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PubMed1989

Use of a nitrotryptophan-containing peptide for photoaffinity labeling the pancreatic cholecystokinin receptor.

We report the preparation and characterization of a new type of intrinsic photoaffinity labeling probe, on the basis of the incorporation of a photolabile nitrotryptophan into a biologically relevant domain of a peptide. The model system used was the pancreatic cholecystokinin (CCK) receptor, previously affinity labeled with a variety of probes. Those studies have suggested that an Mr = 85,000-95,000 protein is more likely to be labeled as the site of covalent attachment approaches the receptor-binding domain of this hormone. Indeed, CCK has a Trp in the center of its receptor-binding region, and replacement of that residue with 6-nitrotryptophan resulted in a photolabile probe which affinity labeled the same Mr = 85,000-95,000 pancreatic membrane protein. This probe, 125I-D-Tyr-Gly-[(Nle28,31,6-NO2-Trp30)CCK-26-33], was synthesized by solid-phase and solution techniques and characterized by mass spectrometry. Following oxidative iodination, it was purified on HPLC to 2000 Ci/mmol. Binding to pancreatic membranes was rapid, temperature dependent, reversible, saturable, and specific and was with high affinity (Kd = 3 nM). While its binding affinity was only 3-fold lower than that of native CCK-8, this probe was 70-fold less potent than native hormone in stimulating amylase secretion (EC50 = 1 nM) and equally efficacious to native hormone. Despite the slight decrease in affinity, this probe demonstrated a high relative efficiency of covalent labeling of the Mr = 85,000-95,000 protein. This confirms that the Mr = 85,000-95,000 protein represents the hormone-binding subunit of the CCK receptor and demonstrates the utility of this type of photoaffinity labeling probe.(ABSTRACT TRUNCATED AT 250 WORDS)

Affinity Labels↗
PubMed1999

Fingerprint patterns from laser-induced azido photochemistry of spin-labeled photoaffinity ATP analogs in matrix-assisted laser desorption/ionization mass spectrometry.

The photochemical reaction of azide derivatives induced by ultraviolet (UV) laser in matrix-assisted laser desorption/ionization mass spectrometry (MALDI) is reported. A novel synthesized class of azide aromatic derivatives, spin-labeled photoaffinity non-nucleoside adenosine triphosphate (ATP) analogs which are useful probes in study of muscle contraction mechanism, is used in this investigation. In the negative ion MALDI spectra of these ATP analogs, "fingerprint" peaks corresponding to [M - 10 - 1]-, [M - 12 - 1]-, [M - 16 - 1]-, [M - 26 - 1]-, [M - 28 - 1]-, [M - 41 - 1]-, and [M - 42 - 1]- were observed with relative intensities depending on the MALDI matrix. Only the [M - 16 - 1]- is present in the similar mass spectra of the analog in which the azido group is replaced by a hydrogen. A model is suggested for the photochemical reactions of azide derivatives under UV laser irradiation. The photoreaction fingerprint information is diagnostically useful in characterization of azido compounds, especially for spin-labeled photoaffinity non-nucleoside ATP analogs.

Adenosine Triphosphate↗
PubMed1996

Understanding the mechanism of sweet taste: synthesis of ultrapotent guanidinoacetic acid photoaffinity labeling reagents.

Azido-functionalized analogs of potently sweet guanidinoacetic acids have been synthesized for use as sweetener receptor photoaffinity labeling reagents. These compounds have been synthesized using readily available starting materials. One of the azido-labeled guanidinoacetic acids has been evaluated in an electrophysiological model in the Rhesus monkey. We found that the photoaffinity-labeling reagent caused irreversible inhibition in electrophysiological response to sweeteners upon exposure of the monkey tongue to a combination of the reagent and UV light.

Affinity Labels↗
PubMed1988

Photoaffinity labeling of pituitary GnRH receptors: significance of the position of photolabel on the ligand.

Photoreactive derivatives of GnRH and its analogues were prepared by incorporation of the 2-nitro-4(5)-azidophenylsulfenyl [2,4(5)-NAPS] group into amino acid residues at positions 1, 3, 6, or 8 of the decapeptide sequence. The modification of Trp3 by the 2,4-NAPS group led to a complete loss of the luteinizing hormone (LH) releasing as well as LH-release-inhibiting activity of the peptide. The [D-Lys(2,4-NAPS)]6 analogue was a very potent agonist that, after covalent attachment by photoaffinity labeling, caused prolonged LH secretion at a submaximal rate. [Orn(2,4-NAPS)]8-GnRH, a full agonist with a relative potency of 7% of GnRH, after photoaffinity labeling caused prolonged maximal LH release from cultured pituitary cells. In contrast, [Orn(2,5-NAPS)]8-GnRH, although being equipotent with the 2,4-NAPS isomer in terms of LH releasing ability, was unable to cause prolonged LH release after photoaffinity labeling. Thus, [Orn(2,4-NAPS)]8-GnRH is a very effective photolabeling ligand of the functionally significant pituitary GnRH receptor. Based on this compound, a pituitary peptidase resistant derivative, D-Phe6,[Orn(2,4-NAPS)]8-GnRH-(1-9)-ethylamide, was synthesized. This derivative showed high-affinity binding to pituitary membranes with a Kd comparable to those of other GnRH analogues. A radioiodinated form of this peptide was used for pituitary GnRH-receptor labeling. This derivative labeled 59- and 57-kDa proteins in rat and 58- and 56-kDa proteins in bovine pituitary membrane preparations, respectively. This peptide also labeled pituitary GnRH receptors in the solubilized state and therefore appears to be a suitable ligand for the isolation and further characterization of the receptor.

Affinity Labels↗
PubMed1978

Azidophenantridinium compounds as photoaffinity labels of cholinergic proteins.

The synthesis of diazidopropidium and diazidoethidium is described. The applicability of these compounds as photoaffinity labels for cholinergic proteins has been investigated: diazidopropidium inhibits neuromuscular transmission. This inhibition is reversible if the compound is applied in the dark but becomes irreversible after irradiation with white light. Inhibition is accompanied by a disappearance of miniature endplate potentials. Electrophysiological analysis of this effect indicates that diazidopropidium acts postsynaptically by blocking the acetylcholine receptors. At the molecular level the action of diazidopropidium and diazidoethidium on acetylcholinesterase has been investigated: both compounds appear to bind to a peripheral acetylcholine binding site of this enzyme. Binding of 125I-labeled alpha-neurotoxin from Naja naja siamensis to purified membranes from Torpedo californica electric tissue rich in acetylcholine receptors is diminished after incubation and irradiation with diazidopropidium. About half of the toxin binding sites appear to be blocked by the photoaffinity label.

Acetylcholine↗
PubMed1994

Differential interaction of glimepiride and glibenclamide with the beta-cell sulfonylurea receptor. II. Photoaffinity labeling of a 65 kDa protein by [3H]glimepiride.

Glimepiride is a novel sulfonylurea for the treatment of type II-diabetic patients exhibiting different receptor binding kinetics to beta-cell membranes with 8-9-fold higher koff rate and 2.5-3-fold higher kon rate compared to glibenclamide (see accompanying paper (Müller, G. et al. (1994) Biochim. Biophys. Acta 1191, 267-277)). To elucidate the molecular basis for this differential behaviour of glimepiride and glibenclamide, direct photoaffinity labeling studies using beta-cell tumor membranes were performed. [3H]Glimepiride was specifically incorporated into a membrane polypeptide of M(r) = 65,000 under conditions, which led to predominant labeling of a 140 kDa protein by [3H]glibenclamide (Kramer, W. et al. (1988) FEBS Lett. 229, 355-359). Labeling of the 140 kDa protein by [3H]glibenclamide was inhibited by unlabeled glimepiride and, vice versa, glibenclamide inhibited labeling of the 65 kDa protein by [3H]glimepiride. The 65 kDa protein was also specifically photolabeled by the sulfonylurea [125I]35623, whereas an 4-azidobenzoyl derivative of glibenclamide, N3-[3H]33055, exclusively labeled a 33 kDa protein. Competitive Scatchard analysis of [3H]glimepiride-binding and [3H]glibenclamide-binding to RINm5F cell membranes using glibenclamide and glimepiride, respectively, as heterologous displacing compounds yielded non-linear plots. These findings may be explained by cooperative interactions between the 140 and 65 kDa sulfonylurea-binding proteins. The possibility that sulfonylureas of different structure have different access to the 140 and 65 kDa receptor proteins due to the beta-cell membrane barrier was investigated by photoaffinity labeling of solubilized beta-cell membrane proteins. Interestingly, solubilization of beta-cell tumor membranes led to a shift of specific [3H]glibenclamide binding from the 140 kDa to the 65 kDa binding protein, exclusively, and to an increased labeling of the 65 kDa protein by [3H]glimepiride. The labeling of a unique protein is in agreement with similar Kd values measured for both sulfonylureas upon solubilization of beta-cell tumor and RINm5F cell membranes (see accompanying paper). Furthermore, competitive Scatchard plots of [3H]glimepiride binding to solubilized RINm5F cell membrane proteins in the presence of glibenclamide and vice versa approximate linearity suggesting loss of cooperativity between the 140 kDa glibenclamide-binding and 65 kDa glimepiride-binding proteins upon solubilization. The physiological significance of the differential interaction of glimepiride and glibenclamide with different binding proteins was also substantiated by photoaffinity labeling of RINm5F cells leading to labeling of a 140 kDa protein by [3H]glibenclamide and of a 65 kDa protein by [3H]glimepiride.(ABSTRACT TRUNCATED AT 400 WORDS)

ATP-Binding Cassette Transporters↗
PubMed1992

Photoaffinity labeling and characterization of isolated inositol 1,3,4,5-tetrakisphosphate- and inositol hexakisphosphate-binding proteins.

We have isolated high affinity inositol (1,3,4,5)-tetrakisphosphate (IP4)- and inositol hexakisphosphate (IP6)-binding proteins from detergent-solubilized rat brain membranes using a P1-tethered IP4 derivative linked to an Affi-Gel support. To determine the identity, binding characteristics, and distribution of the individual IP4 recognition sites, we have synthesized an IP4 photoaffinity label probe, 125I-(D,L)-1-O-[N-(4-azidosalicyloxy)-3-aminopropyl-1-phospho]- IP4 (125I-ASA-IP4). Two apparently distinct IP4-binding proteins (IP4BP), isolated with the IP4 affinity column, display high affinity and selectivity for IP4 over inositol trisphosphate (IP3), inositol pentakisphosphate (IP5), and IP6. The first IP4-binding protein (IP4BP1) which has a KD for IP4 of 4 nM, is comprised of a protein at 182 kDa which is specifically photolabeled with high affinity by 125I-ASA-IP4. The second, IP4BP2, has an affinity for IP4 of 1.5 nM and contains proteins at 84 and 174 kDa, both of which are specifically photoaffinity labeled. A putative IP6-binding protein (IP6BP), also isolated with the IP4 affinity column, binds IP6 with a KD of 14 nM and comprises three proteins of 115, 105, and 50 kDa. The 115- and 105-kDa subunits, but not the 50-kDa subunit, specifically incorporate the photolabel. The IP4BP (182, 174, and 84 kDa) and IP6BP (115 and 105 kDa) proteins are specifically photolabeled in the crude membrane, partially purified, and purified fractions. These receptor-binding proteins vary in inositol phosphate specificity and in the effects of pH, Ca2+, and heparin on IP4 photoaffinity labeling. In addition, IP4BP and IP6BP are enriched in the brain but differ in their regional localizations within the brain.

Affinity Labels↗
PubMed1993

Photoaffinity labeling of subtype 2 angiotensin receptor of human myometrium.

Angiotensin II (AII) binding sites were characterized in human myometrium membrane preparations. The sites were saturable and of high affinity (Kd of 0.09 nM and Bmax of about 200 fmol/mg of protein). PD 123319 completely inhibited 125I-AII binding, with an IC50 of 30 nM, whereas L-158,809 (1 microM) had no significant effect on 125I-AII binding. These results indicate that human myometrium contains almost exclusively the AT2 receptor subtype. Association and dissociation studies performed with 125I-AII on human myometrium membranes revealed that AII had a very high affinity for AT2 receptors, with a Kd of 0.01 nM (association rate constant K1 = 1.056 x 10(12) mol-1 min-1; dissociation rate constant K2 = 0.003 min-1). The photoactivable AII analogue [Sar1, Val5, D-Phe8(N3)]AII displayed a high affinity for AT2 receptors (IC50 of 0.18 nM), but its radioiodinated form showed poor efficiency in photoaffinity labeling experiments. A newly synthesized photoactivatable analogue of AII, [Sar1, p-benzoyl-Phe8]AII, (AII-Bpa), also displayed a high affinity for AT2 receptors of human myometrium (IC50 of 0.3 nM). Photoaffinity labeling experiments were performed with 125I-AII-Bpa, and a high yield (70%) of covalent incorporation to human myometrium membranes was obtained upon photolysis. Covalently labeled receptors were solubilized, denatured, and subjected to polyacrylamide gel electrophoresis. Autoradiography of the polyacrylamide gel revealed a single band, of 68 kDa, and the labeling of this band was completely abolished in the presence of 1 microM PD 123319, indicating selective labeling of the AT2 receptor subtype. These results demonstrate that AII-Bpa is a very efficient tool for selective photoaffinity labeling of the AT2 receptor.

Adrenal Cortex↗
PubMed1993

Photoaffinity labeling of the T cell receptor on living cytotoxic T lymphocytes.

Using a direct binding assay based on photoaffinity labeling, we have studied the interaction of an antigenic peptide with MHC class I molecules and the TCR on living cells. Two photoreactive derivatives of the H-2Kd (Kd) restricted Plasmodium berghei circumsporozoite (PbCS) peptide 253-260 (YIPSAEKI) were used. The first derivative contained an N-terminal photoreactive iodo, 4-azido salicyloyl (IASA) group and biotin on the TCR contact residue Lys259 [IASA-YIPSAEK(biotin)I]. As previously described, this derivative selectively bound to and labeled the Kd molecule. The second photoreactive compound, the isomeric biotin-YIPSAEK(IASA)I, also efficiently bound to the Kd molecule, but failed to label this protein. A CTL clone derived from a mouse immunized with this derivative recognized this conjugate but not the parental P. berghei circumsporozoite peptide or the [IASA-YIPSAEK-(biotin)I] derivative in an Kd-restricted manner. Incubation of the cloned CTL cells with biotin-YIPSAEK(IASA)I, but not its isomer, followed by UV irradiation resulted in photoaffinity labeling of the TCR-alpha chain that was dependent on the conjugate binding to the Kd molecule. The TCR labeling was partially inhibited by anti-LFA 1 and anti-ICAM1 mAb, but was increased by addition of beta 2m or soluble KdQ10. The exquisite labeling selectivity of the two photoprobes opens a new, direct approach to the molecular analysis of antigen presentation and recognition by living CTL.

Affinity Labels↗
PubMed1999

Overexpression, purification and photoaffinity labeling with a 3H-analogue of norfloxacin, of the GyrA and GyrB subunits of the DNA gyrase.

In spite of much work on DNA gyrase and quinolones for many years, our knowledge of the molecular basis of quinolone-gyrase action is still incomplete. We designed a photoaffinity labeling reagent for the quinolone target, and synthesized a norfloxacin analogue with an azide function which, under UV irradiation, becomes covalently linked to its target. For that, a large amount of purified gyrase was needed. Both subunits were purified using exclusion and affinity chromatography. A plasmid was used that allowed the overproduction of GyrA as a fusion-protein with six histidine residues at its carboxy-terminal domain. GyrA-(His)6 was purified after chromatography on a nickel-containing column, and native GyrB after chromatography on immobilized novobiocin. Reconstituted DNA gyrase (A2B2) had supercoiling activity. Photoaffinity labeling showed covalent binding of the 3H-photoaffinity analogue of norfloxacin to the gyrase-DNA complex, and mainly to the GyrA. The specific binding site remains to be explored.

Cloning, Molecular↗
PubMed1985

Stepwise unfolding of chromatin by urea. A flow linear dichroism and photoaffinity labeling study.

The unfolding of chromatin by urea (0-7 M) was studied by means of flow linear dichroism, photoaffinity labeling and nuclease digestion. The linear dichroism results indicate that the unfolding of the DNA is accomplished through two distinct transitions at 1-2 M urea and 6-8 M urea, respectively. The photoaffinity labeling studies indicate that an opening of the nucleosome histone core occurs above 2 M urea, accompanied by general loosening of the structure. Based on the results a model for the unfolding of chromatin fibers by urea is proposed, which includes a stretching of the linker DNA (0-2 M urea) followed by a "loosening" of the nucleosome core, possibly to a one-loop DNA conformation (2-6 M urea), and finally resulting in an almost total stretching of the DNA (greater than 6 M urea).

Affinity Labels↗
PubMed1993

Identification by photoaffinity labeling of a membrane thyroid hormone-binding protein associated with the triiodothyronine transport system in rat erythrocytes.

Photoaffinity labeling with underivatized T3 was used to identify T3-binding proteins in the membrane of rat erythrocytes. UV irradiation of ghosts and peripheral protein-depleted membranes in the presence of [125I]T3 resulted in the covalent attachment of 125I to membrane proteins (analyzed by polyacrylamide gel electrophoresis and autoradiography). In the presence of the free radical scavenger dithiothreitol, 125I was selectively incorporated into a 45,000 mol wt band (p45) that was an integral membrane polypeptide. p45 photolabeling was half-inhibited by 14 nM unlabeled T3. This concentration is similar to the Km for T3 transport in rat erythrocytes and the Kd of the high affinity T3-binding sites under equilibrium binding conditions in the rat erythrocyte membrane. T4 and tryptophan also strongly inhibited p45 labeling, whereas the D-isomer of T3 was less efficient, and leucine had no effect. This corresponds to the specificity of the system T-related T3 transport system and T3-binding sites of rat erythrocytes. The SH-reagent N-ethylmaleimide prevented p45 labeling, unless T3 was present to protect the T3 transport activity and the high affinity T3-binding sites from inactivation. No saturable labeling of p45 or other polypeptides was detected in membranes prepared from human erythrocytes, which have very low T3 transport activity and no measurable high affinity T3-binding sites. p45 is not disulfide linked and is not a degradation product of higher mol wt polypeptides. Substrates and specific inhibitors of known erythrocyte membrane transporters did not alter p45 photolabeling, indicating that p45 is not functionally related to these transporters. We conclude that the photoaffinity-labeled T3-binding protein p45 has the properties expected of the T3-binding component of the T3 transport system in rat erythrocytes.

Affinity Labels↗
PubMed1984

Photoaffinity labeling of the rat isolated portal vein: determination of affinity constants and "spare" receptors for angiotensins II and III.

Photoaffinity labeling of the isolated rat portal vein with [azidobenzoic acid, isoleucine]angiotensin II resulted in selective partial inactivation of angiotensin receptors without affecting norepinephrine receptors. Establishment of dose-response curves to angiotensin II and III before and after photoaffinity labeling has permitted the calculation of "spare" receptors and affinity constants for angiotensins II and III. Spare receptors appear to exist for angiotensin II (greater than 60%) but not for angiotensin III. Furthermore, the data indicate that angiotensin III has a higher binding affinity (K = 6 X 10(-8) M) than angiotensin II (K = 3 X 10(-7) M), but is considerably less potent than angiotensin II in eliciting the contractile response. If angiotensins II and III act at the same receptors in the portal vein, angiotensin III could inhibit the constricting action of angiotensin II and thereby play a role in the angiotensin-mediated control of portal venous capacity.

Affinity Labels↗
PubMed1995

Photoaffinity-labeled ligand binding domains on dopamine transporters identified by peptide mapping.

Binding domains on rat dopamine transporters for cocaine and 1-(2-diphenylmethoxy)ethyl-4-(3-phenylpropyl)piperazine compounds were identified using controlled proteolysis of photoaffinity-labeled protein and epitope-specific immunoprecipitation of the labeled fragments. Rat dopamine transporters were photoaffinity labeled with 1-[2-(diphenylmethoxy)ethyl]-4-[2-(4-azido- 3-[125I]iodophenyl)ethyl]piperazine ([125I]DEEP) [a 1-(2-di- phenylmethoxy)ethyl-4-(3-phenylpropyl)piperazine analog] or 3 beta-(p-chlorophenyl)tropane-2 beta-carboxylic acid, 4'-azido-3'- [125I]iodophenylethyl ester ([125I]RTI 82) (a cocaine analog) and were gel purified to remove contaminating radioactivity. The resulting samples were treated with V8 protease or trypsin and analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The peptide maps generated with each enzyme were different for each of the ligands, suggesting that the ligands were incorporated into different regions of the protein. Identical peptide maps were generated from striatum- and nucleus accumbens-derived transporters, indicating that these polypeptides are highly similar in primary sequence. The proteolytic fragments generated by V8 protease were localized to specific domains of the protein using antipeptide antibodies corresponding to five different regions of the transporter. Fragments of 10 and 7 kDa from [125I]DEEP-labeled transporters were specifically immunoprecipitated with an antibody generated against amino acids 42-59 (near the first putative trans-membrane domain), whereas a 34-kDa fragment from [125I]RTI 82-labeled transporters was precipitated with three different sera corresponding to regions in the carboxyl-terminal two thirds of the protein. None of the V8 fragments smaller than 45 kDa, containing either photolabel, was altered in molecular mass by N-deglycosylation. The results indicate that photoincorporation of [125I]DEEP occurs in the amino half of the dopamine transporter, near the first two transmembrane helices, whereas [125I]RTI 82 labels the carboxyl-terminal region of the protein, between transmembrane domains 4 and 12.

Affinity Labels↗
PubMed1995

Photoaffinity labelling of cyanomethaemoglobin with derivatives of tryptophan and 5-bromotryptophan.

Tryptophan and 5-bromotryptophan (5-BrTrp) are relatively potent inhibitors of sickle-haemoglobin polymerization. The binding sites of these compounds to normal and sickle haemoglobin (HBA and HBS) have been suggested, but not firmly established, through the use of spin-labelled derivatives and/or computer modeling. In the present study we approached the problem by utilizing the technique of photoaffinity labelling. The cyanomet forms of HBA and HBS were subjected to photoaffinity labelling with N alpha-(4-azidotetrafluorobenzoyl)tryptophan and N alpha-(1-ethyl-2-diazomalonyl)-5-bromotryptophan respectively. Both irradiated samples of HBA and HBS were denatured, digested with trypsin, and then separated by reversed-phase HPLC. A labelled tryptic peptide was isolated from the photolabelling of HBS with N alpha-(1-ethyl-2-diazomalonyl)-5-bromotryptophan. The peptide was identified to be Val1(alpha)-Lys7(alpha), with the label attached to Val1(alpha), by virtue of amino acid analysis and sequencing, in conjunction with fast-atom-bombardment MS. The binding mode of N alpha-(1-ethyl-2-diazomalonyl)-5-bromotryptophan is proposed and its relevance to the potency of the 5-BrTrp-based anti-sickling agents is discussed.

Affinity Labels↗
PubMed1993

Photoaffinity labeling of Torpedo acetylcholine receptor at multiple sites.

The acetylcholine receptor from Torpedo californica electroplax was labeled with the photoaffinity reagent bis(3-azidopyridinium)decane perchlorate. All four receptor subunits (alpha, beta, gamma, and delta) were specifically labeled. In the presence of cholinergic agonists the gamma-, beta-, and delta-subunit labeling was decreased significantly, whereas labeling of the alpha subunit was minimally affected. Full occupancy of the two high-affinity sites involving the alpha subunits in the vicinity of alpha-Cys-192-Cys-193 by covalent reaction with bromoacetylcholine also caused a large decrease of gamma-subunit labeling by the photoaffinity reagent and lesser but significant decreases in beta- and delta-subunit labeling. No decrease in labeling of the alpha subunit was seen. Labeling of the alpha subunit could, however, be inhibited by high concentrations of the agonist carbamoylcholine. We conclude that the binding sites of high-affinity reside at interfaces of the alpha subunit and other subunits and that the alpha subunit also contributes to formation of a low-affinity site(s) for cholinergic compounds.

Acetylcholine↗
PubMed1999

Synthesis of a spin-labeled photoaffinity ATP analogue, and its use to specifically photolabel myosin cross-bridges in skeletal muscle fibers.

A spin-labeled photoaffinity ATP analogue 3'(2')-O-[4-[4-oxo-(4-amido-2,2,6,6-tetramethyl-piperidino-1-oxyl)]-benz oyl]benzoyl adenosine 5'-triphosphate (SL-Bz2ATP) was synthesized and used to photolabel myosin in muscle fibers. Previous work has shown that 3'(2')-O-(4-benzoyl)benzoyl adenosine 5'-triphosphate (Bz2ATP) photolabeled Ser-324 of the 50 kDa tryptic fragment of skeletal S1 heavy chain. In this work, [alpha-32P]SL-Bz2ATP was hydrolyzed and trapped as the diphosphate analogue with Co2+ and orthovanadate at the active site of myosin in rabbit psoas muscle fibers. After UV irradiation, the myosin heavy chain was the only protein band found to be significantly photolabeled as assayed by gel electrophoresis and radioactivity counting. The labeling was localized after brief trypsin digestion by SDS-PAGE to be on the 50 kDa tryptic fragment of the S1 heavy chain. Ca. 35% of the myosin in fibers was covalently photolabeled. The fibers photolabeled with SL-Bz2ATP had the same active tension and maximum shortening velocity as the control fibers. The resulting spin label on myosin was too mobile to report the orientation of the heads in fibers. Nonetheless, this is the first work to show the feasibility of utilizing active site binding and photoaffinity labeling to place covalent spectroscopic probes at the myosin active site in fibers with high specificity and yield without affecting mechanical function.

Adenosine Triphosphate↗