Tritiated photoactivatable analogs of the native human thrombin receptor (PAR-1) agonist peptide, SFLLRN-NH2.

Six photoactivatable analogs of the human thrombin receptor activating peptide (TRAP), SFLLRN-NH2, were synthesized by substituting the photoactive amino acid, p-benzoylphenylalanine (Bpa), into each position of the peptide sequence. Platelet aggregation assays indicated that the peptides with Bpa substitutions at positions 3 to 6 retained agonist activity. These peptides were prepared in tritiated form as potential thrombin receptor photoaffinity labels. The [3H]Bpa-containing analogs were constructed by resynthesizing the peptides with the amino acid, 4-benzoyl-2',5'-dibromophenylalanine (Br2Bpa), and subjecting the purified peptides to Pd-catalyzed tritiodebromination. The radiochemical yields for the reductive tritiation were < 2% for peptides with [3H]Bpa in the third and fourth positions, and between 7 and 16% for the peptides with substitutions at the fifth and sixth positions. The low yields were due to over-reduction of the Bpa carbonyl group and nonspecific degradation during reductive tritiation. This report describes the first use of Br2Bpa for the preparation of tritiated photoactivatable peptides.

[3H]MRS 1754, a selective antagonist radioligand for A(2B) adenosine receptors.

MRS 1754 [N-(4-cyanophenyl)-2-[4-(2,3,6,7-tetrahydro-2,6-dioxo-1,3-dipropyl-1H-purin-8-yl)-phenoxy]acetamide] is a selective antagonist ligand of A(2B) adenosine receptors. This is the least well-defined adenosine receptor subtype, and A(2B) antagonists have potential as antiasthmatic drugs. For use as a radioligand, MRS 1754, a p-cyanoanilide xanthine derivative, was tritiated on the propyl groups in a two-step reaction using a p-carboxamido precursor, which was dehydrated to the cyano species using trifluoroacetic anhydride. [3H]MRS 1754 (150 Ci/mmol) bound to recombinant human A(2B) adenosine receptors in membranes of stably transfected HEK-293 cells. Specific binding was saturable, competitive, and followed a one-site model, with a K(D) value of 1.13 +/- 0.12 nM and a B(max) value of 10.9 +/- 0.6 pmol/mg protein. Specific binding utilizing 0.7 nM [3H]MRS 1754 was > 70% of total binding. The affinity calculated from association and dissociation binding constants was 1.22 nM (N = 4). Binding to membranes expressing rat and human A(1) and A(3) adenosine receptors was not significant, and binding in membranes of HEK-293 cells expressing human A(2A) receptors was of low affinity (K(D) > 50 nM). The effects of cations and chelators were explored. Specific binding was constant over a pH range of 4.5 to 6.5, with reduced binding at higher pH. The pharmacological profile in competition experiments with [3H]MRS 1754 was consistent with the structure-activity relationship for agonists and antagonists at A(2B) receptors. The K(i) values of XAC (xanthine amine congener) and CPX (8-cyclopentyl-1,3-dipropylxanthine) were 16 and 55 nM, respectively. NECA (5'-N-ethylcarboxamidoadenosine) competed for [3H]MRS 1754 binding with a K(i) of 570 nM, similar to its potency in functional assays. Thus, [3H]MRS 1754 is suitable as a selective, high-affinity radioligand for A(2B) receptors.

New photoaffinity analogs of paclitaxel.

Two new photoreactive paclitaxel analogs bearing [3H2]-3-(4-benzoyl)phenylpropanoyl group as the photophore as well as radiolabeling unit at the 7 and 10 positions, respectively, are developed. These new photoreactive analogs showed excellent preliminary results on the photoaffinity labeling of tubulin and P-glycoprotein.

Tethered benzophenone reagents for the synthesis of photoactivatable ligands.

A new radiolabeled, bifunctional photoaffinity cross-linking reagent, N-succinimidyl p-benzoyl-[2,3-3H2]dihydrocinnamate, has been synthesized in high yield and with high specific activity. This reagent can be used to append the benzophenone photophore to amino groups of small molecules, such as O-aminoalkylinositol polyphosphates and polypeptides. The resulting tritiated photoaffinity labels can be purified and manipulated in ambient light and can be activated at 360 nm.

Arachidonoyl ethanolamide-[1,2-14C] as a substrate for anandamide amidase.

Arachidonoyl ethanolamide-[1,2-14C] was prepared and evaluated as a substrate for anandamide amidase in a radioenzymatic assay that does not require a thin layer chromatography separation step. Using this substrate the release of ethanolamine-[1,2-14C] is linear for approximately thirty minutes. Anandamide amidase exhibits maximal activity between pH 8 and pH 9 with a steep decline in activity at pH values below 6 and above 10. Arachidonoyl ethanolamide-[1,2-14C] was used for the assay of anandamide amidase from 10 micrograms to 100 micrograms protein, from cow brain homogenate, in a 0.2 ml incubation mixture. When plotted as a rectangular hyperbola of the steady-state Michaelis-Menten equation, an approximate Km of 30 +/- 7 microM and a Vmax of 198 +/- 13 nmoles ethanolamine formed per hour per mg protein homogenate was obtained.

Enzymatic mass determination of high-specific-radioactivity 3H- and 14C-labeled polyunsaturated fatty acids.

The soybean lipoxygenase reaction has been applied to calculating the specific activities of 3H- and 14C-labeled polyunsaturated fatty acids of the omega 3 and omega 6 families. The extent of the soybean lipoxygenase reaction with polyunsaturated fatty acids was determined by measuring the increase in absorbance at 234 nm. A salient feature of this application of of the soybean lipoxygenase assay involves the use of a solution which contains highly purified [1-14C]arachidonic acid of known specific radioactivity as a convenient and versatile standard. Because the amount of arachidonic acid in this standard can be easily measured by liquid scintillation counting, the problems of accurately weighing liquid fatty acid standards are avoided.

Aporphines. 58. N-(2-chloroethyl) [8,9-2H]norapomorphine, an irreversible ligand for dopamine receptors: synthesis and application.

The synthesis of the title compounds (1c and its 2H isomer 1b) from N-(2-hydroxyethyl)norapomorphine was carried out by ring bromination, followed by chlorination to the 2-chloroethyl compound 6. Further reduction with 2H2 or 3H2 and Pd/C gave 1b or 1c. Radiochemically pure (97%) 1c was obtained with a specific activity of 16.3 Ci/mmol. The purity of 1c was determined by LC, HPLC, UV, and NMR. [3H]NCA was shown to label the D2 receptor; however, the D2 signal appears to be only a small portion of the total signal, which may include binding to other dopamine receptor subtypes (D1 and D3).