PHOTOAFFINTTY LABELING

Photoaffinity labeling in activity-based protein profiling

Activity-based protein profiling has come to the fore in recent years as a powerful strategy for studying enzyme activities in their natural surroundings. Substrate analogs that bind covalently and irreversibly to an enzyme active site and that are equipped with an identification or affinity tag can be used to unearth new enzyme activities, to establish whether and at what subcellular location the enzymes are active, and to study the inhibitory effects of small compounds. A specific class of activity-based protein probes includes those that employ a photo-activatable group to create the covalent bond. Such probes are targeted to those enzymes that do not employ a catalytic nucleophile that is part of the polypeptide backbone. An overview of the various photo-activatable groups that are available to chemical biology researchers is presented, with a focus on their (photo)chemistry and their application in various research fields. A number of comparative studies are described in which the efficiency of various photo-activatable groups are compared.

Development of a selective photoactivatable antagonist for corticotropin-releasing factor receptor, type 2 (CRF2)

A novel photoactivatable analog of antisauvagine-30 (aSvg-30), a specific antagonist for corticotropin-releasing factor (CRF) receptor, type 2 (CRF2), has been synthesized and characterized. The N-terminal amino-acid d-Phe in aSvg-30 [d-Phe11,His12]Svg(11-40) was replaced by a phenyldiazirine, the 4-(1-azi-2,2,2-trifluoroethyl)benzoyl (ATB) residue. The photoactivatable aSvg-30 analog ATB-[His12]Svg was tested for its ability to displace [125I-Tyr0]oCRF or [125I-Tyr0]Svg from membrane homogenates of human embryonic kidney (HEK) 293 cells stably transfected with cDNA coding for rat CRF receptor, type 1 (rCRF1) or mouse CRF receptor, type 2beta (mCRF2beta). Furthermore, the ability of ATB-[His12]Svg(12-40) to inhibit oCRF- or Svg-stimulated cAMP production of transfected HEK 293 cells expressing either rCRF1 (HEK-rCRF1 cells) or mCRF2beta (HEK-mCRF2beta cells) was determined. Unlike astressin and photo astressin, ATB-[His12]Svg(12-40) showed high selective binding to mCRF2beta (Ki = 3.1 +/- 0.2 nm) but not the rCRF1 receptor (Ki = 142.5 +/- 22.3 nm) and decreased Svg-stimulated cAMP activity in mCRF2beta-expressing cells in a similar fashion as aSvg-30. A 66-kDa protein was identified by SDS/PAGE, when the radioactively iodinated analog of ATB-[His12]Svg(12-40) was covalently linked to mCRF2beta receptor. The specificity of the photoactivatable 125I-labeled CRF2beta antagonist was demonstrated with SDS/PAGE by the finding that this analog could be displaced from the receptor by antisauvagine-30, but not other unrelated peptides such as vasoactive intestinal peptide (VIP).

Photoaffinity analogues of farnesyl pyrophosphate transferable by protein farnesyl transferase

Farnesylation is a posttranslational lipid modification in which a 15-carbon farnesyl isoprenoid is linked via a thioether bond to specific cysteine residues of proteins in a reaction catalyzed by protein farnesyltransferase (FTase). We synthesized analogues (3-6) of farnesyl pyrophosphate (FPP) to probe the range of modifications possible to the FPP skeleton which allow for efficient transfer by FTase. Photoaffinity analogues of FPP (5, 6) were prepared by substituting perfluorophenyl azide functional groups for the omega-terminal isoprene of FPP. Substituted anilines replace the omega-terminal isoprene in analogues 3 and 4. Compounds 3-5 were prepared by reductive amination of the appropriate anilines with 8-oxo-geranyl acetate, followed by ester hydrolysis, chlorination, and pyrophosphorylation. Additional substitution of three methylenes for the beta-isoprene of FPP gave photoprobe 6 in nine steps. Preparation of the analogues required TiCl(4)-mediated imine formation prior to NaBH(OAc)(3) reduction for anilines with a pK(a) < 1. The azide moiety was not affected by Ph(3)PCl(2) conversion of allylic alcohols 13-16 into corresponding chlorides 17-20. Analogues 3-6 are efficiently transferred to target N-dansyl-GCVLS peptide substrate by mammalian FTase. Comparison of analogue structures and kinetics of transfer to those of FPP reveals that ring fluorination and para substituents have little effect on the affinity of the analogue pyrophosphate for FTase and its transfer efficiency. These results are also supported with models of the analogue binding modes in the active site of FTase. The transferable azide photoprobe 5 photoinactivates FTase. Transferable analogues 5 and 6 allow the formation of appropriately posttranslationally modified photoreactive peptide probes of isoprene function.

Facile and efficient synthesis of 4-azidotetrafluoroaniline: a new photoaffinity reagent

p-Azidotetrafluoroaniline (1) was synthesized in 65-73% yield by two different methods employing a stable carbamate intermediate. The first method trapped the intermediate isocyanate generated via a modified Curtius rearrangement with 2-methyl-2-propanol or 2-(trimethylsilyl)ethanol to form the stable carbamates 2d and 2e, respectively. Benzoic acid 2c was first converted to its acid chloride with PCl(5). Displacement of the chloride by NaN(3) in acetone/water formed the acyl azide. Thermal rearrangement followed by the addition of the appropriate alcohols provided the carbamates. The acid labile carbamate 2d was deprotected with HCl/AcOH to provide 1, while trifluoroacetic acid was required to deprotect 2e and afford 1. In the second path, 1 was synthesized in five steps from pentafluoronitrobenzene (3a) in 65% overall yield. Compound 3a was converted into 4-azidotetrafluoronitrobenzene (3b) with NaN(3) in 93% yield and was used without further purification to form 1, 4-diaminotetrafluorobenzene (3c) by Sn/HCl reduction in 85% yield. The mono-9-fluorenylmethoxycarbonyl (FMOC) derivative 3d was formed from 3c with FMOC-Cl and pyridine in EtOAc in 92% yield. Diazotization of 3d under anhydrous conditions with TFA/NaNO(2) and NaN(3) gave 3e in 87% yield. The aryl azide was formed with concurrent nitration of the 2-position of the fluorenyl system. The protecting group was removed with piperidine to afford 1 in 93% yield. Irradiation of 1 with 254 nm light in cyclohexane gave cyclohexylamine 11, diamine 3c, and azobenzene 12 as the primary products. The formation of C-H insertion product 11 indicates that 1 forms a singlet nitrene upon photolysis. Two heterobifunctional photoaffinity reagents iodoacetamide 9 and dansyl derivative 10 were prepared.

Novel high-affinity photoactivatable antagonists of corticotropin-releasing factor (CRF) photoaffinity labeling studies on CRF receptor, type 1 (CRFR1)

Novel photoactivatable antagonists of human/rat corticotropin-releasing factor (h/rCRF) have been synthesized and characterized. The N-terminal amino acid D-phenylalanine in astressin ¿cyclo(30-33) [D-Phe12, Nle21,38, Glu30, Lys33]h/rCRF-(12-41)¿, a potent CRF peptide antagonist, was replaced by a phenyldiazirine, the 4-(1-azi-2,2,2-trifluoroethyl)benzoyl (ATB) residue. Additionally, His32 of astressin was substituted by either alanine or tyrosine for specific radioactive labeling with 125I at either His13 or Tyr32, respectively. The photoactivatable CRF antagonists were tested for their ability to displace 125I-labeled Tyr0 ovine CRF ([125I-labeled Tyr0]oCRF) in binding experiments and to inhibit oCRF-stimulated adenylate cyclase activity in human embryonic kidney (HEK) 293 cells, permanently transfected with cDNA coding for rat CRF receptor, type 1 (rCRFR1) or human Y-79 retinoblastoma cells known to carry endogenous functional human CRFR1 (hCRFR1). ATB-cyclo(30-33)[Nle21,38, Glu30, Ala32, Lys33]h/rCRF-(13-41) (compound 1) was found to bind with higher affinity to rat or human CRFR1 when compared with ATB-cyclo(30-33)[Nle21,38, Glu30, Tyr32, Lys33]h/rCRF-(13-41) (compound 2) and exhibited higher inhibition of oCRF-stimulated cAMP accumulation in HEK 293 cells stably transfected with cDNA coding for rCRFR1 (HEK-rCRFR1 cells) or Y-79 cells. A highly glycosylated, 66-kDa protein was identified with SDS/PAGE, when the radioactively iodinated compounds 1 or 2 were covalently linked to rCRFR1. The specificity of the photoactivatable 125I-labeled CRF antagonists was demonstrated with SDS/PAGE by the finding that these analogs could be displaced from the receptor by their corresponding nonlabeled form, but not other unrelated peptides such as vasoactive intestinal peptide. The observed molecular size of the receptor was in agreement with the size of CRFR1 found in rat pituitary (66 kDa), but was significantly larger than the size of CRFR1 found in rat cerebellum and olfactory bulb (53 kDa).

Photolabeling of human serum albumin by 4-azido-2-([14C]-methylamino) trifluorobenzonitrile. A high-efficiency, long wavelength photolabel

N-alkyl derivatives of perfluoroarylazides are excellent candidates for photolabeling of proteins since they have absorption spectra in the 340-350 nm range permitting photolabel absorption without direct protein photolysis. The [14C]-N-methylamino derivative of 4-azido-tetrafluorobenzonitrile has been used to demonstrate that 80% of the photo-induced nitrene transient becomes covalently attached to HSA during photolysis. Multiwavelength detection of the photoprobe-protein separation by size exclusion chromatography is shown to be an effective tool for assessing the conjugation of the photoprobe to the protein.

A stable diazo photoaffinity label with high absorptivity and effective photoactivation beyond 300 nm

The sulfosuccinimidyl active ester of 3-(3-carbethoxy-4-diazo-5-oxo-2-pyrrolin-1-yl)propanoic acid (DIAZOPY-SE) has been synthesized for use as a photoaffinity labeling reagent. This compound was obtained from commercial chemicals by a four-step synthesis requiring no complex procedures or special apparatus. The active ester efficiently derivatizes protein amino groups with the chromophore 3-carbethoxy-4-diazo-5-oxo-2-pyrroline (DIAZOPY, epsilon 8800 M-1 cm-1 at lambda max 330 nm), which on irradiation yielded products expected from formation of a reactive carbene intermediate. Brief irradiation of DIAZOPY in 2-propanol using wavelengths greater than 300 nm for photolysis yielded mainly an isopropyl ether resulting from insertion of the carbene into the O-H bond of the alcohol. Formed concurrently and to a somewhat lesser extent was an isopropyl ester, resulting from a ring-contracting Wolff rearrangement of the carbene and subsequent reaction with isopropanol. Analogous products were produced by photolysis in 2-propanol of DIAZOPY-PA (for DIAZOPY propanoic acid), the carboxylic acid precursor of DIAZOPY-SE. Facile protein derivatization by DIAZOPY-SE was demonstrated using actin and sheep IgG. Actin labeled with DIAZOPY-SE and irradiated while in the F-actin (reversibly polymerized) form was crosslinked to yield a covalently-linked dimer, illustrating the potential of the reagent in photoaffinity applications. Advantages of DIAZOPY-SE as a photoaffinity labeling reagent include ease of synthesis, chemical and photostability, efficient photolysis at wavelengths greater than 300 nm, and a capacity for crosslinking by carbene insertion processes.

Efficient and selective photoaffinity labeling of the estrogen receptor using two nonsteroidal ligands that embody aryl azide or tetrafluoroaryl azide photoreactive functions

3-(4-Azido-2,3,5,6-tetrafluorobenzoyl)-6-hydroxy-2-(4- hydroxyphenyl)benzo[b]thiophene 1 (tetrafluoroaryl azide, TFAA) and its protio analogue 3-(4-azidobenzoyl)-6- hydroxy-2-(4-hydroxyphenyl)benzo[b]thiophene 2 (protioaryl azide, PAA), photoaffinity labeling (PAL) reagents for the estrogen receptor (ER), have been prepared in high specific activity tritium-labeled form (19 Ci/mmol) and shown to undergo selective and efficient photocovalent attachment to ER from rat uterus. Both azides 1 and 2 demonstrate high binding affinity for ER as determined by both a competitive binding assay (relative binding affinities: estradiol = 100; TFAA = 9.3; PAA = 66) and a direct binding assay (Kd: estradiol = 0.24 nM; TFAA = 2.64 nM; PAA = 0.37 nM). When unlabeled TFAA and PAA are irradiated at greater than 315 nm, they demonstrate site-specific photoinactivation of ER that reaches 43% and 55%, respectively, by 30 min. Specific photocovalent attachment to ER can be effected by irradiation of the tritium-labeled azides; the covalent attachment efficiency is good (1 = 20-30%, 2 = ca. 25%) and the selectivity of ER labeling is high. Characterization of the photolabeled proteins by SDS-polyacrylamide gel electrophoresis shows specific labeling of a major component at Mr 60,000 and a minor species at Mr 46,000, the same two species that are labeled by [3H]tamoxifen aziridine, a well-characterized affinity label for ER. The ER-specific antibodies H222Sp gamma and D547Sp gamma show a clean precipitation of only these two species. In the MCF-7 human breast cancer cell line, PAA is a full estrogen agonist in terms of stimulation of cell proliferation and induction of progesterone receptor. These two azides provide the first system in which the photocovalent attachment efficiency of an aryl azide can be compared to its tetrafluorosubstituted aryl azide analogue in a complex biological receptor system. Azides 1 and 2 are the most efficient and selective PAL reagents prepared to date for ER, and they should be useful in further studies of the hormone-binding domain of this protein.

Direct photoaffinity labeling of leukotriene binding sites

Due to their conjugated double bonds the leukotrienes themselves are photolabile compounds and may therefore be used directly for photoaffinity labeling of leukotriene binding sites. Cryofixation eliminates unspecific labeling taking place in solution by photoisomers and photodegradation products of leukotrienes. After fixation of receptor ligand interactions by shock-freezing of the samples, irradiation-induced highly reactive excited states and/or intermediates can form covalent bonds with the respective binding site in the frozen state. After cryofixation of a solution of albumin incubated with [3H8]leukotriene E4, irradiation at 300 nm resulted in time-dependent incorporation of radioactivity into the protein. Photoaffinity labeling of rat as well as of human blood serum with [3H8]leukotriene E4 after cryofixation revealed that only one polypeptide with an Mr of 67,000 was labeled. This polypeptide was identified as albumin. Photoaffinity labeling of rat liver membrane subfractions enriched with sinusoidal membranes resulted in the labeling of a polypeptide with an apparent Mr of 48,000, whereas no polypeptide was predominantly labeled in the subfraction enriched with canalicular membranes. Photoaffinity labeling of isolated hepatocytes disclosed different leukotriene E4 binding polypeptides. In the particulate fraction of hepatocytes a polypeptide with an apparent Mr of 48,000 was labeled predominantly, whereas in the soluble fraction several polypeptides were labeled to a similar extent. One of these, with an apparent Mr of 25,000, was identified as subunit 1 of glutathione transferases by immunoprecipitation. The method of direct photoaffinity labeling in the frozen state after cryofixation using leukotrienes as photoactivatable compounds, as exemplified by leukotriene E4, may be most useful for the identification and characterization of various leukotriene binding sites, including receptors, leukotriene-metabolizing enzymes, and transport systems.