Redox compartmentalization and cellular stress

Abstract

Mammalian cells are highly organized to optimize function. For instance, oxidative energy-producing processes in mitochondria are sequestered away from plasma membrane redox signalling complexes and also from nuclear DNA, which is subject to oxidant-induced mutation. Proteins are unique among macromolecules in having reversible oxidizable elements, 'sulphur switches', which support dynamic regulation of structure and function. Accumulating evidence shows that redox signalling and control systems are maintained under kinetically limited steady states, which are highly displaced from redox equilibrium and distinct among organelles. Mitochondria are most reducing and susceptible to oxidation under stressed conditions, while nuclei are also reducing but relatively resistant to oxidation. Within compartments, the glutathione and thioredoxin systems serve parallel and non-redundant functions to maintain the dynamic redox balance of subsets of protein cysteines, which function in redox signalling and control. This organization allows cells to be poised to respond to cell stress but also creates sites of vulnerability. Importantly, disruption of redox organization is a common basis for disease. Research tools are becoming available to elucidate details of subcellular redox organization, and this development highlights an opportunity for a new generation of targeted antioxidants to enhance and restore redox signalling and control in disease prevention.

Fig. 1. The redox hypothesis for oxidative stress

Oxidative stress research has largely focused on free radical mechanisms as a contributing factor in disease development but large-scale double-blind interventional trials with free radical scavenging antioxidants have been disappointing in providing little evidence of health benefit. The redox hypothesis, consisting of four postulates as indicated, was formulated as an alternative interpretation of experimental data [9].

Fig 2. Subcellular compartmentalization of thiol/disulfide redox potentials in cultured cells

Based upon a limited number of cell culture studies, a picture of the normal redox compartmentalization is emerging. The most secure data is for the cytoplasmic GSH redox potential, which varies from about -260 mV in rapidly proliferating cells to about -220 mV in non-dividing cells. A few cell types and metabolic conditions result in more positive (oxidized values), but snap-frozen tissue extracts are typically in this range. Trx-1 redox state has been measured in many cell types and is more reduced than GSH/GSSG. The nuclear Trx1 is more reduced than cytoplasmic Trx1 in monocytic and colonic cell lines. The mitochondrial Trx-2 is more reduced than nuclear and cytoplasmic Trx1, and also more reduced than the mitochondrial GSH/GSSG. The mitochondrial GSH/GSSG is more reduced than cytoplasmic values, and the endoplasmic reticular GSH/GSSG is midway between cytoplasmic and plasma values. Cytoplasmic Cys/CySS redox potential is considerably oxidized relative to the GSH and Trx couples but reduced relative to plasma Cys/CySS.