Read this article to learn about the role of antioxidants in preventing free radical damage.
It is a compound that help prevent free-radical damage is known as antioxidant or “free-radical scavenger“. In foods, antioxidants (see Table 10.1) have been defined as a substance that in small quantities is able to prevent or greatly retard the oxidation of easily oxidizable materials such as fats.
However, in biological systems the definition for antioxidants has been extended to any substance that when present at low concentrations compared to those of an oxidizable substrate significantly delays or prevents oxidation of that substrate like lipids, proteins, DNA, and carbohydrates.
Currently however, biological antioxidants have further assumed a broad definition to include repair systems such as iron transport proteins (e.g. transferrin, albumin, ferritin and caeruloplasmin), antioxidant enzymes, and factors affecting vascular homeostasis, signal transduction and gene expression.
Antioxidants may exert their effects by different mechanisms, such as suppressing the formation of active species by reducing hydro-peroxides (ROO•) and H2O2 and by sequestering metal ions, scavenging active free radicals, repairing and/or clearing damage. Antioxidant works by retarding the oxidation.
In biology, oxidation is often started by free radicals. The role of an antioxidant is to intercept a free radical before it can react with the substrate.
For example, phenol (AOH), the reaction of interest with ROO• is:
AOH + ROO• —AO• + ROOH.
This H-atom transfer reaction effectively stops chain reaction. Therefore, antioxidants of biological/therapeutic importance should have the property that they will react/trap the free radical before it reacts with the susceptible substrate and initiate chain reaction.
Based on several theoretical models and complex calculations, Wright concluded that bond dissociation enthalpy (BDE) gives excellent correlation for this requirement with many known families of antioxidants that have been extensively studied in biological systems, like vitamins E and C, resveratrol, gallocatechins, ubiquinol, etc. He suggested that lower the BDE, the more reactive the antioxidant.
However, it should not be too low to reduce the molecular oxygen, forming HO2 the process of autoxidation. Major understanding of beneficial therapeutic activities of antioxidants has arisen with studies on vitamins E and C and ubiquinol Q10 that serve as excellent reference material.
Progress in sciences is providing crucial insights in the understanding mechanisms of disease pathogenesis and is opening up rich field of potential targets for pharmaceutical intervention. The broader and clearer understanding of the molecular basis of disease processes therefore is paving a way to develop more effective and targeted treatment.
Over the past three decades, free-radical theory has greatly stimulated interest in the role of dietary antioxidants in preventing many human diseases, including cancer, atherosclerosis, stroke, rheumatoid arthritis, neuro-degeneration arid diabetes etc.
These wide varieties of chronic inflammatory diseases form the basis for development of antioxidant based therapeutics. Regardless of their initiating pathological events, these diseases share a series of steps that lead to a common mechanistic pathway of oxidative stress through regulatory oxidative signals (Figure 10.1).
Chemistry of Antioxidant Phenolic Phytochemicals:
Phenolic phytochemicals are one of the most abundant groups of natural metabolites and form an important part of both human and animal diets.
These phenolic metabolites function to protect the plants against biological and environmental stresses and therefore are synthesized in response to pathogenic attack such as fungal or bacterial infection or high-energy radiation exposure such as prolonged UV exposure.
Common fruits such as apples, cranberries, grapes, raspberries, and strawberries and their beverages like red wine, apple and orange juices are rich sources of phenolic phytochemicals.
In addition to fruits, vegetables such as cabbage, tomato, garlic, onion etc.; food grains such as sorghum, millet, barley, peas, and other legumes are also described as important sources of phenolic phytochemicals.
There are numerous different types of phenolic phytochemicals which are classified according to their ring structure and the number of carbon atoms substituting the ring and linking them together (Table 10.2).
Metabolic processing of phenolic phytochemicals in plants for their final biological function has led to chemical variations in basic phenolic structure. They vary structurally from being simple molecules (e.g. phenolic acids with a single ring structure), biphenyls and flavonoids having 2-3 phenolic rings.
Another abundant group of phenolic phytochemicals in fruits and vegetables often referred to as polyphenols contain 12-16 phenolic groups. These polyphenols are classified as condensed pro-anthocyanidins, tannins which include galloyl and hexahydroxydiphenoyl (or ellagoyl) esters and their derivatives, or phlorotannins. Some common phenolic structures are cited in Fig. 10.2.
