Sulfur Metabolism Under Stress

Abstract

Significance: In humans, imbalances in the reduction-oxidation (redox) status of cells are associated with many pathological states. In addition, many therapeutics and prophylactics used as interventions for diverse pathologies either directly modulate oxidant levels or otherwise influence endogenous cellular redox systems. Recent Advances: The cellular machineries that maintain redox homeostasis or that function within antioxidant defense systems rely heavily on the regulated reactivities of sulfur atoms either within or derived from the amino acids cysteine and methionine. Recent advances have substantially advanced our understanding of the complex and essential chemistry of biological sulfur-containing molecules. Critical Issues: The redox machineries that maintain cellular homeostasis under diverse stresses can consume large amounts of energy to generate reducing power and/or large amounts of sulfur-containing nutrients to replenish or sustain intracellular stores. By understanding the metabolic pathways underlying these responses, one can better predict how to protect cells from specific stresses. Future Directions: Here, we summarize the current state of knowledge about the impacts of different stresses on cellular metabolism of sulfur-containing molecules. This analysis suggests that there remains more to be learned about how cells use sulfur chemistry to respond to stresses, which could in turn lead to advances in therapeutic interventions for some exposures or conditions.

Keywords: disulfide reductase systems; drug metabolism; methionine cycle; oxidative stress; trans-sulfuration.

FIG. 1.

Basal mammalian S-metabolism. Major cellular pathways are shaded to distinguish those that are NADPH dependent (blue), those in which the S must come entirely from extracellular Met sources (red), those in which the sulfur can come from either Met in an NADPH-independent process or from cystine in an NADPH-dependent process (green), and those in which the S can come from Met, cystine, or inorganic sulfite or sulfate (brown). Red boxes denote the following: [1] the Met cycle, [2] trans-sulfuration, and [3] the polyamine synthesis (methionine salvage) pathway. Dashed arrows denote multistep processes. Abbreviations are listed in Abbreviations Used section. Different cell types will have different activities of the various pathways; arrow weights reflect predictions of approximate flux in liver hepatocytes under basal states. As discussed in the text, stresses will variously affect flux rates at many points in these pathways. Color images are available online.

FIG. 2.

Biological redox states and transitions of S. Most activities of S in mammalian cells transition between the most reduced state (far left) and a 2-electron oxidation of that state, fueled by the NADPH-dependent GSR and TRXR1 disulfide reductase systems. A further 2-electron oxidation yields sulfinic acids, which are not reducible in mammalian cells except in the special situation of sulfiredoxin-catalyzed repair. Further oxidation states cannot be reduced in mammalian cells, with the exception of persulfide-linked higher oxidation states. In the figure, “Cys” is used to refer to the free amino acid, Cys in the context of a protein or other Cys-containing molecule, or a Cys-derived S in the context of another metabolite. See text for more details. GSR, glutathione reductase; TRXR1, thioredoxin reductase-1.

FIG. 3.

Oxidation and reduction of Met. Oxidation of Met will yield Met-SO in either R or S diastereomeric forms. On free Met, this will occur randomly; however in the context of a protein or under influence of an enzyme, preference for one or the other could occur. Reduction of the R form can only be catalyzed by MSRA, whereas the S isoform can only be reduced by MSRB. These two enzymes are differentially expressed. It has been proposed that diastereomer-specific oxidation and resolution of protein-Met-SO forms might participate in redox regulation (see text). Met-SO, Met-sulfoxide; MSR, Met-SO reductase.

FIG. 4.

Different toxic stresses have different impacts on S metabolism. (A) Toxins like rotenone disrupt mitochondrial respiration, leading to ROS production that will be addressed primarily by GPX and PRX enzymes. The pathways used generally recycle their S-metabolites and therefore will not deplete cellular S amino acids, but rather will consume energetic nutrients to maintain NADPH pools. (B) Toxins like aflatoxin B1 that are glutathionylated and exported will predominantly consume S amino acids to maintain GSH pools. Compared with (A), these will have lower consumption of NADPH and energetic resources, though some NADPH and ATP will be used to reduce cystine, synthesize GSH, and export the glutathionylated toxin through ABC exporters. (C) Electrophilic toxins such as arsenite (As³⁺) will be glutathionylated, but will also disrupt nucleophilic active site Cys or Sec residues, thereby dominantly inhibiting redox homeostasis systems. Typically, arsenite is a potent low-dose exposure, so GSH depletion is less important than is the disruption of redox enzymes. (D) Electrophilic compounds that redox cycle, like paraquat, can induce catalytic production of superoxide radical → H₂O₂. Whereas this alone will not deplete S amino acid pools, it can consume large amounts of NADPH and can generate considerable ROS/oxidative damage. (E) At high doses, APAP can exhibit toxicity at many levels by many mechanisms, as summarized in the text. In the online version of this panel, red denotes energetic impacts; green denotes impacts on S amino acid metabolism; orange denotes inhibition of nucleophilic active sites in redox enzymes; and purple indicates possible redox cycling. To summarize, APAP that enters hepatocytes is first conjugated to sulfate (not shown) or glycogen-derived glucuronic acid and then exported by ABC transporters. This can rapidly deplete all glycogen reserves in the hepatocytes [1]. Once glycogen reserves are depleted, excess APAP is reduced by CYP enzymes, consuming NADPH and generating the electrophilic quinone NAPQI. NAPQI is then conjugated to GSH by GST enzymes and exported through ABC transporters, thereby depleting GSH and consuming S amino acids [2]. Once GSH pools are exhausted, accumulating NAPQI dominantly inhibits nucleophilic active sites in redox enzymes [3] and likely redox cycles in some NADPH-dependent oxidoreductases [4]. ABC, ATP-binding cassette; APAP, acetaminophen; CYP, cytochrome p450; GPX, glutathione peroxidase; GSH glutathione; GST, GSH-S-transferase; NAPQI, N-acetylbenzoquinoneimine; PRX, peroxiredoxin. Color images are available online.

FIG. 5.

Redox sensing and metabolic reprioritization by GAPDH. GAPDH, a critical enzyme on the energy generating triosephosphate pathway, has a highly reactive Cys-thiolate ion in the active site (S⁻). In normal conditions, GAPDH is active and flux of glucose through the triose-P pathway for energetics versus through the pentose-P pathway for generating NADPH is balanced. During oxidative stress, H₂O₂ oxidizes the active site thiolate, which inactivates glycolytic activity of GAPDH, lowering glycolytic flux and making more glucose available for the pentose-P pathway; and converts GAPDH into a nuclear-localized transcription factor that further favors antioxidant activities of NADPH arising from the pentose-P pathway. GAPDH, glyceraldehyde-3-phosphate dehydrogenase.

FIG. 6.

Hypothetical persulfidation-based thiol protection. a, In the unstressed state, Cys residues in proteins or metabolites will generally be in the reduced thiol form. b, Exposure to H₂O₂ yields the unstable sulfenic acid. c, Whereas this will typically react with another thiol to yield a disulfide, in the presence of H₂S, a persulfide can form, which will spontaneously ionize to the reactive perthiolate anion at physiological pH. d, The reactive perthiolate anion can react with H₂O₂, yielding persulfenylate anion. This might further react with H₂O₂, yielding persulfinylate and persulfonylate anions (in combination, these overoxidized forms designated “-SSOx”). e, Reduction of the disulfide bond in any of the -SSOx species by TRX1, TRP14, or GRX will regenerate the active thiol on the original substrate and liberate an oxidized inorganic S product, designated “SOx.” GRX, glutaredoxin; H₂O₂, hydrogen peroxide; H₂S, hydrogen sulfide; TRP14, thioredoxin-related protein of 14 kDa; TRX1, thioredoxin-1.