title: Competition between lithium and magnesium ions for the G-protein transducin in the guanosine 5'-diphosphate bound conformation.
abstract: Li(+) is the most effective drug used to treat bipolar disorder; however, its exact mechanism of action has yet to be elucidated. One hypothesis is that Li(+) competes with Mg2+ for the Mg2+ binding sites on guanine-nucleotide binding proteins (G-proteins). Using 7Li T1 relaxation measurements and fluorescence spectroscopy with the Mg2+ fluorophore furaptra, we detected Li(+)/Mg(2+) competition in three preparations: the purified G-protein transducin (Gt), stripped rod outer segment membranes (SROS), and SROS with purified Gt reattached (ROS-T). When purified ROS-T, SROS or transducin were titrated with Li+ in the presence of fixed amounts of Mg(2+), the apparent Li(+) binding constant decreased due to Li(+)/Mg(2+) competition. Whereas for SROS the competition mechanism was monophasic, for G(t), the competition was biphasic, suggesting that in G(t), Li(+)/Mg(2+) competition occurred with different affinities for Mg(2+) in two types of Mg(2+) binding sites. Moreover, as [Li(+)] increased, the fluorescence excitation spectra of both ROS-T and G(t) were blue shifted, indicating an increase in free [Mg(2+)] compatible with Li(+) displacement of Mg(2+) from two low affinity Mg(2+) binding sites of G(t). G(t) release from ROS-T membrane was also inhibited by Li(+) addition. In summary, we found evidence of Li(+)/Mg(2+) competition in G(t)-containing preparations.
authors.affiliation: Department of Chemistry, Loyola University Chicago, 6525 N. Sheridan Road, Chicago, IL 60626, USA.
authors.forename: Chandra
authors.initials: C
authors.lastname: Srinivasan
authors.forename: Jason
authors.initials: J
authors.lastname: Toon
authors.forename: Louis
authors.initials: L
authors.lastname: Amari
authors.forename: Abde M
authors.initials: AM
authors.lastname: Abukhdeir
authors.forename: Heidi
authors.initials: H
authors.lastname: Hamm
authors.forename: Carlos F G C
authors.initials: CF
authors.lastname: Geraldes
authors.forename: Yee-Kin
authors.initials: YK
authors.lastname: Ho
authors.forename: Duarte
authors.initials: D
authors.lastname: Mota de Freitas
mesh_terms: D000818:Animals; D001667:Binding, Competitive; D001714:Bipolar Disorder / Q000188:drug therapy / Q000378:metabolism; D002417:Cattle; D006153:Guanosine Diphosphate / Q000378:metabolism*; D006160:Guanosine Triphosphate / Q000378:metabolism; D006801:Humans; D066298:In Vitro Techniques; D007700:Kinetics; D008094:Lithium / Q000378:metabolism*; D008274:Magnesium / Q000378:metabolism*; D009682:Magnetic Resonance Spectroscopy; D011487:Protein Conformation; D012374:Rod Cell Outer Segment / Q000378:metabolism; D013050:Spectrometry, Fluorescence; D015293:Transducin / Q000737:chemistry / Q000378:metabolism*
chemical_list: D006153:Guanosine Diphosphate; D006160:Guanosine Triphosphate; D008094:Lithium; D015293:Transducin; D008274:Magnesium