A Molecular Model for Lithium's Bioactive Form

Abstract

Lithium carbonate, a drug for the treatment of bipolar disorder, provides mood stability to mitigate recurrent episodes of mania and/or depression. Despite its long-term and widespread use, the mechanism by which lithium acts to elicit these psychological changes has remained unknown. Using nuclear magnetic resonance (NMR) methods, in this study we characterized the association of lithium with adenosine triphosphate (ATP) and identified a bimetallic (Mg·Li) ATP complex. Lithium's affinity to form this complex was found to be relatively high (Kd ∼1.6 mM) compared with other monovalent cations and relevant, considering lithium dosing and physiological concentrations of Mg(2+) and ATP. The ATP·Mg·Li complex reveals, for the first time, to the best of our knowledge, that lithium can associate with magnesium-bound phosphate sites and then act to modulate purine receptor activity in neuronal cells, suggesting a molecular mode for in vivo lithium action.

Figure 1

Mode of Li⁺ binding to ATP·Mg. (A) Schematic representation of ATP with Mg²⁺ bound to the β and γ phosphates is shown. (B) Three possible modes of Li⁺ binding to ATP·Mg (phosphates, red; Mg²⁺, blue; Li⁺, green) are shown. Scheme 1 represents Li_f + ATP·Mg → ATP·Mg + Li_f. Scheme 2 represents Li_f + ATP·Mg → ATP·Li + Mg_f. Scheme 3 represents Li_f + ATP·Mg → ATP·Mg·Li. (C) Representative ⁷Li T1 relaxation data in the absence (open circles) and presence (closed circles) of 50 μM MnCl₂ in a sample of 10 mM ATP, 11 mM MgCl₂, and 10 mM LiCl; smooth curves are single-exponential fits.

Figure 2

Measured affinity of Li⁺ binding to ATP·Mg, ADP·Mg, and TP·Mg. ⁷Li T1 relaxation times (black circles) measured for 2 mM LiCl at 10°C as a function of the concentration of (A) ATP·Mg, (B) ADP·Mg, or (C) TP·Mg are shown. Fits using a quadratic binding equation (curves) yielded Li⁺ equilibrium dissociation constants, K_d: 1.60 ± 0.21 mM for ATP·Mg; 3.24 ± 0.56 mM for ADP·Mg; and 0.71 ± 0.23 mM for TP·Mg (reported uncertainties are standard errors calculated from least-squares fitting). Sample integrity was monitored using ³¹P NMR (Fig. S4). (D) A proposed molecular model for the ternary ATP·Mg·Li complex is shown. Mg²⁺ is 6-coordinate, with the β and γ phosphate oxygens replacing two water molecules. Li⁺ is 4-coordinate, with a γ phosphate oxygen replacing one water and a water bridge shared with Mg²⁺. To see this figure in color, go online.

Figure 3

Intracellular Ca²⁺ signals evoked by stimulating neuronal purinergic receptors with ATP, ATP·Mg, ATP·Li, and ATP·Mg·Li. (A) Schematic representation of purinergic receptors on the cell surface: P2X receptors are ion channels that permit influx of Ca²⁺ ions when activated by purinergic ligands like ATP; P2Y receptors are G-protein-coupled and trigger Ca²⁺ release from intracellular stores when activated by ATP. (B) Ca²⁺ signals are reliably triggered by consecutive 20 s pulses of 100 μM ATP. Repeated responses in the same cell are not significantly different (compare 1st and 2nd responses: t_100-20 = 114, 112 s, respectively, p = 0.83, n = 9). (C) Ca²⁺ signals elicited by 20 s pulses of 100 μM ATP·Mg, 100 μM ATP, or 100 μM ATP·Li. ATP·Li without Mg²⁺ lengthens the Ca²⁺ response by 39% (compare 2nd and 3rd responses (ATP vs. ATP·Li, [Mg²⁺] = 0 mM): mean t_100-20 = 139, 193 s, respectively, p = 0.015, n = 12). (D) Ca²⁺ responses evoked by 20 s pulses of 100 μM ATP·Mg or 100 μM ATP·Mg·Li. Response evoked by ATP·Mg·Li was longer than that evoked by ATP·Mg by 2.2-fold (compare 1st and 2nd responses (ATP·Mg vs. ATP·Mg·Li): t_100-20 = 86, 191 s, respectively, p = 2.4 × 10⁻⁵, n = 20; compare 1st and 3rd response (1st ATP·Mg vs. 2nd ATP·Mg; internal control): t_100-20 = 86, 88 s, respectively, p = 0.78, n = 20). (E) Ca²⁺ responses triggered by 100 s pulses of 100 μM ATP·Mg or 100 μM ATP·Mg·Li in Ca²⁺-free buffer. The P2Y component of ATP response does not depend significantly on Li⁺ (compare 1st and 2nd responses (ATP·Mg vs. ATP·Mg·Li): t_100-20 = 41, 50 s, respectively, p = 0.17338, n = 28; compare 1st and 3rd responses (1st ATP·Mg vs. 2nd ATP·Mg; internal control): t_100-20 = 41, 38 s, respectively, p = 0.73, n = 28). In panels B–D, extracellular Ca²⁺ was 2.5 mM. Application of Li⁺ alone elicited no response (not shown).