New & Noteworthy
July 11, 2013
You don’t have to be a scientist to get the message that oxidation is bad and antioxidants are good. Just go to the vitamin aisle of your local supermarket, or listen to the ads on late-night TV. You’ll quickly find out that oxidation caused by free radicals is the reason for aging, and antioxidants are the fountain of youth. Of course you shouldn’t believe everything you hear…
Things just aren’t that clear when you take a good hard look at aging. Yes, oxidation happens, but there actually isn’t solid experimental proof that it causes aging. In mice, this connection has not panned out at all: lowering the ability to sop up oxidants, by knocking out an antioxidant enzyme, does not shorten the mouse’s life.
In a recent eLife paper, joint first authors Brandes and Tienson and their coworkers used our favorite experimental subject, Saccharomyces cerevisiae, to see if oxidation is a cause or just a consequence of aging. They generated a ton of data about oxidation during aging and did not find any evidence for causation. Instead they came to the surprising conclusion that the trigger for aging may actually be a sudden drop in the levels of the coenzyme NADPH.
The first step, published previously by this group, was to come up with a very sensitive assay for protein oxidation. The amino acid cysteine can act as a sensor for levels of oxidation, as its sulfur-containing thiol group can be oxidized and reduced. Their technique, known as OxICAT, detects the ratio of reduced to oxidized thiol groups on cysteine residues for individual proteins. They can do this for hundreds of proteins at the same time.
In the current study, they looked at the oxidation state of cysteine residues in about 300 different proteins and also measured the levels of several different metabolites related to the redox state of the cell. All of these data were collected over time in aging yeast cells, both under normal conditions and under conditions simulating caloric restriction or starvation. These last conditions were included because a lower-calorie diet has been shown to slow down aging, in yeast as well as in animals.
Oxidation of proteins definitely did increase over time. But if oxidation were the cause of cell death, you would expect that it would increase steadily and at some maximum point, the cells would die. Surprisingly, that didn’t happen.
Instead, different groups of proteins were oxidized with different kinetics. The most sensitive proteins (about 10% of the set that they studied) were oxidized 48 hours before the cells started to lose viability. This set included some conserved proteins that are important in maintaining oxidation-reduction balance in the cell, such as the thioredoxin reductase Trr1p.
But it wasn’t only those especially sensitive proteins that were oxidized. In a second wave of oxidation, almost all the remaining proteins (80%) were oxidized at 24 hours before death. And even with so many proteins oxidized the cells were still metabolically active, with ATP levels near normal. So massive oxidation did not equal instant death for these cells.
As predicted, a low-calorie diet slowed down the whole process. The pattern looked a lot like it did in cells on a normal diet, but there was more time between the waves of oxidation and before the end of viability.
The authors also looked at what happened to different metabolites during aging. One key metabolite is the coenzyme NADPH: it donates electrons to the thioredoxin system that helps balance oxidation and reduction. They found that even before any changes in oxidation are detectable, levels of NADPH decrease very suddenly. The authors speculate that this decrease starts the collapse in redox potential that ends in the death of the cell. The oxidation of protein thiols is an effect rather than a cause, and could actually be a way for the cell to sense its redox state and possibly regulate it. NADPH levels have been seen to decrease in aging rats as well, suggesting that this could be a universal part of the aging process.
The results of this study are too voluminous to describe fully here, but they raise a lot of intriguing questions. Some proteins never got oxidized – what protected them? Are NADPH levels really the trigger for aging, and if so, what causes the sudden decrease? Is oxidation of cysteines actually part of a sensory mechanism? And if that’s true, would preventing oxidation really be such a good thing? This may be another good reason to turn off late-night TV.