Modification of human metallothioneins by garlic organosulfur compounds, allicin and ajoene: direct effect on zinc homeostasis with relevance to immune regulation

Abstract

Garlic (Allium sativum) is a key culinary ingredient and agricultural product that releases its pungent aroma when cut or crushed due to the presence of volatile organosulfur compounds. These compounds, specifically allicin and ajoene, rapidly modify human metallothioneins (MTs), leading to the dissociation of Zn(II) ions. This study employed a top-down protein MS analysis to identify the most reactive cysteine sites in MT2, confirming that both compounds induce S-thioallylation and alter Zn(II) binding. Spectrophotometric studies indicated that both compounds mobilized nearly six molar equivalents of Zn(II) from MT2 after one hour, with ajoene demonstrating slightly higher dissociation efficiency than allicin. Moreover, allicin mobilized Zn(II) from all human isoforms of MT (MT1-MT4), with the lowest efficiency observed for MT4. Mass spectrometry analysis revealed a diverse array of modified complexes, with ajoene resulting in more complex formations than allicin. Reduced glutathione interacts with both untreated and allicin- or ajoene-treated MTs, forming mixed and partially oxidized complexes, highlighting the relationship between these compounds depending on the cell’s redox state. The use of reducing agents demonstrated that MTs’ S-thioallylation is reversible, allowing the protein to rebind Zn(II) after reduction, thereby maintaining its role in zinc homeostasis and rendering the effects of allicin temporary. The health implications of the modifications are significant, given the established role of Zn(II) in inhibiting pro-inflammatory pathways and enhancing antioxidant responses. Zn(II) ions were shown to inhibit the activation of the NF-κB pathway, which regulates pro-inflammatory cytokines. Our findings indicate that the modification of metallothioneins (MTs) by garlic-derived organosulfur compounds may play a crucial role in zinc homeostasis, with implications for immune regulation in humans, and warrant further investigation into their potential therapeutic applications.

Graphical Abstract