Glucose Damage: Browning like Toast
By Danné Montague-King, Founder DMK
My college house mate’s mother worked as a butcher at a local market. Hers was a wonderful job for us because she brought us home fine steaks night after night. The meat needed to be disposed of anyway, because it was too old to be legally sold. It tasted fine just the same.
I remember quite well that this meat always had a rather peculiar appearance. Before cooking it was faintly brown on top as if it had been lightly seared. Meat changes colour as it is cooked, of course, becoming golden then brown and I know that this old stored meat had undergone very slowly some of the same chemical reactions that meat goes rapidly through during cooking. And as surprising as it may seem, we undergo some of these same chemical changes as we age, we brown.
Meat browns and we brown because of glucose, the other major molecule of aerobic metabolism. This peculiar phenomenon was discovered in 1912 by Louis Maillard, a French chemist, who observed that a mixture of glucose to protein components, when heated, would turn from clear to yellow to a deep brown. This was subsequently called the Maillard Reaction and has ever since been of great interest to food chemists concerned with making food as tasty and tempting in appearance as possible. The Maillard Reaction is just the chemical attachment of glucose to proteins at places it doesn’t normally belong, which makes a yellow brown product. Glucose is sticky stuff, so it attaches spontaneously, given the appropriate temperature and amount of time.
Until recently, however no one understood that this same process could occur at any body temperature. But in the 1970’s it was noticed that people who have high levels of blood glucose due to poorly controlled diabetes also have glucose attached to some of their haemoglobin, a protein. That is their haemoglobin is modified as if by a Maillard Reaction. Doctors had noticed for years that uncontrolled diabetics seemed to undergo something resembling accelerated ageing. Many of the common ailments of ageing, such as cataracts, atherosclerosis, heart attacks, strokes, Lung and joint stiffening, appeared earlier in diabetics together with the Maillard Reaction and concluded that ageing itself might be partially due to Maillard, or Browning, products accumulating at a slow rate in the body. What’s more, he noted that the end result of this reaction was a series of unalterable new chemical structures in our tissues, which he called AGEs, a clever acronym from his more opaque chemical term, Advaced Glycosylation End-products.
But why would simply attaching glucose to proteins at places it doesn’t belong lead to the sort of deterioration, we call ageing?
The first reason is that some proteins form the structure and support of our bodies. Many of the most important structural proteins, such as myopossums collagen, live for decades in our bodies without their molecules turning over. Apparently
general characteristics of ageing take place in exactly those tissues that are stuffed with long-lived, non-renewing cells and molecules.
So let’s think about collagen again. Remember that it is a flexible protein composed of three strands coiled around one another like a cable. It’s flexibility is what makes it so useful for forming the foundation of arteries, veins, lungs and skin; for making tendons and ligaments that twist and bend without breaking; for forming cartilage that cushions our joints with its resiliency. But as glucose attaches to collagen, it forms bridges or crosslinks between strands of a single molecule and between molecules. As these bridges multiply over time, collagen’s flexibility gradually disappears. It turns yellow and stiff and no longer makes such wonderful lungs, tendons, ligaments or support for artery walls. What’s more, collagen with attached glucose in the walls of the arteries acts like an opened-jawed bear trap. It seizes and holds on to passing proteins. In this fashion browning may play a part in trapping and accumulation of LDL cholesterol in the artery walls-an early stage of atherosclerosis.
Proteins do many things in the body besides provide its body structure. They turn genes on and off, direct cell replication, and chaperone other molecules to their appropriate site of action. As enzymes, they are essential for virtually all of the chemical activity of a cell. The fidelity with which proteins carry out the functions they were designed for depends on their being chemically unaltered. When sugars attach to protein inappropriately, they can impair their function and therefore disrupt the proper working of the cell. Attached sugars also make proteins less soluble in the body more likely to solidify and become non-functional, and less likely to be broken down by chemicals designed to destroy damaged molecules.
The impairment of proteins isn’t the only potential problem associated with glucose. Glucose can also bind directly to DNA, although it does so more slowly than it binds to proteins. Nevertheless, in non-dividing cells, such as those composing much of the brain and heart. DNA is a long-lived molecule on which AGEs can potentially accumulate. In principle, AGE attachment to DNA could disrupt the production of new cellular proteins, could interfere with DNA repair, and could even cause mutations. As of now, however, relatively little is known about these particular processes.
Chemical theorists tell us that the Maillard Reaction should proceed at a rate pretty much determined by the concentration of sugars and proteins, and the temperature at which these ingredients are kept.
To the extent that browning is a central process of ageing, the anti-aging impact of caloric restriction might be partially explained by a reduced browning rate. If you feed laboratory rats only 60 percent of the calories they would eat if given unlimited food supplies, their blood glucose level is reduced by about 25 percent, their body temperature declines by several degrees, and their ageing is retarded by about 20 to 25 percent.
But the larger picture is more complex. Most mammals maintain about the same concentration of sugars and proteins in their bodies and live near the same 98.6 degrees that humans do, and yet some mammals accumulate browning products rapidly and live only a year or two, while others brown slowly and live many decades. How can this be?
As was the case with oxidants, the production rate of browning products is apparently only part of the story. If they accumulate at different rates in different species, then there must exist, protective mechanisms – anti-ageing processes that are well developed in some species and poorly developed in others. Discovering the nature of these anti-ageing processes and being able to modify them pharmaceutically, should hold great promise for understanding and modifying the age rate itself.
Currently, we know very little about these processes. We know that certain natural plant compounds, as well as synthetic drugs, can inhibit the formation of browning products in a test tube. So it seems likely that our bodies will also contain an array of anti-browning chemicals, although they are as yet unknown. But at least one of the body’s phagocytes, or scavenger cells, seems specialised to devour proteins or cells that have been browned
If oxidation and browning are two important general processes of ageing, it might come as no surprise that they seem to operate co-operatively to our detriment. The damage caused by one affects the other. So glucose and its derivative products can react with other chemicals to produce free radicals, and free radicals can accelerate browning. Also, glucose can attach to cellular antioxidant enzymes and by doing so inactivate them leading to higher levels of free radicals and the damage they cause.
These two processes are likely to be centrally involved in ageing, but there is no reason to expect that they completely explain it. Evolutionary theory suggests that there will be many mechanisms of ageing. Browning and rusting may just be among the most general and easiest to identify. One other general process that has short-term benefits but long-term hazards is the continuing ability of cells to divide throughout life. The failure of proper regulation of this process leads, of course to cancer, which turns out to be perhaps them most general disease of ageing in the animal kingdom.
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