A hydrolysate of poly-trans-[(2-carboxyethyl)germasesquioxane] (Ge-132) suppresses Ca(v)3.2-dependent pain by sequestering exogenous and endogenous sulfide

Poly-trans-[(2-carboxyethyl)germasesquioxane] (Ge-132), an organogermanium, is hydrolyzed to 3-(trihydroxygermyl)propanoic acid (THGP) in aqueous solutions, and reduces inflammation, pain and cancer, whereas the underlying mechanisms remain unknown. Sulfides including H(2)S, a gasotransmitter, generated from l-cysteine by some enzymes including cystathionine-γ-lyase (CSE), are pro-nociceptive, since they enhance Ca(v)3.2 T-type Ca(2+) channel activity expressed in the primary afferents, most probably by canceling the channel inhibition by Zn(2+) linked via coordinate bonding to His(191) of Ca(v)3.2. Given that germanium is reactive to sulfur, we tested whether THGP would directly trap sulfide, and inhibit sulfide-induced enhancement of Ca(v)3.2 activity and sulfide-dependent pain in mice. Using mass spectrometry and (1)H NMR techniques, we demonstrated that THGP directly reacted with sulfides including Na(2)S and NaSH, and formed a sulfur-containing reaction product, which decreased in the presence of ZnCl(2). In Ca(v)3.2-transfected HEK293 cells, THGP inhibited the sulfide-induced enhancement of T-type Ca(2+) channel-dependent membrane currents. In mice, THGP, administered systemically or locally, inhibited the mechanical allodynia caused by intraplantar Na(2)S. In the mice with cyclophosphamide-induced cystitis and cerulein-induced pancreatitis, which exhibited upregulation of CSE in the bladder and pancreas, respectively, systemic administration of THGP as well as a selective T-type Ca(2+) channel inhibitor suppressed the cystitis-related and pancreatitis-related visceral pain. These data suggest that THGP traps sulfide and inhibits sulfide-induced enhancement of Ca(v)3.2 activity, leading to suppression of Ca(v)3.2-dependent pain caused by sulfide applied exogenously and generated endogenously.