significant T11 lymphocyte receptor in addition to 125I labeling of other membrane proteins to which the lectin binds was detected by polyacrylamide gel electrophoresis under reducing conditions.

N-(3-(p-azido-m-[125I]iodophenyl)propionyl)-succinimide--a heterobifunctional reagent for the synthesis of radioactive photoaffinity ligands: synthesis of a carrier-free 125I-labeled cardiac glycoside photoaffinity label

A new heterobifunctional reagent, N-(3-(p-azido-m-iodophenyl)propionyl)-succinimide (AIPPS), was synthesized and chemically characterized. The radiochemical form of the reagent, [125I]AIPPS, should be of general use as a photoactive reagent for the derivatization of free amino groups on a large variety of biologically active compounds, including many hormones. Amino-containing ligands can be derivatized with [125I]AIPPS in a method which is similar to that used for the 125I-labeled Bolton-Hunter reagent (N-(3-(p-hydroxyphenyl)propionyl)-succinimide). The added advantage with [125I]AIPPS, however, is that the ligand derivative is made both photoactive and radioactive in a single step. As an example of how this reagent can be used, we have prepared carrier-free [125I]AIPPS and reacted it with the amino-containing cardiac glycoside, 4-amino-4,6-dideoxyglucosyl digitoxigenin (GluD). The radioiodinated cardiac glycoside, [125I]AIPP-GluD, was purified by thin-layer chromatography and was carrier-free with a specific radioactivity of 2175 Ci/mmol. [125I]AIPP-GluD was an effective photoaffinity label for Na,K-ATPase as shown by specific photoaffinity labeling of purified canine kidney enzyme and human erythrocyte enzyme.