Interactions of b-Lactoglobulin with Serotonin and Arachidonyl Serotonin Asghar Taheri-Kafrani, 1,2 Yvan Choiset, 1 Dzhigangir A. Faizullin, 3 Yuri F. Zuev, 3 Vladimir V. Bezuglov, 4 Jean-Marc Chobert, 1 Abdol-Khalegh Bordbar, 2 Thomas Haertle ´ 1 1 UR 1268, INRA BIA-FIP, B.P. 71627, 44316 Nantes Cedex 03, France 2 Department of Chemistry, Laboratory of Biophysical Chemistry, University of Isfahan, Isfahan 81746-73441, I.R. Iran 3 Kazan Institute of Biochemistry and Biophysics, Russian Academy of Sciences, Kazan, Russia 4 Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, Moscow, 117997, Russia Received 28 February 2011; revised 9 June 2011; accepted 9 June 2011 Published online 5 July 2011 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/bip.21690 This article was originally published online as an accepted preprint. The ‘‘Published Online’’ date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at biopolymers@wiley. com INTRODUCTION S erotonin (5-hydroxytryptamine, 5-HT, Figure 1A) is a monoamine neurotransmitter. 5-HT and its deriva- tives are found extensively in the gastrointestinal tract of animals, and about 80% to 90% of the total 5-HT in the human body is located in the Interactions of b-Lactoglobulin with Serotonin and Arachidonyl Serotonin Additional Supporting Information may be found in the online version of this article. Correspondence to: Thomas Haertle ´; e-mail: haertle@nantes.inra.fr or Abdol-Khalegh Bordbar; e-mail: khalegh_bordbar@yahoo.com ABSTRACT: b-Lactoglobulin (b-LG) is a lipocalin, which is the major whey protein of cow’s milk and the milk of other mammals. However, it is absent from human milk. The biological function of b-LG is not clear, but its potential role in carrying fatty acids through the digestive tract has been suggested. b-LG has been found in complexes with lipids such as butyric and oleic acids and has a high affinity for a wide variety of compounds. Serotonin (5-hydroxytryptamine, 5-HT), an important compound found in animals and plants, has various functions, including the regulation of mood, appetite, sleep, muscle contraction, and some cognitive functions such as memory and learning. In this study, the interaction of serotonin and one of its derivatives, arachidonyl serotonin (AA-5HT), with b-LG was investigated using circular dichroism (CD) and fluorescence intensity measurements. These two ligands interact with b-LG forming equimolar complexes. The binding constant for the serotonin/b-LG interaction is between 10 5 and 10 6 M 21 , whereas for the AA-5HT/b-LG complex it is between 10 4 and 10 5 M 21 as determined by measurements of either protein or ligand fluorescence. The observed binding affinities were higher in hydroethanolic media (25% EtOH). The interactions between serotonin/b- LG and AA-5HT/b-LG may compete with self-association (micellization) of both the ligand and the protein. According to far- and near-UV CD results, these ligands have no apparent influence on b-LG secondary structure, however they partially destabilize its tertiary structure. Their binding by b-LG may be one of the peripheral mechanisms of the regulation of the content of serotonin and its derivatives in the bowel of milk-fed animals. # 2011 Wiley Periodicals, Inc. Biopolymers 95: 871–880, 2011. Keywords: b-lactoglobulin; serotonin; arachidonyl serotonin; micellization; binding constant; fluorimetry; circular dichroism Contract grant sponsor: Research Council of Isfahan University and of the Franco- Iranian Partenariat Hubert Curien Gundishapur Contract grant number: 10114VC Contract grant sponsor: French Ministry of Foreign Affairs in the frame of EcoNet programme for the DAF Contract grant sponsor: Russian Academy of Sciences, the programme Molecular and Cellular Biology for DAF and YFZ V V C 2011 Wiley Periodicals, Inc. Biopolymers Volume 95 / Number 12 871