Occurrence of cholesterol 7 alpha- and 7 beta-hydroperoxides in rat skin as aging markers

Cholesterol is more readily oxidized than phospholipid linoleates in cell membranes to produce cholesterol hydroperoxides

Cholesterol is an essential component of cell membranes and serves as an important precursor of steroidal hormones and bile acids, but elevated levels of cholesterol and its oxidation products have been accepted as a risk factor for maintenance of health. The free and ester forms of cholesterol and fatty acids are the two major biological lipids. The aim of this hypothesis paper is to address the long-standing dogma that cholesterol is less susceptible to free radical peroxidation than polyunsaturated fatty acids (PUFAs). It has been observed that cholesterol is peroxidized much slower than PUFAs in plasma but that, contrary to expectations from chemical reactivity toward peroxyl radicals, cholesterol appears to be more readily autoxidized than linoleates in cell membranes. The levels of oxidation products of cholesterol and linoleates observed in humans support this notion. It is speculated that this discrepancy is ascribed to the fact that cholesterol and phospholipids bearing PUFAs are localized apart in raft and non-raft domains of cell membranes respectively and that the antioxidant vitamin E distributed predominantly in the non-raft domains cannot suppress the oxidation of cholesterol lying in raft domains which are relatively deficient in antioxidant.

Xenobiotica-metabolizing enzymes in the skin of rat, mouse, pig, guinea pig, man, and in human skin models

Studies on the metabolic fate of medical drugs, skin care products, cosmetics and other chemicals intentionally or accidently applied to the human skin have become increasingly important in order to ascertain pharmacological effectiveness and to avoid toxicities. The use of freshly excised human skin for experimental investigations meets with ethical and practical limitations. Hence information on xenobiotic-metabolizing enzymes (XME) in the experimental systems available for pertinent studies compared with native human skin has become crucial. This review collects available information of which—taken with great caution because of the still very limited data—the most salient points are: in the skin of all animal species and skin-derived in vitro systems considered in this review cytochrome P450 (CYP)-dependent monooxygenase activities (largely responsible for initiating xenobiotica metabolism in the organ which provides most of the xenobiotica metabolism of the mammalian organism, the liver) are very low to undetectable. Quite likely other oxidative enzymes [e.g. flavin monooxygenase, COX (cooxidation by prostaglandin synthase)] will turn out to be much more important for the oxidative xenobiotic metabolism in the skin. Moreover, conjugating enzyme activities such as glutathione transferases and glucuronosyltransferases are much higher than the oxidative CYP activities. Since these conjugating enzymes are predominantly detoxifying, the skin appears to be predominantly protected against CYP-generated reactive metabolites. The following recommendations for the use of experimental animal species or human skin in vitro models may tentatively be derived from the information available to date: for dermal absorption and for skin irritation esterase activity is of special importance which in pig skin, some human cell lines and reconstructed skin models appears reasonably close to native human skin. With respect to genotoxicity and sensitization reactive-metabolite-reducing XME in primary human keratinocytes and several reconstructed human skin models appear reasonably close to human skin. For a more detailed delineation and discussion of the severe limitations see the Conclusions section in the end of this review.

Xenobiotic-metabolizing enzymes in the skin of rat, mouse, pig, guinea pig, man, and in human skin models

The exposure of the skin to medical drugs, skin care products, cosmetics, and other chemicals renders information on xenobiotic-metabolizing enzymes (XME) in the skin highly interesting. Since the use of freshly excised human skin for experimental investigations meets with ethical and practical limitations, information on XME in models comes in the focus including non-human mammalian species and in vitro skin models. This review attempts to summarize the information available in the open scientific literature on XME in the skin of human, rat, mouse, guinea pig, and pig as well as human primary skin cells, human cell lines, and reconstructed human skin models. The most salient outcome is that much more research on cutaneous XME is needed for solid metabolism-dependent efficacy and safety predictions, and the cutaneous metabolism comparisons have to be viewed with caution. Keeping this fully in mind at least with respect to some cutaneous XME, some models may tentatively be considered to approximate reasonable closeness to human skin. For dermal absorption and for skin irritation among many contributing XME, esterase activity is of special importance, which in pig skin, some human cell lines, and reconstructed skin models appears reasonably close to human skin. With respect to genotoxicity and sensitization, activating XME are not yet judgeable, but reactive metabolite-reducing XME in primary human keratinocytes and several reconstructed human skin models appear reasonably close to human skin. For a more detailed delineation and discussion of the severe limitations see the “Overview and Conclusions” section in the end of this review.

Metabolism of fatty acids and lipid hydroperoxides in human body monitoring with Fourier transform Infrared Spectroscopy

BACKGROUND

The metabolism of dietary fatty acids in human has been measured so far using human blood cells and stable-isotope labeled fatty acids, however, no direct data was available for human peripheral tissues and other major organs. To realize the role of dietary fatty acids in human health and diseases, it would be eager to develop convenient and suitable method to monitor fatty acid metabolism in human.

RESULTS

We have developed the measurement system in situ for human lip surface lipids using the Fourier transform infrared spectroscopy (FTIR) - attenuated total reflection (ATR) detection system with special adaptor to monitor metabolic changes of lipids in human body. As human lip surface lipids may not be much affected by skin sebum constituents and may be affected directly by the lipid constituents of diet, we could detect changes of FTIR-ATR spectra, especially at 3005 to approximately 3015 cm(-1), of lip surface polyunsaturated fatty acids in a duration time-dependent manner after intake of the docosahexaenoic acid (DHA)-containing triglyceride diet. The ingested DHA appeared on the lip surface and was detected by FTIR-ATR directly and non-invasively. It was found that the metabolic rates of DHA for male volunteer subjects with age 60s were much lower than those with age 20s. Lipid hydroperoxides were found in lip lipids which were extracted from the lip surface using a mixture of ethanol/ethylpropionate/iso-octane solvents, and were the highest in the content just before noon. The changes of lipid hydroperoxides were detected also in situ with FTIR-ATR at 968 cm(-1).

CONCLUSION

The measurements of lip surface lipids with FTIR-ATR technique may advance the investigation of human lipid metabolism in situ non-invasively.

Combination of TLC Blotting and Gas Chromatography–Mass Spectrometry for Analysis of Peroxidized Cholesterol

We have established a sensitive and convenient method for analysis of cholesterol hydroperoxides (Chol-OOHs) as trimethylsilyloxyl derivatives using diphenylpyrenylphosphine (DPPP)-thin-layer chromatography (TLC) blotting and gas chromatography-electron ionization-mass spectrometry/selected-ion monitoring (GC-EI-MS/SIM). Chol-OOH standards were prepared by photosensitized oxidation and azo radical-induced peroxidation of cholesterol. Trimethylsilyloxyl derivatives of cholesterol 5alpha-hydroperoxide (Chol 5alpha-OOH), cholesterol 7alpha-hydroperoxide (Chol 7alpha-OOH), and cholesterol 7beta-hydroperoxide (Chol 7beta-OOH) could be separated from one another in the SIM chromatogram using a fragment ion with elimination of trimethylsilanol from the molecular ion. This method was used to characterize peroxidized cholesterol from azo radical-exposed human low-density lipoprotein and UVA-irradiated human keratinocytes in the presence of hematoporphyrin. Finally, we succeeded in the quantification of each Chol-OOH isomer present in hairless mouse skin with and without UVA irradiation by use of beta-sitosterol hydroperoxide as internal standard. The accumulation of Chol 5alpha-OOH with Chol 7alpha/betaOOH in the skin indicates that singlet molecular oxygen ((1)O(2)) participated in the peroxidation of skin cholesterol, because Chol 5alpha-OOH is known to be a (1)O(2) specific cholesterol peroxidation product. We concluded that the combination of DPPP-TLC blotting and GC-EI-MS/SIM is useful for quantifying peroxidized cholesterol in biological samples and confirming the participation of (1)O(2) in oxidative stress.

Chemistry and reactions of reactive oxygen species in foods

Reactive oxygen species (ROS) are formed enzymatically, chemically, photochemically, and by irradiation of food. They are also formed by the decomposition and the inter-reactions of ROS. Hydroxy radical is the most reactive ROS, followed by singlet oxygen. Reactions of ROS with food components produce undesirable volatile compounds and carcinogens, destroy essential nutrients, and change the functionalities of proteins, lipids, and carbohydrates. Lipid oxidation by ROS produces low molecular volatile aldehydes, alcohols, and hydrocarbons. ROS causes crosslink or cleavage of proteins and produces low molecular carbonyls from carbohydrates. Vitamins are easily oxidized by ROS, especially singlet oxygen. The singlet oxygen reaction rate was the highest in ss-carotene, followed by tocopherol, riboflavin, vitamin D, and ascorbic acid.

Effect of sunlight exposure and aging on skin surface lipids and urate

Free fatty acids (FFA), squalene, squalene hydroperoxide, and uric acid in the methanol extracts from human skin surface were measured. Levels of FFA and squalene were significantly lower in the older (83.7 +/- 9.4 years) than in the younger (22.2 +/- 3.9 years) group. FFA are mostly saturated, and linoleic acid is an exclusive polyunsaturated fatty acid. The composition of linoleic acid decreased in the older group by 40%, suggesting age-dependent loss of oxidatively vulnerable polyunsaturated fatty acid. Even monounsaturated acids such as palmitoleic and oleic acids decreased significantly in the older group. This could be interesting because 2-nonenal is the oxidation product of palmitoleic acid and has been identified as the major aged body odor component. Sunlight exposure for 1.5 h did not change levels of FFA and squalene, or FFA composition. However, squalene hydroperoxide increased by 60-fold, as reported previously, suggesting that hydroperoxide is produced by singlet oxygen. Uric acid increased by two-fold, which may be the adaptive response against photo-oxidative stress because uric acid is a good scavenger of singlet oxygen and oxygen radicals.

Oxysterol stimulation of epidermal differentiation is mediated by liver X receptor-beta in murine epidermis

Liver X receptor-alpha and -beta are members of the nuclear hormone receptor superfamily that heterodimerize with retinoid X receptor and are activated by oxysterols. In recent studies we found that treatment of cultured human keratinocytes with oxysterolstimulated differentiation, as demonstrated by increased expression of involucrin and transglutaminase, and inhibited proliferation. The aims of this study were to determine: (i) whether oxysterols applied topically to the skin of mice induce differentiation in normal epidermis; (ii) whether this effect is mediated via liver X receptor-alpha and/or liver X receptor-beta; and (iii) whether oxysterols normalize epidermal morphology in an animal model of epidermal hyperplasia. Topical treatment of normal hairless mice with 22(R)-hydroxycholesterol or 24(S),25-epoxycholesterol resulted in a decrease in epidermal thickness and a decrease in keratinocyte proliferation assayed by proliferating cell nuclear antigen staining. Moreover, oxysterol treatment increased the levels of involucrin, loricrin, and profilaggrin protein and mRNA in the epidermis, indicating that oxysterols stimulate epidermal differentiation. Additionally, topical oxysterol pretreatment improved permeability barrier homeostasis. Whereas liver X receptor-alpha-/- mice revealed no alterations in epidermal differentiation, the epidermis was thinner in liver X receptor-beta-/- mice than in wild-type mice, with a reduced number of proliferating cell nuclear antigen positive cells and a modest reduction in the expression of differentiation markers. Topical oxysterol treatment induced differentiation in liver X receptor-alpha-/- mice whereas in liver X receptor-beta-/- mice there was no increase in the expression of differentiation markers. Whereas both liver X receptor-alpha and liver X receptor-beta are expressed in cultured human keratinocytes and in fetal rat skin, only liver X receptor-beta was observed on northern blotting in adult mouse epidermis. Finally, treatment of hyperproliferative epidermis with oxysterols restored epidermal homeostasis. These studies demonstrate that epidermal differentiation is regulated by liver X receptor-beta and that oxysterols, acting via liver X receptor-beta, can induce differentiation and inhibit proliferation in vivo. The ability of oxysterols to reverse epidermal hyperplasia suggests that these agents could be beneficial for the treatment of skin disorders associated with hyperproliferation and/or altered differentiation.

Role of active oxygen species and antioxidants in photoaging

The mechanism for the formation of active oxygen species and their reactions with antioxidants is described. The importance of the free radical chain oxidation and the singlet oxygen-dependent oxidation is suggested by a decrease in skin levels of alpha-tocopherol, ubiquinol-10, and ascorbic acid with a concomitant formation of lipid hydroperoxides during UV irradiation of murine skin, and the formation of squalene hydroperoxides in human skin upon UV exposure, respectively.

Age-related changes in oxidative damage to lipids and DNA in rat skin

Skin is a tissue exposed most frequently to oxidative stress from the environment in daily life. Age-related changes of oxidative damage and antioxidant enzyme activity in the skin were examined in male Fischer 344 rats aged 6 to 30 months. The contents of phosphatidylcholine hydroperoxide (PCOOH) and thiobarbituric acid-reacting substances (TBARS) increased linearly with age. The content of cholesterol hydroperoxide increased until 24 months of age and then decreased. The content of 8-oxo-2'-deoxyguanosine (8-oxodG) increased gradually with age, and was significantly higher at 30 months of age than at 6 months of age. Superoxide dismutase activity tended to decrease with age. The activities of catalase and glutathione peroxidase showed no changes with age. We examined the effect of dietary restriction on the accumulation of oxidative damage in rat skin. The increase in PCOOH content in the skin of dietary-restricted rats was suppressed until 30 months of age. The TBARS and cholesterol hydroperoxide contents in the skin of dietary-restricted rats were significantly lower than in the skin of ad libitum-fed rats, while the 8-oxodG content was somewhat lower in the dietary-restricted rats than the ad libitum-fed rats. These results indicate that oxidative damage to the lipids and DNA in rat skin increases with age and that dietary restriction delays the accumulation of oxidative damage in skin.

Cholesterol 7-hydroperoxides in rat skin as a marker for lipid peroxidation

Concentrations of cholesterol 7alpha- and 7beta-hydroperoxides (Ch 7-OOHs) in the skin of rats were determined by HPLC with a chemiluminescence detector. We demonstrated that (a) the concentrations of Ch 7-OOHs in rat skin were highly correlated with rat age (r = 0.929; N = 51, 1 to 55 weeks old), (b) the concentrations of Ch 7-OOHs in the skin of rats in an ambient light room were not significantly different from those found in rats kept in a dark room for 12 weeks, and (c) lipid peroxidation in vitro induced by ADP-Fe2+ caused an increase in the concentrations of Ch 7-OOHs in homogenates of rat skin. These results indicated that levels of Ch 7-OOHs in skin might be a good marker for aging of rats and might be independent of housing illumination, thus a good marker for endogenous lipid peroxidation. Furthermore, we observed that ultraviolet light B (UVB) irradiation markedly enhanced the concentrations of Ch 7-OOHs in the skin of rats in vivo depending on the duration of the irradiation, and the increases in Ch 7-OOHs were inhibited by radical scavengers, i.e. tocopherols. Therefore, it was suggested that the levels of Ch 7-OOHs in the skin could also be a good marker for UVB-dependent lipid peroxidation.

Photogeneration of 3beta-hydroxy-5alpha-cholest-6-ene-5-hydroperoxide in rat skin: evidence for occurrence of singlet oxygen in vivo

We identified singlet oxygen adduct of cholesterol, 3beta-hydroxy-5alpha-cholest-6-ene-5-hydroperoxide (5alpha-OOH), in skin of rats pretreated with oral doses of pheophorbide a and subsequent visible irradiation, that have been known to induce photosensitive diseases in animals and humans. In a living animal body, this is the first demonstration of presence of 5alpha-OOH, that is known to be formed exclusively by reaction in vitro between singlet oxygen and cholesterol. By the quantitative determination with high performance liquid chromatography equipped with a chemiluminescence detector, we observed time-dependent increase in concentrations of 5alpha-OOH in skin of rats pretreated with oral doses of pheophorbide a and subsequent visible irradiation, suggesting the occurrence of a labile activated oxygen species, singlet oxygen, in this system.

Marked expression of glutathione S-transferase A4-4 detoxifying 4-hydroxy-2(E)-nonenal in the skin of rats irradiated by ultraviolet B-band light (UVB)

Enzyme, Western blot, and immunohistochemical analyses indicated that rat skin cytosol contained no detectable level of the homodimeric, alpha-class glutathione S-transferase (rGST) A4-4 which catalyzes the GSH conjugation of the toxic product, 4-hydroxy-2(E)-nonenal (HNE), nonenzymatically formed from n-6 polyunsaturated fatty acid residues of lipids by lipid peroxidation. Rats irradiated by single doses (4000-24,000 mJ/cm(2)) of ultraviolet B-band light (UVB, 200 mJ/cm(2)/min) markedly expressed rGSTA4-4 in the skin at a level one-fifth that of the liver in apparent specific activity toward HNE at a single dose of 24,000 mJ/cm(2). Skin rGSTA4-4 was isolated, purified to homogeneity, and identified with hepatic rGSTA4-4 by reverse-phase partition HPLC and by amino acid sequence analysis of its CNBr fission peptides. Immunohistochemistry with polyclonal antibody raised against rGSTA4-4 demonstrated the selective expression of rGSTA4-4 in epidermis and sebaceous glands localized in dermis after UVB irradiation.

Quantitative determination of cholesterol 5alpha-, 7alpha-, and 7beta-hydroperoxides in rat skin

An assay method for determination of cholesterol 5alpha-, 7alpha-, and 7beta-hydroperoxides (ChOOHs) in rat skin using high-performance liquid chromatography (HPLC) with a chemiluminescence detector has been developed. In the assay method, free form and free plus ester forms of ChOOHs could be separately determined by HPLC in combination with the treatment of a tissue extract by cholesterol esterase. Lower limits of quantitation for cholesterol 5alpha-, 7alpha-, and 7beta-hydroperoxides were 0.2, 0.1, and 0.5 nmol/g skin, respectively. This assay method showed that (i) good absolute recoveries of ChOOHs from rat skin (80-90% of radiolabeled ChOOHs added to rat skin); (ii) negligible autoxidation of cholesterol caused by the assay procedure (<9.4x10(-5)% of radiolabeled cholesterol added to rat skin); and (iii) good correlation between ChOOHs added to rat skin and ChOOHs determined, indicating this assay method is applicable to quantify ChOOHs in rat skin. By using this assay method, we observed that (i) cholesterol 5alpha-hydroperoxide was detected in skin of rats pretreated with oral doses of pheophorbide a and subsequent visible irradiation; (ii) concentrations of cholesterol 7-hydroperoxides in skin of rats in an ambient light room were not significantly different from those in a dark room for 12 weeks; and (iii) ultraviolet light B irradiation markedly enhanced the concentrations of cholesterol 7-hydroperoxides in the skin of rats.

Cholesterol-derived hydroperoxides in alcoholic liver disease

Human liver samples from 33 patients were collected at autopsy (controls, n = 9; fatty liver, n = 12; liver cirrhosis, n = 12), and samples homogenized. Lipids extracted with chloroform and methanol were injected into the octyl column of a high-performance liquid chromatograph with post-column chemiluminescence. Liquid chromatography-mass spectrometry was developed to identify 7-hydroperoxycholest-5-en-3 beta-ol (7-OOH). We found that two cholesterol-derived hydroperoxides, 7 alpha-hydroperoxycholest-5-en-3 beta-ol (7 alpha-OOH) and 7 beta-hydroperoxycholest-5-en-3 beta-ol (7 beta-OOH), are present in significantly elevated amounts (12.4 and 25.0 nmol/g tissue, respectively) in lipid extracts from alcoholic fatty liver, but not in extracts from alcoholic cirrhotic liver. 7 alpha-OOH and 7 beta-OOH are early intermediates produced during free radical-mediated cholesterol oxidation and can serve as molecular indicators of chain peroxidative damage in cell membranes. This is the first demonstration of 7 alpha-OOH and 7 beta-OOH accumulations in human liver, and it is presumed to reflect greater oxidative stress pathology in alcoholic fatty liver.

Increases in cholesterol 7-hydroperoxides in lipids of human skin by sunlight exposure

Free and ester forms of cholesterol 7alpha- and 7beta-hydroperoxides (Ch 7-OOHs) in skin lipids of humans were separated and determined by high performance liquid chromatography with a chemiluminescence detector. We first demonstrated the presence of Ch 7-OOHs in lipids of human skin. The levels of Ch 7-OOHs found in skin lipids of healthy Japanese volunteers (n = 5) ranged from 2.78 to 25.2 pmol/cm2 skin, indicating large inter-individual differences. However, the intra-individual differences of Ch 7-OOHs levels in skin lipids between right and left arms were less than 25% (-16.4% to 24.0%). Inter-day differences of Ch 7-OOHs in 5 subjects at 1 week interval were also small (-36.7% to 47.7%). Additionally, we investigated effects of sunlight exposure on the levels of Ch 7-OOHs in skin lipids of healthy Japanese volunteers (n = 24). The levels of Ch 7-OOHs in skin lipids significantly increased from 10.0+/-6.7 to 38.9+/-38.0 pmol/cm2 skin by sunlight exposure (10-40 mJ/cm2/min) for 3 h. Therefore, natural sunlight exposure causes lipid peroxidation in skin lipids of humans. These results suggest that the level of Ch 7-OOHs is a good marker for lipid peroxidation in human skin.

Cholesterol hydroperoxides in erythrocyte membranes of alcoholic patients

Evidence for the presence of 5alpha-hydroperoxycholest-6-en-3beta-ol (cholesterol 5alpha-hydroperoxide, Ch 5alpha-OOH) and 7alpha- and 7beta-hydroperoxycholest-5-en-3beta-ols (cholesterol 7-hydroperoxides: Ch 7alpha-OOH and Ch 7beta-OOH, respectively) in human erythrocyte membrane was found. Blood samples were collected from alcoholic patients and healthy volunteers (controls), and their cholesterol hydroperoxides were analyzed by high-performance liquid chromatography postcolumn chemiluminescence and roughly identified by liquid chromatography-mass spectrometry. Ch 7alpha-OOH and Ch 7beta-OOH were present in each sample, being significantly higher in alcoholic samples than in control samples. Ch 5alpha-OOH was present in some alcoholic samples, but not in the control ones. The accumulation of cholesterol hydroperoxides suggests enhanced lipid peroxidation by active oxygen species and/or a reduced elimination system for lipid peroxide in alcoholic patients.

Chemiluminescent determination of cholesterol hydroperoxides in human erythrocyte membrane

A method for separating, detecting, and quantifying cholesterol hydroperoxide (Ch-OOH) based on extraction, purification by solid-phase extraction cartridge, high-performance liquid chromatography with chemiluminescent detection (HPLC-CL), and liquid chromatography-mass spectrometry has been developed for human erythrocyte membrane. We prepared standard compounds of the cholesterol 5alpha-, 7alpha-, and 7beta-hydroperoxides (Ch 5alpha-OOH, Ch 7alpha-OOH, and Ch 7beta-OOH). An octyl silica column with methanol/water/acetonitrile 89:9:2 (by vol) as eluent was used to determine Ch-OOH. HPLC-CL that incorporated cytochrome c and luminol as the post-column luminescent reagent was used. We also investigated the optimal assay conditions and how to prevent formation of artifact Ch-OOH. Analysis of erythrocyte membranes from seven healthy volunteers identified Ch 7alpha-OOH and Ch 7beta-OOH, but not Ch 5alpha-OOH, as commonly occurring components. The respective mean concentrations of Ch 7alpha-OOH and Ch 7beta-OOH were 2.5+/-1.6 and 5.4+/-3.5 pmol/mL blood.

Subunit Ya-specific glutathione peroxidase activity toward cholesterol 7-hydroperoxides of glutathione S-transferases in cytosols from rat liver and skin

Dermal 7alpha- and 7beta-hydroperoxycholest-5-en-3beta-ols (cholesterol 7alpha- and 7beta-hydroperoxides), regarded as good aging markers in the rat (Ozawa, N., Yamazaki, S., Chiba, K., Aoyama, H., Tomisawa, H., Tateishi, M., and Watabe, T. (1991) Biochem. Biophys. Res. Commun. 178, 242-247), were reduced in the presence of glutathione (GSH) with concomitant formation of GSSG by cytosol from rat liver in which no detectable level of the hydroperoxides had been demonstrated to occur. The GSH peroxidase (GSH Px) activity toward the toxic steroid hydroperoxides was exerted to almost the same extent by both Alpha-class GSH S-transferases (GSTs), Ya-Ya and Ya-Yc, and by selenium-containing GSH Px (Se-GSH Px) in rat liver cytosol. None of three Mu-class GSTs, Yb1-Yb1, Yb1-Yb2, and Yb2-Yb2, and a Theta-class GST, Yrs-Yrs, from rat liver and a Pi-class GST, Yp-Yp, from rat kidney showed any appreciable GSH Px activity toward the hydroperoxides. The subunit Ya-bearing GSTs and Se-GSH Px purified from rat liver cytosol showed marked differences in apparent specific activity toward the cholesterol hydroperoxides (GSTs Ya-Ya > Ya-Yc >> Se-GSH Px). However, a kinetic study indicated that Se-GSH Px had a higher affinity for steroid hydroperoxides than did the GSTs, so that Se-GSH Px could catalyze the reduction of lower concentrations of cholesterol 7-hydroperoxides with approximately equal Vmax/Km values to those by the GSTs. Rat skin had no GST bearing the subunit Ya but contained only a very low concentration of Se-GSH Px, possibly resulting in the accumulation of cholesterol 7-hydroperoxides in the skin but not in the liver. From rat skin cytosol, GSTs Yc-Yc, Yb1-Yb1, Yb1-Yb2, Yb2-Yb2, and Yp-Yp were isolated, purified to homogeneity, and identified with the corresponding GSTs from liver and kidney. The GSTs accounted for 0.23% of total skin cytosolic protein, and the most abundant isoform of skin GSTs was Yb2-Yb2, followed by Yc-Yc, Yp-Yp, Yb1-Yb1, and Yb1-Yb2 in decreasing order.

Review of progress in sterol oxidations: 1987-1995

Material dealing with the chemistry, biochemistry, and biological activities of oxysterols is reviewed for the period 1987-1995. Particular attention is paid to the presence of oxysterols in tissues and foods and to their physiological relevance.

7 beta-hydroperoxycholest-5-en-3 beta-ol, a component of human atherosclerotic lesions, is the primary cytotoxin of oxidized human low density lipoprotein

Modification of low density lipoprotein (LDL) by free radical oxidation renders this molecular complex cytotoxic. Oxidized lipoproteins exist in vivo in atherosclerotic lesions and in the plasma of diabetic animals, suggesting that lipoprotein-induced tissue damage may occur in certain diseases. We undertook purification and identification of the major cytotoxin in oxidized LDL. The lipid extract from oxidized LDL was subjected to multiple HPLC separations, and the fractions were assayed for cytotoxicity. Mass spectrometry and nuclear magnetic resonance identified the purified toxin as 7 beta-hydroperoxycholest-5-en-3 beta-ol (7 beta-OOH-Chol). This molecule accounted for approximately 90% of the cytotoxicity of the lipids of oxidized LDL. We also found 7 beta-OOH-Chol in human atherosclerotic lesions from endarterectomy specimens obtained immediately after excision. These results are consistent with the hypothesis that the oxidized LDL present in lesions has the capacity to induce cell and tissue injury, leading to progression of the disease and the generation of the necrotic core of the lesion.

Existence of 7 alpha- and 7 beta-hydroperoxycholest-5-en-3 beta-ols in lipoproteins from diabetic patients and normal subjects

We report evidence for the presence of 7 alpha- and 7 beta-hydroperoxycholest-5-en-3 beta-ols (cholesterol 7-hydroperoxides, Ch 7 alpha-OOH and Ch 7 beta-OOH, respectively) in human plasma lipoproteins in vivo, which had been reported to be markers of aging in rat skin. A comparative study was carried out focusing on the detection of Ch 7-OOHs in plasma lipoproteins from diabetic patients whose plasma has been suggested to be under high oxidative stress. Blood samples were collected from healthy volunteers (control) and diabetics with and without hypercholesterolemia. Ch 7-OOHs were isolated from low and high density lipoproteins (LDL and HDL, respectively) in the plasma of these subjects, identified, and determined by high-performance liquid chromatography with a chemiluminescence detector. The percent detection of Ch 7-OOHs in LDL from diabetics without hypercholesterolemia was similar to that in the control group. However, it was significantly higher in diabetics with hypercholesterolemia than in those without hypercholesterolemia. The percent detection of Ch 7-OOHs in HDL from diabetics without hypercholesterolemia was higher than both that in LDL from the same group and that in HDL from the control group.

Identification of core aldehydes among in vitro peroxidation products of cholesteryl esters

Synthetic cholesteryl 5-oxovalerate and 9-oxononanoate were used as reference standards for the isolation and identification of cholesteryl ester core aldehydes from tert-butyl hydroperoxide/Fe++ oxidation of synthetic and natural cholesteryl esters. The core aldehydes were recovered from the peroxidation products by thin-layer chromatography as the free aldehydes or the 2,4-dinitrophenylhydrazones and were identified, respectively, by gas-liquid chromatography (GLC) and by GLC combined with mass spectrometry (GC/MS) or by reverse-phase high-performance liquid chromatography (HPLC) and by HPLC with MS (LC/MS). The core aldehydes produced by peroxidation of cholesteryl linoleate were identified as mainly 9-oxononanoates of cholesterol and oxycholesterols, with smaller amounts of the 8-oxooctenoates, 10-oxodecenoates, 11-oxoundecenoates and 12-oxododecenoates. Peroxidation of cholesteryl arachidonate yielded 5-oxovalerates of cholesterol and the oxycholesterols as the main products with smaller amounts of the 4-oxobutyrates, 6-oxohexenoates, 7-oxoheptenoates, 8-oxooctenoates, 9-oxononenoates, 9-oxononadienoates and 10-oxodecadienotes. The oxycholesterols resulting from the peroxidation of the steroid ring were identified as mainly 7-keto-, 7 alpha-hydroxy- and 7 beta-hydroxy-cholesterols and 5 alpha,6 alpha- and 5 beta,6 beta-epoxy-cholestanols. Cholesteryl palmitate and oleate did not yield core aldehydes in the present peroxidation system. In these esters, the sterol and linoleic acid moieties appeared to be oxygenated at about the same rate, while the arachidonic acid moiety reacted more rapidly than did the sterol moiety.