Zinc thiolate reactivity toward nitrogen oxides: insights into the interaction of Zn2+ with S-nitrosothiols and implications for nitric oxide synthase

Abstract

Zinc thiolate complexes containing N(2)S tridentate ligands were prepared to investigate their reactivity toward reactive nitrogen species, chemistry proposed to occur at the zinc tetracysteine thiolate site of nitric oxide synthase (NOS). The complexes are unreactive toward nitric oxide (NO) in the absence of dioxygen, strongly indicating that NO cannot be the species directly responsible for S-nitrosothiol formation and loss of Zn(2+) at the NOS dimer interface in vivo. S-Nitrosothiol formation does occur upon exposure of zinc thiolate solutions to NO in the presence of air, however, or to NO(2) or NOBF(4), indicating that these reactive nitrogen/oxygen species are capable of liberating zinc from the enzyme, possibly through generation of the S-nitrosothiol. Interaction between simple Zn(2+) salts and preformed S-nitrosothiols leads to decomposition of the -SNO moiety, resulting in release of gaseous NO and N(2)O. The potential biological relevance of this chemistry is discussed.

Chart 1

N₂S tridentate chelating ligands and the related Zn²⁺ coordination compounds.

Figure 1

UV-Vis reactivity profiles for anaerobic 1.5 mM ZnPAThCl + excess NO (g), left, and 1.5 mM ZnPAThCl + excess NO (g) + air, right. Recorded in acetonitrile.

Figure 2

Disappearance of characteristic S-nitrosothiol peaks occurs upon exposure of 1.8 mM PAThNO (left) and 1.5 mM APAThNO (right) to 1 equiv of Zn(OTf)₂ in acetonitrile. Reactions were monitored for 1 h.

Scheme 1

Synthesis of PATh species.

Scheme 2

Synthesis of APATh species.

Scheme 3

Traceless ligation of APAThNO by phosphine-mediated N=O bond scission.^,