Cellular Sulfur Metabolism and Signaling in Physiology and Pathology

Dear Colleagues,

Sulfur is an element widespread in nature. It is present in the cells of all living organisms in various oxidation states: from -2 to +6. Methionine and cysteine contain sulfur in its lowest oxidation state (-2). The sulfur atom present in sulfhydryl groups of low molecular weight compounds is at a low oxidation state and can protect other compounds form oxygen toxicity and radiation damage. This is possible because sulfhydryl groups are involved in numerous redox reactions due to their ease of oxidation. Reduced sulfur is employed in the activation of substrates in some catalytic molecules and cofactors, such as biotin, coenzymes: A, B, M, glutathione, ferredoxins and other iron-sulfur proteins, molybdenum cofactor, lipoic acid, and thiamine phosphate. At higher oxidation states, sulfur occurs as sulfates (SO₄^2-; 6+) and sulfonates (RSO₃^-; 4+) which are components of sulfur-containing macromolecules, such as sulfatides and sulfate polysaccharides—major constituents of extracellular structures. The products of the oxidation of the reduced sulfur-containing compounds into higher oxidations states are reactive sulfur species (RSS). Recent discoveries characterize RSS as key players in redox regulation as important as reactive oxygen (ROS) and nitrogen (RNS) species. Examples of RSS include persulfides (RSSH, -1), polysulfides (RS(S)_nH; 0), and thiosulfate (S₂O₃^2-; +2). Persulfides and polysulfides are oxidation products of hydrogen sulfide (H₂S; -2) and they contain one or more sulfur atoms at the zero oxidation state called sulfane sulfur (S; 0). According to some researchers, the definition of RSS is much broader and includes small-molecule biological thiols, such as glutathione, cysteine, homocysteine, and H₂S. It is worth mentioning that S-nitrosothiols (RSNO) are also defined as RSS. However, the chemical nature of RSS is complex and remains poorly understood within various pathophysiological conditions. Thus, it seems likely that the presence of other chemical intermediates or derivatives of sulfide may mediate many biological functions. A wide range of sulfide metabolites are produced by sulfur-containing amino acids metabolism via transsulfuration enzymes, such as cystathionine β-synthase (EC 4.2.1.22, CBS) and cystathionine γ-lyase (EC 4.4.1.1, CSE). Moreover, numerous in vitro and in vivo studies indicate that sulfur-containing compounds have great therapeutic potential, and when used supportively, have a positive effect on the treatment of cardiovascular diseases, neurological deficits, metabolic syndrome, disorders of the immune system, and cancer.

In the current Special Issue of Antioxidants, we welcome original research papers and reviews focused on the biological role of sulfur and sulfur-containing compounds in living organisms, the impact of these compounds on changes in sulfur amino acids metabolism, and pathologies resulting from its dysregulation. New therapeutic approaches and strategies are also welcome. Papers that involve studies with humans, animals, plants, bacteria or cell lines can be published. Papers containing new analytical methods for the determination of sulfur compounds in biological samples will also be considered.

Dr. Anna Bilska-Wilkosz
Dr. Małgorzata B. Iciek
Dr. Marta Kaczor-Kaminska
Guest Editors

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