Carbon monoxide, reactive oxygen signaling, and oxidative stress

Abstract

The ubiquitous gas, carbon monoxide (CO), is of substantial biological importance, but apart from its affinity for reduced transition metals, particularly heme-iron, it is surprisingly nonreactive-as is the ferrous-carbonyl-in living systems. CO does form strong complexes with heme proteins for which molecular O2 is the preferred ligand and to which are attributed diverse physiological, adaptive, and toxic effects. Lately, it has become apparent that both exogenous and endogenous CO produced by heme oxygenase engender a prooxidant milieu in aerobic mammalian cells which initiates signaling related to reactive oxygen species (ROS) generation. ROS signaling contingent on CO can be segregated by CO concentration-time effects on cellular function, by the location of heme proteins, e.g., mitochondrial or nonmitochondrial sites, or by specific oxidation-reduction (redox) reactions. The fundamental responses to CO involve overt physiological regulatory events, such as activation of redox-sensitive transcription factors or stress-activated kinases, which institute compensatory expression of antioxidant enzymes and other adaptations to oxidative stress. In contrast, responses originating from highly elevated or protracted CO exposures tend to be nonspecific, produce untoward biological oxidations, and interfere with homeostasis. This brief overview provides a conceptual framework for understanding CO biology in terms of this physiological-pathological hierarchy.

Figure 1

A plot of the putative concentration-time relationships involved in the physiological, adaptive, and toxic effects of carbon monoxide (CO) in cells and tissues. As intracellular CO concentration increases from the picomolar into the nanomolar range, adaptive responses to the cell stress overlay the physiological homeostatic effects and the period for overt toxicity to supervene becomes progressively shorter.

Figure 2

Box plot of average cellular CO concentrations in different organs in mice. The mice were either air controls or had received one hour of CO inhalation at 500 ppm to achieve blood carboxyhemoglobin levels of ∼25%. One hour later, after euthanasia, tissues were harvested after perfusion of the vascular system with phosphate buffered saline (pH 7.4) to remove the hemoglobin. CO measurements were made in tissue homogenates using reduction gas chromatography as reported in references 44-46. The boxes represent the middle quartiles and the bars are the data ranges for 5 to 10 mice per tissue. Basal physiological CO levels are low, except for the spleen, but demonstrate significant intra- and inter-organ variability. CO exposure at the mouse's injury threshold increases cellular CO concentration by 4 to 10 fold; significant by ANOVA in all organs except the liver and the spleen (asterisks indicate P<0.05).

Figure 3

Diagram of the heme oxygenase (HO) enzyme system in the context of heme synthesis and heme protein turnover showing endogenous CO production and sites in the pathways of pro-oxidant and anti-oxidant effects.

Figure 4

Schematic diagram of the effects of cytochrome a,a₃-CO binding on mitochondrial electron transport and superoxide (O₂^-) and H₂O₂ production by Complex III. The increase in the reduction state of the cytochrome bc₁ region of the respiratory chain has been demonstrated in vivo. The reduced, oxidized, and semi-quinone forms of ubiqinone are indicated by QH₂, Q, and Q. respectively.

Figure 5

Role of CO-dependent mitochondrial H₂O₂ signaling in mitochondrial biogenesis. The egress of H₂O₂ from mitochondria allows unopposed Akt kinase activity via inactivation of thiol moieties in counter-regulatory phosphatases. Akt phosphorylates NRF-1, which translocates to the nucleus and transactivates the gene for mitochondrial transcription factor-A (Tfam), required by the bi-genomic mitochondrial biogenesis program for mtDNA transcription and replication.