Power, Sex, Suicide: Mitochondria and the Meaning of Life (50 page)

It is always difficult to interpret negative evidence. We are reminded, in that smug phrase, that ‘absence of evidence is not evidence of absence’. The fact, if it is a fact, that antioxidants don’t work, may always be related to the difficulties of targeting: the dose is wrong, or the antioxidant is wrong, or the distribution is wrong, or the timing is wrong. At what point are we entitled to walk away saying: ‘No, this isn’t a pharmacological problem—antioxidants really don’t work’? The answer depends on the temperament, and there are some distinguished researchers who have yet to turn away. But the field as a whole did turn away in the 1990s. As two well-known free-radical gurus, John Gutteridge and Barry Halliwell, put it a few years ago: ‘By the 1990s it was clear that antioxidants are not a panacea for ageing and disease, and only fringe medicine still peddles this notion.’

There are stronger reasons to challenge the standing of antioxidants too, and these come from comparative studies. I mentioned the prediction that animals with long lives should have high levels of antioxidants. For a time this prediction seemed to be true, but only after subjecting the data to a little innocent statistical jiggery-pokery. In the 1980s, Richard Cutler, at the National Institute of Ageing in Baltimore reported, somewhat misleadingly, that long-lived animals harbour more antioxidants than short-lived animals. The trouble was that he presented his data relative to the metabolic rate, and in so doing, he air-brushed out the far stronger association between metabolic rate and lifespan. In other words, a rat has a lower level of antioxidants than a human being, but
only
when antioxidant concentration is divided by metabolic rate, which is seven times faster in the rat; no wonder the poor rat seems so bereft of help. This manoeuvre concealed the true relationship between antioxidant levels and lifespan: a rat has actually far more antioxidants in its cells than a human being. A dozen independent studies have since confirmed that there is in fact a
negative
correlation between antioxidant levels and lifespan In other words, the higher the antioxidant concentration, the shorter the lifespan.

Perhaps the most intriguing aspect of this unexpected relationship is how closely antioxidant levels balance the metabolic rate. If the metabolic rate is high, then antioxidant levels are also high, presumably to prevent oxidation of the cell; yet lifespan is still short. Conversely, if the metabolic rate is low, then antioxidant levels are also low, presumably because there is a lower risk of cell oxidation; yet lifespan is still long. It seems that the body doesn’t waste any time and energy in manufacturing more antioxidants than it needs—it uses them simply to maintain a balanced redox state in the cell (which means the dynamic equilibrium between oxidized and reduced molecules is kept optimal for the cell’s function).
¹
The cells of short-lived and long-lived animals maintain a similar, flexible, redox state by counterpoising the antioxidant concentration against the rate of free-radical generation; but lifespan is not affected by antioxidant concentration in any way. We are forced to conclude that antioxidants are virtually irrelevant to ageing.

These ideas are borne out by the birds, which live long lives in relation to their metabolic rate. According to the original version of the mitochondrial theory of ageing, birds ought to have higher antioxidant levels, but again this is not true. The relationship is inconsistent, but in general birds have lower antioxidant levels than mammals, reversing the predictions. Another test-case
is calorie restriction. To date, calorie restriction is the only mechanism proved to extend the lifespan of mammals like rats and mice. Exactly how it works is debated, but the relationship with antioxidant levels in different species is ambiguous. Sometimes antioxidant concentrations go up, sometimes they go down, but there is no clearly consistent relationship. Even a piece of encouraging work from the early 1990s, suggesting that fruit flies live longer when genetically modified to express higher levels of antioxidant enzymes, turned out to be unrepeatable, at least in the hands of the original researchers (who make a distinction between strains that are long-lived and strains that are short-lived: higher antioxidant levels might extend the life of short-lived breeds of flies, in other words, they may correct a genetic deficiency). If any solid conclusion emerges from all this, it is certainly not that high levels of antioxidants prolong lifespan in healthy, well-nourished animals.

We’ve been confounded by the lure of antioxidants for a simple reason: the proportion of free radicals escaping from the respiratory chains is
not
constant—Harman’s original assumption was wrong. While free-radical leakage often does reflect oxygen consumption, it can also be modulated up or down. In other words, far from being an uncontrolled and unavoidable by-product of cell respiration, the rate of free-radical leakage is controlled and largely avoidable. According to the pioneering work of Gustavo Barja and his colleagues at the Complutense University in Madrid, birds live long lives because they leak fewer free radicals from their respiratory chains in the first place. As a result, they don’t need to have so many antioxidants, despite consuming large amounts of oxygen. Importantly, it seems that calorie restriction might work in a similar way. While there are various genetic changes, one of the most significant is a restriction in free-radical leakage from the mitochondria, despite similar oxygen consumption. In other words, in both long-lived birds and mammals the proportion of free radicals that leaks from the respiratory chains
decreases
.

This answer seems inoffensive enough but it is actually troublesome and hacks a hole in the established evolutionary theory of ageing. The problem is this. Animals that live a long time do so by restricting the free-radical leakage from their mitochondria. Because genes control the rate of ageing, it follows that in birds (and presumably in humans to a lesser degree) there has been selection to lower the rate of free-radical leakage. Fine. But if free radicals were simply damaging, why wouldn’t a rat also do better by restricting its free-radical leakage? There seems to be no cost, indeed quite the contrary—there would be no need for the rat to go on manufacturing all those extra antioxidants to prevent itself from being oxidized. And surely it would have everything to gain, because a long-lived rat would have more time, and could leave behind more offspring. So rats, and by the same token humans, could live longer,
cost-free
, if they simply restricted free-radical leakage.

So why don’t they? Is there a hidden cost, or do our ideas of ageing stand in need of radical revision? The cost of a long life is usually said to be a degree of impairment in sexuality. According to the disposable soma theory, first proposed by Tom Kirkwood, at the University of Newcastle, longevity is balanced against fecundity: long-lived species tend to have smaller litters, and rear them rather less frequently, than short-lived species. This is certainly true, at least in most known cases. The reason is less certain. Kirkwood suggested that the reason relates to the balance of resource use in individual cells and tissues: resources diverted towards attaining reproductive maturity, and raising litters, detract from those required to ensure longevity of the cell, such as DNA repair, antioxidant enzymes, and stress resistance—there are only so many ways to divide limited resources. Barja’s data challenge this idea. Restricting free-radical leakage should have no cost on fecundity, as cellular damage is restricted without any need for better stress-resistance—the cost imputed in the disposable soma theory is negated. So, if the disposable soma theory is correct, there ought to be a hidden cost to restricting free-radical leakage; we shall see, in the final chapter, that there is indeed a hidden cost, and it holds vital connotations for our own quest to live longer.

To understand why, we need to consider another prediction of Harman’s mitochondrial theory, which has also caused trouble. This held that free radicals don’t necessarily damage the cell in general very much—they’re mopped up by antioxidants—but they do specifically damage the mitochondria, especially their DNA. Harman actually mentioned mitochondrial DNA only in passing, but its involvement later became a fundamental tenet of the theory. Let’s see why, for the gap between the predictions and hard prosaic reality reveals a great deal about what’s really going on.

Harman argued that, because free radicals are so reactive, those escaping from the respiratory chains should mainly affect the mitochondria themselves—they should react on the spot, where they were produced, and not damage distant locations very much. He then asked, quite perceptively, whether the gradual decay of mitochondria with increasing age ‘might be mediated in part through alteration of mitochondrial DNA functions?’ The chain of effect would be as follows: free radicals escape the respiratory chains and attack the adjacent mitochondrial DNA, causing mutations that undermine mitochondrial function. As mitochondria decay, the performance of the cell as a whole declines, leading to the traits of ageing.

Harman’s perceptive question was addressed more explicitly a few years later by Jaime Miquel and his colleagues in Alicante, Spain. Their formulation
in 1980 is still the most familiar version of the mitochondrial theory of ageing today, even though many aspects don’t really fit the data, as we’ll see. It goes something like this. Damage to proteins, carbohydrates, lipids, and so on can be repaired, and is not dangerous unless the rate of damage is overwhelmingly fast (as it might be, for example, after radiation poisoning). DNA is different. Although DNA damage can also be repaired, there are occasions when the damage confounds the original sequence and mutations occur. Mutations are heritable changes in DNA sequence. Except by random back-mutation to the original sequence, or recombination with another strand of un-mutated DNA, there is no way that the original sequence can be recovered. Not all mutations affect protein structure and function, but some of them certainly do. In the usual way of things, the more mutations, the greater the chance of detrimental effects.

In theory, mitochondrial mutations accumulate with age. As they do so, the efficiency of the system as a whole begins to break down. It’s not possible to fashion a perfect protein from an imperfect set of instructions, so a certain degree of inefficiency is built in. Worse, if the mutations affect the respiratory chains in mitochondria, then the rate of free-radical leakage rises, spinning the whole vicious cycle faster and faster. Such positive feedback ultimately builds up into an ‘error catastrophe’, in which the cell loses all control over its function. When this fate has overcome a sizeable proportion of the cells in a tissue the organs fail, placing the remaining functional organs under still greater strain. The inevitable outcome is ageing and death.

So what are the chances that mutations would affect the respiratory chain proteins? It’s overwhelmingly likely. We have seen that thirteen of the core respiratory proteins are encoded by mitochondrial DNA, which is anchored to the membrane right next to the respiratory chains. Any escaping free radicals are virtually bound to react with this DNA: it’s only a matter of time before mutations occur. And we have seen that proteins encoded in the mitochondria interact intimately with those encoded in the nucleus. An alteration in either party can erode this intimacy, and affects the function of the respiratory chain as a whole.

If this sounds grim, it gets worse. A succession of dire findings made the whole set-up seem like a bad joke perpetrated by a satanic biochemical deity. We were told that mitochondrial DNA is not just stored in the incinerator, but it’s also stripped of the normal defences: it’s not wrapped in protective histone proteins; it has little ability to repair oxidative damage; and the genes are packed together so tightly, without the cushioning of ‘junk’ DNA, that a mutation anywhere is likely to cause havoc. This dire scenario was set off by a sense of pointlessness: most mitochondrial genes had already been transferred to the nucleus, and the handful that remained seemed to be in the wrong place. Aubrey de Gray, one of the most original and dynamic thinkers in this field, has
even suggested we could cure the ravages of ageing by transferring the rest of the mitochondrial genes across to the nucleus. I disagree, for reasons that we’ll come to, but it’s easy to feel his point.

Why on earth did such a daft system evolve? That depends on one’s view of evolution. Stephen Jay Gould used to vent his frustrations about what he called the ‘adaptationist program’ in biology—the assumption that all is adaptation, in other words, everything has a reason, however innocent of function it may appear, and is shaped by the processes of natural selection. Even today, biologists can be split into those who are reluctant to believe that nature does anything for nothing, and those who believe that some things are just beyond direct control. Is ‘junk’ DNA really junk or does it have some unknown purpose? We don’t know for sure, and the answer you get will depend on whom you ask. Similarly, the ‘point’ of ageing is disputed. The most widely accepted view is that we are less likely to reproduce as we grow older, so natural selection is less able to weed out the genetic variants that cause damage late in life. Because mutations in mitochondrial DNA build up late in life, natural selection is unable to come up with an efficient mechanism of eliminating them. Only when the expected lifespan increases, as it does in animals isolated on an island without predators, or in birds that can fly away, or humans who use their large brains and social structures, can selection act to prolong life. If we subscribe to this view, then the sheer lunacy of storing badly protected mitochondrial genes in the incinerator is just one of those things: an accident of evolutionary history.

Is this nihilistic vision correct? I don’t think so. The fault is that the line of reasoning is too stiffly chemical: it doesn’t take into account the dynamism of biology. We’ll see the difference this makes later. Nonetheless, it is a bold theory, and it has the great merit of making some explicit testable predictions. There are two in particular that we’ll look into. First, the theory predicts that mitochondrial mutations are sufficiently corrosive to bring about the whole sorry trajectory of ageing. This, we’ll see, is likely to be true. But the second prediction is probably false, at least in the full diabolical sense in which it was originally put forward—that mitochondrial mutations should accumulate with age. As they do they ought, ultimately, to bring about an ‘error catastrophe’. There’s little strong evidence to say that this happens. And therein lies the secret.