Role of sulfane sulfur species in elemental tellurium nanorod formation in mammalian cells

Abstract

Tellurium (Te) is an industrially useful element but its oxyanions, such as tellurite and tellurate, are naturally occurring chemical forms that can become a potential source of toxicity to humans and animals. As a means of mitigating the toxicity of Te oxyanions, the formation of less toxic zero-valent elemental Te (Te⁰) nanostructures has been observed in various species including bacteria, fungi, green algae, and higher plants. In this study, we investigated the formation of Te⁰ nanorods in human hepatoma HepG2 cells. We detected electron-dense Te nanorods in lysosomes after exposure to potassium tellurite. The amount of Te nanorods in the cells gradually increased with the exposure period. Interestingly, the amount of Te in the insoluble fraction of the culture supernatant was approximately 10 times higher than that in HepG2 cells, suggesting that extracellular reducing agents originating from HepG2 cells transformed tetravalent Te (TeO₃²⁻) into Te⁰ in the culture medium. As an extracellular reducing agent, sulfane sulfur species were considered responsible for the reduction of Te(IV). Then, by inhibiting cystathionine γ-lyase with propargylglycine (PPG), we were able to reduce the amount of sulfane sulfur species generated in the cells. In the presence of PPG, the amount of insoluble Te in the culture supernatant, which was possibly composed of Te⁰ nanorods, was significantly decreased. The results suggest that sulfane sulfur species are involved in the formation of Te⁰ nanorods from tellurite in mammalian cells and play a critical role in the amelioration of Te oxyanion toxicity.