Competition between lithium and magnesium ions for the G-protein transducin in the guanosine 5'-diphosphate bound conformation.
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.
Srinivasan
Chandra
C
Department of Chemistry, Loyola University Chicago, 6525 N. Sheridan Road, Chicago, IL 60626, USA.
Toon
Jason
J
Amari
Louis
L
Abukhdeir
Abde M
AM
Hamm
Heidi
H
Geraldes
Carlos F G C
CF
Ho
Yee-Kin
YK
Mota de Freitas
Duarte
D
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*
D006153:Guanosine Diphosphate; D006160:Guanosine Triphosphate; D008094:Lithium; D015293:Transducin; D008274:Magnesium