Reactive Oxygen Species (ROS)
ROS and Lung Health
Each breath brings new sources of gases, including the ROS, ozone, in contact with the 'air side' of a lung cell. The blood carries ROS and other oxidants to lung's cell's 'blood side', and the cell's own metabolic reactions produce additional ROS.
In certain situations, ROS cause oxidative stress on lung cells. Under substantial oxidative stress, lung cells are not repaired effectively and lung function is reduced, leading to lung disorders like Acute Respiratory Distress Syndrome (ARDS) or chronic obstructive pulmonary disease (COPD) (Todokoro, 2004). Understanding what happens as a result of the ROS contacting the cell and what factors change a cell's threshhold of tolerance will help us determine how we might combat, reduce, or prevent these lung diseases.
What are ROS?
ROS stands for reactive oxygen species. It is a general term for free radicals and peroxides that have unpaired electrons and that will therefore oxidize various compounds. A common oxidation reaction is a rusting nail. The oxidant has 'grabbed' electrons from the nail to fill up its unpaired set (see the single electron in the Lewis structure of superoxide). Over time, as this reaction happens more, the nail literally begins to fall apart. In a cell this sort of 'electron grabbing' is an important way that components of the cell pass energy. However, ROS aren't picky, they grab any electron that isn't held on to tightly enough. That means that if there are more than enough ROS to go around the oxidant molecules will end up grabbing electrons and pull apart molecules that the cell needs (cell membrane part, receptors, enzymes, etc).
How are ROS produced?
- ROS are produced naturally as a product of cellular metabolism. During oxidative phosphorylation (the process in which cells produce ATP, the universal energy molecule), electrons are passed to electron carriers called cytochromes. The final electron acceptor is an oxygen molecule. Typically this reaction will combine with hydrogen to form water, however in small amounts, peroxide is produced, one of the common reactive oxygen species (Poyton, 2009).
- ROS are also produced by environmental oxidants such as ozone when they interact with molecules within the body or oxidize cells directly (Todokoro et al., 2004)
- Reactive oxygen species can cause damage to all components of cells (Cederbaum 2009)
- They oxidize fatty acids and amino acids, the building blocks of fats and proteins (Cederbaum 2009)
- They inactive enzymes by oxidizing cofactors effectively 'breaking' these cellular machines (Cederbaum 2009)
- They damage DNA (Cederbaum 2009)
- Under mild to moderate oxidative stress, cells will undergo apoptosis (programmed cell death) (Todokoro et al., 2004)
- Under severe oxidative stress, cells will undergo necrosis (premature cell death) (Todokoro et al., 2004)
Defense from ROS
Cells in our bodies naturally produce antioxidants and enzymes to defend against reactive oxygen species. One of the most common antioxidants is glutathione. These antioxidants provide a buffer between the cell and ROS by providing electrons for the oxidants to 'grab' and thus protecting the cells from the harm of oxidative degradation. In addition, the enzymes also latch on to ROS and metabolize them into harmless forms (like seen in the detoxiciation of H2O2 the image above) (Brown, 2001). Other enzymes are responsibe for regulating how many of the antioxidant molecules are made or for recycling the oxidized glutathione so that it can be used again.