Antioxidant and pro-oxidant activities of carotenoids and their oxidation products

Carotenoids are lipid soluble plant pigments that make part of human daily diet. These compounds are able to react with diverse reactive species and originate a myriad of oxidation products that have similar or even more reactivity than their parent compounds. These products may be short-chain carbonyl compounds and, the commonly designated, apo-carotenoids. The effects fo these products in the human body are not yet fully understood, but it is believed that they are intimately related with their oxidative potential. As so, carotenoids and their oxidation products have undeniable potential as bioactive compounds. However, the multifactorial elements that influence their activity/reactivity make really hard, or probably impossible, the task to classify them as anti- or pro-oxidant compounds. This manuscript provides a thorough review on the reactions of carotenoids with the most physiologically relevant reactive oxygen and nitrogen species, and scrutinizes the resulting oxidation products that have been hitherto identified. Finally, the dual anti- vs. pro-oxidant activities of carotenoids are discussed, mostly based on studies which use physiologic relevant reactive species in oxidative stress. Definitely, there is an open window to the development of deeper studies in this field with a greater number of carotenoids, especially with the ones hitherto not studied.