PHOTOCHEMICAL-REACTIONS AND PHOTOTOXICITY OF STEROLS - NOVEL SELF-PERPETUATING MECHANISM FOR LIPID PHOTOOXIDATION

Sterols are important lipid components that may contribute to phototox icity. We have found that phototoxic response in earthworms is related to sterols extractable with lipophilic solvents, The photochemically active compounds in worm lipids are 5,7,9(11),22-ergostatetraen-3 beta -ol (9-DHE) and 5,7,9(11)-cholestatrien-3 beta-ol (9-DDHC), respective ly. Human skin lipids art: known to contain 9-DHE, We have also found 9-DDHC in human skin, which is reported here for the first rime, In th e presence of an excess of the corresponding 5,7-dienes (ergosterol or 7-dehydrocholesterol), these photoactive sterols constitute a self-re generating source of singlet molecular oxygen (O-1(2) during irradiati on in vivo or in vitro with UVA (315-400 nm), The quantum yield for ph otosensitization of O-1(2) by 9-DHE was Estimated to be 0,09. The O-1( 2), is scavenged by the dienes and tile rate constant for O-1(2) quenc hing by ergosterol was found to be 1.2 X 10(7) M-1 s(-1) in methyl t-b utyl ether (MTBE). This scavenging ultimately leads to the production of 5,8-endo-peroxide and hydrogen peroxide, Photochemically induced su peroxide radical was also produced on irradiation of sterol 5,7,9-trie nes and trapped with the spin trap 5,5-dimethyl-1-pyrroline N-oxide (D MPO). The production of singlet oxygen, peroxides and radicals by the sterols map be significant in the cell damaging and tumor promoting ac tion of WA light on skin.