Chemical Reaction: Age, Rage and the Skin

The antiaging skin care juggernaut has crushed any lotion incapable of turning back the ravages of time, or at least promising to do so. The downside of UV exposure, smoking and pollution has long been known. Sugar formation in the cells, however, presents another front in the battle against aging. Chemically speaking, inside every cell, the Hayflick limit—the number of times a cell divides before it dies (approximately 50)—conspires against dreams of immortality. Receptor for advanced glycation end products (RAGE), too, is not simply the frustration of baby boomers with wrinkles—it is a new enemy of the intracellular matrix, and now it’s time to add the Maillard reaction, the Amadori product and collagen glycation to cosmetic chemists’ list of enemies.

French biochemist Louis-Camille Maillard (1878–1936) is the founding father of the most recent war on skin aging. In 1912, Maillard undertook studies of the reaction between amino acids and sugars, which now bear his name.

Food chemistry is a common part of everyone’s life and clearly illustrates the Maillard reaction. An egg white, for example, is not white before it is cooked. Heating denatures the protein, making it insoluble and, thus, changing it from clear to white. Raw meat is red, cooked meat is brown—and much tastier. The difference is primarily due to the Maillard reaction—the contribution of colors, flavors and odors—and it is where cooking and modern skin treatment find common ground.

The Maillard reaction (see Figure 1) starts when the carbonyl group of a sugar reacts with an amino group of a protein, producing N-substituted glycosylamine and water. Maillard reactions generally only begin to occur above 140°C (284°F). If the cooking temperature never gets above the boiling point of water (100°C or 212°F), the Maillard reactions will not take place. The resultant glycosylamine of the reaction is unstable and undergoes the Amadori rearrangement to form ketosamines. The formation of an advanced glycation end product begins with the formation of a Schiff base.

For example, the aldehyde group of a glucose molecule will combine with the amino group of a lysine molecule to form an imine, which is a double bond between the carbon atom of the glucose and the nitrogen atom of the lysine. The Amadori product is a rearrangement from the Schiff base wherein the hydrogen atom from the hydroxyl group adjacent to the carbon-nitrogen double bond moves to bond to the nitrogen, leaving a ketone. When the Amadori product is oxidized, most often by transition metal catalysis, an advanced glycation end product (AGE) is formed. The first two steps in this reaction are both reversible; the last step is irreversible.

The advanced glycation end product produced by the reaction of pentosidine with arginine and lysine is illustrated in Figure 2. Pentosidine levels increase with age, and collagen has a long life in the skin. Even though the nonenzymatic process is slow, it accumulates relentlessly as we grow older.

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