Radiolabeled photoaffinity labels

We have developed a new photoreactive analogue of paclitaxel, 3 -N-BzDC-3 -N-debenzoylpaclitaxel (109) and its ditritiated derivative ([3H]-109) has been evaluated for its ability to photolabel tubulin and P-glycoprotein.66 Radiolabeled photoreactive analogue [3H]-109 was synthesized by N-acylation of 3 -N-deben-zoyl-2, 7-bis(0-TES)paclitaxel (108) with N-(2,3-ditritio-3-(4-benzoyl-phenyl)propanoyloxy)succinimide ([3H]-107), followed by purification on a reversed phase semipreparative HPLC using a C-18 column (Scheme 21).66 Photoaffinity label [3H]-109 was assessed to possess >99.9% radiochemical purity and a high specific radioactivity (34 Ci/mmol). 

Several controls related to those required in photoaffinity labeling experiments with radiolabeled ligands (see Section 4.7) must also be performed in studies of photoinactivation. In particular inactivation should not occur on irradiation in the absence of the photoaffinity label, and the receptor site should be protected against photoinactivation if it is first blocked with a photochemically inert molecule. 

Herbicides that inhibit photosynthetic electron flow prevent reduction of plastoquinone by the photosystem II acceptor complex. The properties of the photosystem II herbicide receptor proteins have been investigated by binding and displacement studies with radiolabeled herbicides. The herbicide receptor proteins have been identified with herbicide-derived photoaffinity labels. Herbicides, similar in their mode of action to 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) bind to a 34 kDa protein, whereas phenolic herbicides bind to the 43-51 kDa photosystem II reaction center proteins. At these receptor proteins, plastoquinone/herbicide interactions and plastoquinone binding sites have been studied, the latter by means of a plastoquinone-deriv-ed photoaffinity label. For the 34 kDa herbicide binding protein, whose amino acid sequence is known, herbicide and plastoquinone binding are discussed at the molecular level. 

In conclusion, observations made in the last few years, especially the binding studies with radiolabeled herbicides, the photoaffinity labeling technique, and the advances of molecular biology have substantially added to our knowledge of the mechanism of action of photosynthetic herbicides. However, many questions also remain to be answered. 

Although binding activity could be easily demonstrated in crude cell free mycelial extracts of phenylamide-sensitive Phytophthora strains, binding activity decreased considerably when the extracts were subjected to Heparin Sepharose affinity chromatography, a first step in the purification of RNA polymerases (16. Purification of the phenylamide receptor, therefore, cannot be followed using the binding assay. An alternative approach would be to covalently label the receptor with a photoaffinity probe. Purification of the labeled complex can then be followed either by a radioassay or an immunoassay, which respectively require a radiolabeled photoaffinity probe or antiserum that selectively recognizes covalently bound phenylamide residues. 

Azidoacridine was used as photoaffinity label for nueleotide- and aromaticbinding sites in proteins [56,57]. The photochemistry of this azide was discussed in detail above. A radiolabeled [ H]7-azido-4-isopropylacridone upon irradiation specifically labeled (boimd covalently to) Cysl59 of the bovine mitochondrial ADP/ATP-carrier protein [103]. 

The distinctly different behavior of the phenol-type herbicides following trypsin treatment suggests that different determinants within the PS II protein complex establish the "domains" that regulate the binding properties of these inhibitors. In spite of the fact that phenol-type herbicides will displace bound radiolabeled herbicides such as diuron, these inhibitors show noncompetitive inhibition (29, 30). At present, there are three lines of evidence which favor TH e involvement of two domains within the PS II complex that participate in creating the binding sites for these herbicides (a) isolated PS II particles can be selectively depleted of a polypeptide with parallel loss of atrazine sensitivity, but not dinoseb inhibition activity (33) (b) in resistant weed biotypes, chloroplast membranes that exhibit extreme triazine resistance have increased sensitivity to the phenol-type herbicides (13) and (c) experiments with azido (photoaffinity) derivatives of phenol and triazine herbicides result in the covalent labeling of different PS II polypeptides (, 31). 

---