Reactivity of Zn-, Cd-, and Apo-Metallothionein with Nitric Oxide Compounds: In Vitro and Cellular Comparison

Abstract

The reactivity of Zn(7)- and Cd(7)-metallothionein (MT) with S-nitrosopenicillamine (SNAP), S-nitrosoglutathione (GSNO), and 2-(N,N-diethylamino)-diazenolate-2-oxide (DEA/NO) was investigated to explore the hypothesis that metallothionein is a signficant site of cellular reaction of nitric oxide or NO compounds. Zn(7)-MT reacted with SNAP or GSNO only under aerobic conditions and in the presence of light, which stimulates the decomposition of S-nitrosothiolates to NO. Zn(2+) is released, and protein thiols are modified. DEA/NO, which degrades spontaneously to release NO, also reacted with Zn(7)-MT only when oxygen was present. Anaerobically, DEA/NO reacted with Zn(7)-MT in the presence of 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, which converts NO to NO(2). Glutathione competed effectively with Zn(7)-MT for reactive nitrogen oxide species in reaction mixtures. Reaction of Cd(7)-MT with SNAP also required oxygen and light to react. In this case, only a fraction of the Cd(2+) bound to Cd(7)-MT was displaced by SNAP. Apo-metallothionein was much more reactive with SNAP and DEA-NO than Zn(7)- or Cd(7)-MT. TE671 and LLC-PK(1) cell lines were incubated with DEA/NO to examine the role that MT might play in the cellular reactions of this NO donor compound. Incubation of cells with 0-80 microM Zn(2+) for 24 h resulted in progressively increasing concentrations of Zn-unsaturated MT. One hour of cellular exposure to a range of DEA/NO concentrations followed by 24 h of incubation caused no evident acute toxicity at less than 0.45 mM. Preinduction of MT did not alter this response. The effects of DEA/NO on proteomic, metallothionein, and low molecular weight (LMW) thiol pools, including glutathione (GSH), were measured. Substantial fractions of the proteomic and LMW thiol pools underwent reaction with little dislocation of Zn(2+). In addition, one-third of the MT thiol pool reacted without labilizing any of the bound Zn(2+). These results demonstrated that it was free thiols associated with MT that reacted with DEA/NO not those bound to Zn(2+). Moreover, under the conditions of the experiments, DEA/NO reacted with the spectrum of cellular thiols in proportion to their fraction in the cytosol and did not preferentially react with MT sulfhydryl groups.