Calorie restriction attenuates age-related alterations in the plasma membrane antioxidant system in rat liver

Aging is associated with increased production of reactive oxygen species and oxidation-induced damage to intracellular structures and membranes. Caloric restriction (CR) is the only non-genetic method proven to extend lifespan in mammals. Although the mechanisms of CR remain to be clearly elucidated, reductions in oxidative stress have been shown to increase lifespan in several model systems. Oxidative stress can be attenuated by CR. Mitochondria and plasma membrane (PM) are normal sources of free radicals. The PM has a trans-membrane redox system that provides electrons to recycle lipophilic antioxidants, such as alpha-tocopherol and coenzyme Q (CoQ). The idea developed in this study is that the PM is intimately involved in cellular physiology controlling the relationship of the cell to its environment. PM is the key for protecting cellular integrity during aging. Specifically, we have investigated age-related alterations and the effects of CR in the trans-PM redox (antioxidant) system in rat liver. We found that age-related declines in the ratio of CoQ(10)/CoQ(9) and alpha-tocopherol in liver PM were attenuated by CR compared to those fed ad libitum (AL). CoQ-dependent NAD(P)H dehydrogenases were increased in CR old rat liver PMs. As a consequence, the liver PM of CR old rats was more resistant to oxidative stress-induced lipid peroxidation than AL rats. Thus, our results suggest that CR induces a higher capacity to oxidize NAD(P)H in the PM of old rat livers and as a result, a higher resistance to oxidative stress-induced damage.

Dietary compliance among salt-sensitive and salt-insensitive normotensive adults

Little is known about the customary level of sodium intake by salt-sensitive people and the nature of obstacles they face in the adoption of a reduced-sodium diet. These issues were addressed with 12 salt-sensitive (SS) and 9 salt-insensitive (SI) normotensive adults. Information about sodium consumption, taste, and blood pressure and concerns about following a diet reduced in sodium were collected at baseline and monthly while participants followed a 100 mmol Na/day diet for 4 months. Mean sodium intakes of both groups were comparable at baseline and were reduced significantly during diet. The principal dietary concerns were reduced food availability, increased food costs, and reduced food palatability. There were no group differences. Ratings declined over time, but only the food palatability issue did so significantly because of a shift by the SI only. While the predictive value of SS classification remains uncertain, these data indicate that dietary change is feasible in SS subjects.

Vitamin E and selenium deficiency induces expression of the ubiquinone-dependent antioxidant system at the plasma membrane

We have used a model of dietary deficiency that leads to a chronic oxidative stress to evaluate responses that are adaptations invoked to boost cellular defense systems. Long-Evans hooded rats were fed with a diet lacking vitamin E (E) and selenium (Se) for 7 wk from weaning leading to animals deficient in both nutrients (-E -Se). In the absence of an electron donor, liver plasma membranes from these rats were more sensitive to lipid peroxidation, although they contained 40% greater amounts of ubiquinone than the plasma membranes from rats consuming diets with sufficient vitamin E and Se (+E +Se). The incubation of plasma membranes with NAD(P)H resulted in protection against peroxidation, and this effect was more pronounced in -E -Se membranes. Deficiency was accompanied by a twofold increase in redox activities associated with trans plasma membrane electron transport such as ubiquinone reductase and ascorbate free radical reductase. Staining with a polyclonal antibody against pig liver cytochrome b5 reductase, which acts as one ubiquinone reductase in the plasma membrane, showed an increased expression of the enzyme in membranes from -E -Se rats. Little DT-diaphorase activity was measured in +E +Se plasma membranes, but this activity was dramatically increased in -E -Se plasma membranes. No such increase was found in liver cytosols, which contained elevated activity of calcium-independent phospholipase A2. Thus, ubiquinone-dependent antioxidant protection in +E +Se plasma membranes is based primarily on NADH-cytochrome b5 reductase, whereas additional protection needed in -E -Se plasma membranes is supported by the increase of ubiquinone levels, increased expression of the cytochrome b5 reductase, and translocation of soluble DT-diaphorase to the plasma membrane. Our results indicate that, in the absence of vitamin E and Se, enhancement of ubiquinone-dependent reductase systems can fulfill the membrane antioxidant protection.

Auxin-Modulated Protein Disulfide-Thiol-Interchange Activity from Soybean Plasma Membranes

The renaturation of scrambled (oxidized and inactive) RNase A is catalyzed by soybean (Glycine max cv Williams 82) plasma membranes. The catalysis is stimulated by the auxin herbicide 2,4-dichlorophenoxyacetic acid or by the natural auxin indole-3-acetic acid. The inactive auxin analog, 2,3-dichlorophenoxyacetic acid, is without effect. The activity occurs in the absence of external electron acceptors or donors and therefore appears to be a true disulfide-thiol-interchange activity between protein disulfides and thiols of RNase A and those of plasma membrane proteins. The activity is not affected by a mixture of reduced and oxidized glutathione. However, no auxin-stimulated activity was observed in the presence of either oxidized glutathione or reduced glutathione alone, a response characteristic of the previously described auxin-stimulated NADH oxidase activity of soybean plasma membranes. Taken together, the results suggest the operation in the plant plasma membrane of a protein disulfide-thiol-interchange activity that is stimulated by auxins. The auxin stimulations of the interchange activity are prevented by glutathione, reduced glutathione, and brefeldin A at concentrations that also prevent auxin stimulation of NADH oxidation by isolated plasma membranes and inhibit, as well, the auxin-stimulated elongation of excised segments of soybean hypocotyls.