Substance P derivatives with photoactivatable labels in the N-terminal part of the molecule

Photoactivatable substance P (SP) derivatives containing the p-benzoylbenzoic moiety at the N-terminal alpha-amino group of Arg 1 or at the epsilon-amino group of Lys 3 were prepared. Both derivatives also had a p-hydroxyphenylpropionyl group for radioiodination. To obtain the analogue with the photolabel at Arg 1, SP was first reacted with N-hydroxysuccinimide p-hydroxyphenylpropionate, the Lys 3-modified derivative was isolated by reversed-phase high-performance liquid chromatography (HPLC), reacted with N-hydroxysuccinimide p-benzoylbenzoate and purified by HPLC. To place the photolabel at Lys 3, the order of the reactions was reversed. The structure of the derivatives obtained was confirmed by mass spectrometry. The interaction of the derivatives obtained and of their 125I-labeled forms with the NK-1 neurokinin receptor from the rat brain, as well as with the nicotinic acetylcholine receptor from Torpedo electrocytes was analyzed. The results obtained supported by the data from the literature indicate that benzoylbenzoic acid derivatives should not be considered as universal photolabels, which ensure in all cases a high level of photo-cross-linking.