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.
by Maria Costanzo, Ph.D., Senior Biocurator, SGD
Categories: Research Spotlight
Tags: aging, oxidation, redox, Saccharomyces cerevisiae
February 17, 2012
Let’s face it: low alcohol beer just doesn’t taste that great. This is because the alcohol is either diluted or removed chemically after fermentation. Both methods wreak havoc with a beer’s flavor.
Dr. John Morrissey of University College Cork is trying to change this. His lab is working to generate a strain of yeast that turns some but not all of its sugar into alcohol. That way the beer process is the same, just with less alcohol at the end.
This is different from stopping fermentation early. In that case there are still sugars in the final product which ruin a beer’s taste even more than removing the alcohol! Here the same amount of sugars are used up, it is just that only part of that energy has gone into making the alcohol. Same sugar content, less alcohol.
Although we don’t have all the details because of intellectual property issues, what we do know is that he compared the genomes of yeast species that make a lot of alcohol and those that don’t. In an email he stated that he focused on genes that would affect carbon metabolism without perturbing redox balance in a significant way. Presumably he then swapped the appropriate genes between strains and created his low alcohol strain.
This is not only a godsend for low alcohol beer, but it may be useful for other fermentation processes as well. For example, maybe something similar can be done for low or no alcohol wines which, apparently, are even less tasty than low alcohol beer. Designated drivers everywhere will be thanking Dr. Morrissey profusely if he can make decent tasting, low alcohol drinks a reality.
And apparently it isn’t just designated drivers that want this stuff. Judging by recent upticks in sales of the relatively low quality low alcohol beers currently on the market, there is definitely a market out there for such beverages. A cool science project, decent low alcohol beer and nice profits to boot! Who could ask for more?
How beer is made, from Modern Marvels, http://www.history.com
by D. Barry Starr, Ph.D., Director of Outreach Activities, Stanford Genetics
Categories: Research Spotlight
Tags: carbon metabolism, fermentation, low alcohol beer, redox, yeast