Subcellular localization of riboflavin (RF, Vitamin B2) in human normal liver L02 cells has been studied by means of fluorescence microscopy. The observed distribution of riboflavin inside the nucleus indicates for the first time that the photo-and radio- sensitized target is predominant inside the cells, While at higher concentrations, riboflavin can also be found in the cellular membrane, which demonstrates that cellular membrane is also a target site for riboflavin photo-and radio-sensitization. Here in this work, linoleic acid was used as a lipid model system in order to study the damaging potential of photoexcited flavin. By using time-resolved 308 nm laser flash photolysis with transient absorbance detection, the formation process of radical anion of riboflavin (RF^. -/RFH^.) or flavin adenine dinucleotide (FAD^. -/FADH^.) was found to be synchronous with the decay of triplet sates of riboflavin (3^RF^*) or flavin adenine dinucleotide (3^FAD^*). Moreover, the decay of 3^RF^* or 3^FAD^* was pseudo-first-order in the concentration of linoleic acid. These observations provide for the first time direct evidence of electron transfer from linoleic acid to the triplet states of riboflavin and flavin adenine dinucleotide.

Key words: LINOLEIC ACID, HEPATOLAVIN (=VITAMIN B2), EXCITED STATE, CHARGE TRANSFER, NUCLEOTIDES