Glycation (sometimes called non-enzymatic glycosylation) is the result of the covalent bonding of a sugar molecule, such as glucose or fructose, to a protein or lipid molecule, without the controlling action of an enzyme. Glycation may occur either inside the body (endogenous glycation) or outside the body (exogenous glycation). It is a haphazard process that impairs the functioning of biomolecules, and does not require the expenditure of ATP (the energy storing molecule).
Glycosylation, in contrast, is the enzyme-mediated ATP-dependent attachment of sugars to protein or lipid molecules at defined sites on the target molecule. It is an important form of post-translational modification of proteins and is required for the functioning of the mature molecule.
Much of the early laboratory research work on fructose glycations used inaccurate assay techniques that led to drastic underestimation of the importance of fructose in glycation.
Exogenous, meaning outside the body, may also be referred to as dietary or pre-formed. Exogenous glycations and advanced glycation endproducts (AGEs) are formed when sugars are cooked with proteins or fats. Temperatures over 120 °C (~248 °F) greatly accelerate the reactions, but lower temperatures with longer cooking times also promote their formation.
These compounds are absorbed by the body during digestion with about 10% efficiency. Browning reactions (usually Maillard type reactions) are evidence of pre-formed glycations. Indeed, sugar is often added to products such as french fries and baked goods to enhance browning. Glycation may also contribute to the formation of acrylamide, a potential carcinogen, during cooking. Until recently, it was thought that exogenous glycations and AGEs were negligible contributors to inflammation and disease states, but recent work has shown that they are important.
Food manufacturers have added AGEs to foods, especially in the last 50 years, as flavor enhancers and colorants to improve appearance. Foods with significant browning, caramelization, or directly added preformed AGEs can be high in these compounds. A very partial listing of foods with very high exogenous AGEs includes donuts, barbecued meats, cake, and dark colored soda pop.
Endogenous glycations occur mainly in the bloodstream to a small proportion of the absorbed simple sugars: glucose, fructose, and galactose. It appears that fructose has approximately ten times the glycation activity of glucose, the primary body fuel. Glycation is the first step in the evolution of these molecules through a complex series of very slow reactions in the body known as Amadori reactions, Schiff base reactions, and Maillard reactions; which lead to advanced glycation endproducts (AGEs). Some AGEs are benign, but others are more reactive than the sugars they are derived from, and are implicated in many age-related chronic diseases such as cardiovascular diseases (the endothelium, fibrinogen, and collagen are damaged), Alzheimer's disease (amyloid proteins are side-products of the reactions progressing to AGEs), cancer (acrylamide and other side-products are released), peripheral neuropathy (the myelin is attacked), and other sensory losses such as deafness (due to demyelination). This range of diseases is the result of the very basic level at which glycations interfere with molecular and cellular functioning throughout the body and the release of highly oxidizing side-products such as hydrogen peroxide.
Long-lived cells (such as nerves and different types of brain cell), long-lasting proteins (such as crystallins of the lens and cornea), and DNA may accumulate substantial damage over time. Cells such as the retina cells in the eyes, and beta cells (insulin-producing) in the pancreas are also at high risk of damage. Damage by glycation results in stiffening of the collagen in the blood vessel walls, leading to high blood pressure, especially in diabetes. Glycations also cause weakening of the collagen in the blood vessel walls, which may lead to micro- or macro-aneurisms; this may cause strokes if in the brain.
Red blood cells have a consistent lifespan of 120 days and are easily accessible for measurement of glycated hemoglobin. Measurement of HbA1c—the predominant form of glycated hemoglobin—enables medium-term blood sugar control to be monitored in diabetes.
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