Proteins are complex molecules, made up of amino acids, which perform a wide variety of functions in the human body, from chemical reaction-controlling enzymes, to structural molecules like collagen, to necessary components in human memory. In order to function properly, proteins must assume a correct three-dimensional configuration. In their proper
shape, some proteins act as little machines, some with moving
parts, which serve to position other molecules for various metabolic reactions. Collagen is made of protein and is the major structural molecule of the body (comprising 30 percent of total body protein). Collagen provides the principal supporting tissue in most connective tissue, muscles, blood vessels, etc. As we age, there is a progressive increase in cross-linking of these important body constituents.
Some proteins contain sulfur, which is an important site for oxidation (losing an electron) and reduction (gaining an electron) reactions. As the body ages, the components of the body tend to become more oxidized. When the sulfur-hydro gen bonds (sulfhydryl) in proteins are oxidized, they can react with another sulfhydryl group in the same or another protein
molecule to form a disulfide (sulfur to sulfur) bond. This is how rubber is vulcanized, turning soft latex into hard rubber combs. Scientists call these attachments cross-links. There are also oxygen- and nitrogen-based cross-links which behave in a similar manner. These bonds hold the proteins in rigid structural relation to each other. A certain number of these disulfide bonds are necessary to keep proteins in their proper shape and proper position with respect to each other. However, beyond this requirement, cross-links interfere with proper protein function and movement. Skin eventually becomes tough, hard, inelastic, brittle, and wrinkled. Collagen
shrinks and loses flexibility. A good example of cross-linking is the tanning of soft cattle skin into hard, tough leather. Shakespeare wrote of tanning (a cross-link process) in Hamlet (Act V, Scene I):
HAMLET: “How long will a man lie i’ the earth ere he
rot?”
GRAVEDIGGER: “I’ faith, if he be not rotten before he
die—as we have many pocky corses now-a-days, that will
scarce hold the laying in—he will last you some eight year
or nine year: tanner-willlast: you nine year.”
HAMLET: “Why he more than another?”
GRAVEDIGGER: “Why,-sir, his hide is so tanned with his
trade, that he will keep out water a great while; and your
water is a sore decayer of your whoreson dead. body.”
The reason the tanner’s body took longer to rot than others is that cross-linked proteins dissolve less easily and are more difficult for microorganisms to digest. In older animals, collagen is less soluble, has lower water absorption swelling capacity, and is digested more slowly by enzymes.
Cross-linking of arteries causes them to lose flexibility and become hard, contributing to susceptibility to cerebral blowouts (hemorrhages). This is exactly what happens to the radiator hoses in your car as they age, becoming progressively more cross-linked and brittle until they leak or burst.
When arterial walls have been hardened to a degree by excess cross-linking, there is a reduced ability of these arteries to pulsate with blood flow. As a result of this rigidity, the endothelium (inner lining) of the arteries is damaged and increases in permeability to the plasma (liquid part of blood).
Plasma elements then diffuse into the arterial wall. This damage is greatest at the points of greatest stress—the bends and branch points of the arteries.
Many agents found in the body are cross-linkers. Ketones, found in the blood of diabetics, are potent cross-linkers. Many metal ions are cross-linkers, including cadmium, aluminum, copper, and titanium. Aldehydes (found in smog, cigarette smoke, and formed in the liver from alcohol), ultraviolet light, radiation, and free radicals are very potent cross-linkers. The actual chemical processes of forming the pathologic cross-
links generally require free radical intermediates. Ketones and aldehydes, for example, cross-link unsaturated fats via free radical reactions.
The loss of ability of aging tissue to hold bound water can be duplicated outside the body by adding cross-linking agents to a gel. Note that as jelled desserts, such as Jell-O®, become stiffer, they lose the ability to hold water (they “weep”). Such desserts would last longer if the water used in making them were treated so that metal ions are removed (as in soft water).
Chelating agents, such as EDTA, have been effective in treating humans who have ingested heavy metals like lead, which can promote cross-linking reactions. The combination of EDTA and vitamin C has been found to be particularly effective in removing lead from the brain, where it does the most harm.
Rotifers, multicellular microscopic organisms, lived longer in experiments where they were given regular brief immersions in solutions of one of the following chelating agents: sodium citrate (found in citrus fruits), sodium tartrate (found in wine), EDTA (a food additive), and EGTA. The treatments removed significant quantities of calcium, the levels of which increased throughout the life span of the animals.
Chelation with EDTA has also been used in atherosclerotic patients with abnormally high plasma lipids. Some patients’ lipid levels were reduced to a normal or near normal range by this treatment. Levels rose again after the discontinuation of EDTA therapy, but fell again when it was reinitiated. In
atherosclerotic patients with normal plasma lipid levels, there
was little or no plaque reduction with EDTA.
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