Reactive oxygen species, riboflavin

Reoxidation of reduced flavin coenzymes is the major source of oxygen radicals in the body, and riboflavin is also capable of generating reactive oxygen species nonenzymically. As protection against this, there is very strict control over the body content of riboflavin. Absorption is limited, and any in excess of requirements is rapidly excreted. 

The first step in XJVR-induced skin cancer is UVR-initiated DNA mutation, which causes the transformation of the normal cells to malignant cells. For UVR to initiate a biological reaction, it has to be absorbed by endogenous molecules (chromophores). UVB is absorbed directly by the DNA, and therefore can directly induce DNA mutation (224), in the form of thymine dimer formation (289). Some protein components may also ad as chromophores for UVB (224). UVA is absorbed by the reduced forms of the co-enzymes nicotinamide adenine dinucleotide (NADH) and nicotinamide adenine dinucleotide phosphate (NADPH), tryptophan, riboflavin, and melanin (224,290). UVA-induced DNA damage is believed to be mediated by oxygen reactive species that are released after the absorption of UVA by those endogenous chromophores and results in photooxidation of selected bases... 

Riboflavin or vitamin makes a prosthetic group of flavin enzymes where it can be reversibly reduced by hydrogen atoms (formula [38]). When exposed to light, riboflavin absorbs energy and reacts, via a triplet exited state, with other molecules such as protonated substrates or molecular oxygen, generating reactive species. In the type I photodynamic reaction, energy is transferred from a triplet sensi-tiser to O2 with the formation of O2. 

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