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There are numerous studies concerning the effect of UVB light on skin cells but fewer on other skin components such as the interstitial fluid. This review highlights high-density lipoprotein (HDL) and low-density lipoprotein (LDL) as important targets of UVB in interstitial fluid. Tryptophan residues are the sole apolipoprotein residues absorbing solar UVB. The UVB-induced one-electron oxidation of Trp produces (•)Trp and (•)O2 (-) radicals which trigger lipid peroxidation. Immunoblots from buffered solutions or suction blister fluid reveal that propagation of photooxidative damage to other residues such as Tyr or disulfide bonds produces intra- and intermolecular bonds in apolipoproteins A-I, A-II, and B100. Partial repair of phenoxyl tyrosyl radicals (TyrO(•)) by α -tocopherol is observed with LDL and HDL on millisecond or second time scales, whereas limited repair of α -tocopherol by carotenoids occurs in only HDL. More effective repair of Tyr and α -tocopherol is observed with the flavonoid, quercetin, bound to serum albumin, but quercetin is less potent than new synthetic polyphenols in inhibiting LDL lipid peroxidation or restoring α -tocopherol. The systemic consequences of HDL and LDL oxidation and the activation and/or inhibition of signalling pathways by oxidized LDL and their ability to enhance transcription factor DNA binding activity are also reviewed.

In the minor fraction of HDL3 containing α-tocopherol (αTocOH), selective one-electron oxidation of Trp and Tyr residues of apolipoproteins A-I and A-II by •Br2- radical-anions produces the corresponding semioxidized species, TyrO• and •Trp. Repair of TyrO• by endogenous αTocOH generates the α-tocopheroxyl radical (αTocO•). Fast spectroscopic studies show that two populations representing 80% of αTocO• initially formed are repaired over several seconds with rate constants of 3.0 × 106 and 1.5 × 105 M-1 s-1 by quercetin bound to human serum albumin (HSA) at physiologically relevant concentration. Formation of HSA-bound quercetin radicals (•Qb) is observed. In the major fraction of HDL3 particles lacking αTocOH, TyrO• and •Trp are repaired by free and HSA-bound quercetin. In LDL particles which all contain αTocOH, αTocO• radicals are formed in the millisecond time scale by repair of TyrO• radicals produced in apolipoprotein B. Then, 75% of initial TocO are repaired over seconds by HSA-bound quercetin (rate constant: 2.0 × 106 M-1 s-1). HSA-bound quercetin can also repair •Trp radicals. In O2-saturated solutions, the fraction of αTocO• radicals (more than 50%) not repaired by superoxide radical-anions can be repaired by HSA-bound quercetin with formation of •Qb but to a much lesser extent in LDL than in HDL. [ABSTRACT FROM AUTHOR]

It has been recently shown that the inhibition of apolipoprotein A-I (apoAI) reverse cholesterol transport activity during oxidation of HDL by myeloperoxidase may involve myeloperoxidase electron transfer pathways other than those leading to tyrosine chlorination. To better understand how such mechanisms might be initiated, the role of semioxidized Tyr and Trp residues in loss of apoAI and apolipoprotein A-II (apoAII) integrity has been assessed using selective Trp and Tyr one-electron oxidation by ̇Br2- radical-anions in HDL3 as well as in unbound apoAI and apoAII. Behavior of these radicals in apolipoprotein B of LDL has also been assessed. Formation of semioxidized Tyr in HDL3 is followed by partial repair during several milliseconds via reaction with endogenous α-tocopherol to form the α-tocopheroxyl radical. Subsequently, 2% of α-tocopheroxyl radical is repaired by HDL3 carotenoids. With LDL, a faster repair of semioxidized Tyr by α-tocopherol is observed, but carotenoid repair of α-tocopheroxyl radical is not. Only a small fraction of HDL3 particles contains α-tocopherol and carotenoids, which explains limited repair of semioxidized Tyr by α-tocopherol. All LDL particles normally contain multiple a-tocopherol and carotenoid molecules, and the lack of repair of α-tocopheroxyl radical by carotenoids probably results from hindered mobility of carotenoids in the lipid core. Western blots of v-irradiated HDL3 comparable to those reported for apoAI myeloperoxidase oxidation show that the incomplete repair of semioxidized Tyr and Trp induces apoAI and apoAII permanent damage including formation of a heterodimer of one apoAI with a monomeric apoAII at about 36 kDa. [ABSTRACT FROM AUTHOR]

Selective oxidative damage to apolipoprotein B in LDL can be effected radiolytically by •Br[sub 2][sup -] radicals. Twenty-seven Trp residues constitute major primary sites of oxidation, but two-thirds of oxidized Trps (•Trp) that are formed are repaired by intramolecular electron transfer from Tyr residues with formation of phenoxyl radicals (TyrO•). Analysis of •Trp and TyrO• transient absorbance changes suggests that other apolipoprotein B residues, probably Cys, are oxidized. LDL-bound quercetin can efficiently repair this damage. Absorption studies show that a LDL particle has the capacity to bind ∼10 quercetin molecules through interaction with apolipoprotein B. The repair occurs by intramolecular electron transfer characterized by a rate constant of 2 × 10³ s[sup -1]. In contrast, rutin, a related flavonoid which does not bind to LDL, cannot repair oxidized apolipoprotein B. Urate is a hydrophilic plasma antioxidant which displays synergistic antioxidant properties with flavonoids. Urate radicals produced by •Br[sub 2][sup -] can also be repaired by LDLbound quercetin. This repair occurs with a reaction rate constant of 6.8 × 10[sup 7] M[sup -1] s[sup -1]. Comparison with previous studies conducted with human serum albumin-bound quercetin suggests that quercetin analogues tailored to be carded preferentially by lipoproteins might be more powerful plasma antioxidants than natural quercetin carded by serum albumin. [ABSTRACT FROM AUTHOR]