Helen Wiseman

Nitric oxide and oxygen radicals: a question of balance

The production of superoxide and nitric oxide individually has been associated with the development of several diseases but only recently has it been realised that interactions between them may also be important in disease pathology. The central hypothesis which is emerging is that the balance between nitric oxide and superoxide generation is a critical determinant in the aetiology of many human diseases including atherosclerosis, neurodegenerative disease, ischaemia‐reperfusion and cancer. These ideas are discussed in this short overview and placed in the context of the current and future status of therapies which could modulate the balance between nitric oxide and superoxide.

Interindividual Variation in Metabolism of Soy Isoflavones and Lignans: Influence of Habitual Diet on Equol Production by the Gut Microflora

The soy isoflavones, daidzein and genistein, and the lignans, matairesinol and secoisolariciresinol, are phytoestrogens metabolized extensively by the intestinal microflora. Considerable important evidence is already available that shows extensive interindividual variation in isoflavone metabolism, and we have investigated the extent of this variation in a crossover study of a soy-containing food low or high in isoflavones (each treatment period lasted for 17 days, and the 2 treatment periods were separated by a 25-day washout period) in 24 healthy subjects [19 women and 5 men, mean age 30 yr, range 19-40, mean body mass index 22.5 ±3.5 (SD) kg/m2]. There was a 16-fold variation in total isoflavonoid excretion in urine after the high-isoflavone treatment period. The variation in urinary equol excretion was greatest (664-fold), and subjects fell into two groups: poor equol excretors and good equol excretors (36%). A significant negative correlation was found between the proportion of energy from fat in the habitual diet and urinary equol excretion (r = -0.55; p = 0.012). Good equol excretors consumed less fat as percentage of energy than poor excretors (26 ± 2.3% compared with 35 ± 1.6%, p < 0.01) and more carbohydrate as percentage of energy than poor excretors (55 ± 2.9% compared with 47 ± 1.7%, p < 0.05). Interindividual variation in the urinary excretion of O-desmethylangolensin (O-DMA) was also apparent (76-fold after the high-isoflavone treatment period), but there was no relationship between equol excretion and O-DMA excretion. Enterolactone was the major lignan metabolite in urine and plasma but showed less interindividual variation than equol and O-DMA. It is suggested that the dietary fat intake decreases the capacity of gut microbial flora to synthesize equol.

Antioxidant action and potential antidiabetic properties of an isoflavonoid-containing soyabean phytochemical extract (SPE)

The potential role of oestrogenic agents, antioxidants and intestinal glucose‐uptake inhibitors in the treatment of diabetes is briefly reviewed. Reports in the literature suggest that oestrogen replacement therapy may favourably modulate glucose homeostasis. A soya phytochemical extract (SPE) containing the isoflavone phytoestrogens genistein and daidzein (mostly in their glycone forms as genistin and daidzin) was investigated as an antioxidant and modulator of intestinal glucose‐transport. In the present study, SPE was found to protect against glucose‐induced oxidation of human low density lipoproteins (LDL) in vitro. Equol (a gut bacterial metabolite of daidzein) was a more effective antioxidant than daidzein or genistein in this system and was of similar antioxidant potency to the dietary flavonols quercetin and kaempferol and to the endogenous antioxidant 17β‐oestradiol. SPE was found to be an inhibitor of glucose uptake into rabbit intestinal brush border membrane vesicles in vitro, though of weaker potency than the classical sodium dependent glucose transporter (SGLT) inhibitor, phlorizin. Thus SPE displays a range of properties which may be of benefit in diabetes, namely as an oestrogenic agent, an inhibitor of intestinal glucose‐uptake and a preventive agent for glucose‐induced lipid peroxidation. Copyright

Improved cognitive function in postmenopausal women after 12 weeks of consumption of a soya extract containing isoflavones.

We previously reported that a high soya diet improved memory and frontal lobe function in young volunteers, and since soya isoflavones are agonists at oestrogen receptors, they may improve these functions in postmenopausal women. Thirty-three postmenopausal women (50-65 years) not receiving conventional hormone replacement therapy (HRT) were randomly allocated in a double-blind parallel study to receive a soya supplement (60 mg total isoflavone equivalents/day) or placebo for 12 weeks. They received a battery of cognitive tests and completed analogue rating scales of mood and sleepiness, and a menopausal symptoms questionnaire before the start of treatment and then after 12 weeks of treatment. Those receiving the isoflavone supplement showed significantly greater improvements in recall of pictures and in a sustained attention task. The groups did not differ in their ability to learn rules, but the isoflavone supplement group showed significantly greater improvements in learning rule reversals. They also showed significantly greater improvement in a planning task. There was no effect of treatment on menopausal symptoms, self-ratings of mood, bodily symptoms or sleepiness. Thus, significant cognitive improvements in postmenopausal women can be gained from 12 weeks of consumption of a supplement containing soya isoflavones that are independent of any changes in menopausal symptoms, mood or sleepiness.

Vitamin D is a membrane antioxidant Ability to inhibit iron-dependent lipid peroxidation in liposomes compared to cholesterol, ergosterol and tamoxifen and relevance to anticancer action

Vitamin D is a membrane antioxidant: thus Vitamin D3 (cholecalciferol) and its active metabolite 1,25‐dihydroxycholecalciferol and also Vitamin D2 (ergocalciferol) and 7‐dehydrocholesterol (pro‐Vitamin D3) all inhibited iron‐dependent liposomal lipid peroxidation. Cholecalciferol, 1,25‐dihydroxycholecalciferol and ergocalciferol were all of similar effectiveness as inhibitors of lipid peroxidation but were less effective than 7‐dehydrocholesterol; this was a better inhibitor of lipid peroxidation than cholesterol, though not ergosterol. The structural basis for the antioxidant ability of these Vitamin D compounds is considered in terms of their molecular relationship to cholesterol and ergosterol. Furthermore, the antioxidant ability of Vitamin D is compared to that of the anticancer drug tamoxifen and its 4‐hydroxy metabolite (structural mimics of cholesterol) and discussed in relation to the anticancer action of this vitamin.

Dietary influences on membrane function: Importance in protection against oxidative damage and disease

Membrane function is vital to many cellular processes including the role of membrane enzymes and receptors in cell growth and signaling. A number of factors are thought to modulate membrane function including dietary components. Membrane function can either be influenced directly, e.g., by altering fluidity, or indirectly, e.g., by modulation of the free radical-mediated process of membrane lipid peroxidation, which can arise from oxidative stress and result in oxidative membrane damage. Vitamins E, C (ascorbic acid), and D, β-carotene, trace metals, flavonoids, isoflavonoid-type phytoestrogens, food additives, and fatty acids are all dietary factors that may influence membrane function in one or more of these ways. Modulation of the activity of membrane enzymes and receptors by dietary components may be important in disease states such as cancer. The possibility of dietary protection of membranes (including mitochondrial membranes) against the oxidative damage implicated either directly or indirectly via DNA damage in neurodegeneration, aging, and malignant progression to cancer (e.g., following chronic inflammation) will also be considered. Dietary modulation of the susceptibility of the phospholipid monolayer of low-density lipoprotein (LDL) particles to the oxidative damage implicated in atherosclerosis also appears to be a possibility and thus will be explored.

DNA damage and cancer: Measurement and mechanism

There is currently great interest in the possible role of reactive nitrogen species and reactive oxygen species in causing DNA damage that leads to cancer. It appears likely that certain reactive oxygen species can act as complete carcinogens. However, the development of human cancer will depend on other factors such as the extent of DNA damage, antioxidant levels and DNA repair systems. The true picture will only be seen if we have reliable and sensitive techniques for the measurement of DNA damage. In this article we outline various methods for measuring DNA damage base, with special emphasis on HPLC and GC-MS based systems.

The therapeutic potential of phytoestrogens.

Phytoestrogens, such as the soya isoflavones genistein and daidzein, are currently being extensively investigated through both molecular, preclinical and clinical studies to determine their potential health benefits. Phytoestrogens may protect against chronic diseases such as hormone-dependent cancer (e.g., breast and prostate cancer), cardiovascular disease and osteoporosis. Investigations of phytoestrogen metabolism and bioavailability are also of great relevance. Conversion by gut microflora of daizein to its isoflavan metabolite equol, which is a more potent oestrogen and anti-oxidant, occurs only in some individuals (about 35% of subjects tested are equol excretors). This has considerable implications for daidzein bioavailability and also for cancer risk. Oxidative damage has been implicated in the development of heart disease and cancer and soya phytoestrogens have been reported to decrease plasma F2-isoprostane concentrations (biomarker for in vivo lipid peroxidation) and increase low density lipoprotein oxidation resistance. This anti-oxidant action of phytoestrogens could potentially contribute to their therapeutic efficacy. The findings from the current ongoing studies are all likely to contribute to determining the potential use of phytoestrogens as therapeutic agents.

Tamoxifen and related compounds decrease membrane fluidity in liposomes : mechanism for the antioxidant action of tamoxifen and relevance to its anticancer and cardioprotective actions ?

Tamoxifen and related compounds decrease membrane fluidity in ox‐brain phospholipid liposomes: their order of effectiveness is, 4‐hydroxytamoxifen > 17β‐oestradiol > tamoxifen > cis‐tamoxifen > N‐desmethyltamoxifen > cholesterol. A good positive correlation was demonstrated between the decrease in membrane fluidity by these compounds and their antioxidant ability as inhibitors of liposomal and microsomal lipid peroxidation (correlation coefficient, r = 0.99, P < 0.001, in both cases). The ability of tamoxifen to decrease membrane fluidity is suggested to be the mechanism of its antioxidant action and is discussed in relation to its anticancer and cardioprotective actions.

Cognitive improvement after 6 weeks of soy supplements in postmenopausal women is limited to frontal lobe function.

Objective: To assess the effects of 6 weeks of treatment with soy supplements on mood, menopausal symptoms, and cognition in postmenopausal women not taking other forms of hormone therapy. Design: In a double-blind, placebo-matched parallel groups study, 50 postmenopausal women (aged 51-66 y) were randomly allocated to receive daily treatment with a soy supplement (Novasoy, 60 mg total isoflavone equivalents/day) or matching placebo capsules. They were tested at baseline before treatment began and after 6 weeks of treatment in tests of attention, memory, and frontal lobe function, and completed questionnaires to assess sleepiness, mood, and menopausal symptoms. Results: After 6 weeks of treatment, there was a significant (P < 0.02) reduction in somatic menopausal symptoms in the group taking soy supplements, but there were no other significant effects of soy on menopausal symptoms or mood. On the test of nonverbal short-term memory, the soy group showed greater improvement than the placebo group (P < 0.03), but there were no effects of soy on long-term memory, category generation, or sustained attention. However, the soy treatment produced significantly better performance on the two tests of frontal lobe function, those of mental flexibility (simple rule reversal, P < 0.05; complex rule reversal, P < 0.03) and of planning ability (P < 0.05). Conclusions: The results suggest that the main improvement after 6 weeks of soy supplementation was in frontal lobe function. Significant improvements in the same three measures of frontal lobe function were previously found after 12 weeks of soy supplements in postmenopausal women. The effects of soy on memory seem less robust.