Physiology Sponsored by
Figure 1: Formation of ascorbic acid from glucose
Figure 2: The process of collagen formation
Figure 3: The hydroxylation process
Figure 4: Vitamin C regenerates vitamin E
Figure 5: The action of GSH on hydrogen peroxide
Vitamin C, also known as ascorbic acid, is the prince of vitamins and the first dietary substance associated with curing a disease known as scurvy, a deadly and painful illness that was prevalent before James Lind, MD, discovered the cure in 1747. He established that the absence of a compound in the diet was the cause of scurvy. Lind died in 1794 at 78, and the following year, the British Admiralty adopted the use of citrus foods as the prevention of scurvy. In 1935, vitamin C was synthesized in pure form.
Vitamin C is also an important part of a variety of bodily functions, ranging from bone formation to scar tissue repair. It is the major water-soluble antioxidant, it destroys free radicals, it plays a critical role in hydroxylation reactions that are essential for the formation of collagen, and carnitine synthesis uses vitamin C as a reducing agent. It is directly involved in the formation of norepinephrine and serotonin, two important substances needed for the proper function of the nervous system; and it is also involved in the synthesis of hormones, hormone-releasing factors and neurotransmitters.
The chemistry of vitamin Ca and its role in the body can be more easily understood if you remember that it is a reducing agent. This means that it donates hydrogen to other compounds. Vitamin C is formed from glucose in the bodies of most animals, except primates and guinea pigs. Humans lack the enzyme L-gulonolactone oxidase, which converts glucose to ascorbic acid. Although other enzymes are also required for the conversion, this one is definitely lacking. Take a look at the vitamin C molecule in Figure 1. It is a simple sugar molecule with a slight modification. The important parts of this figure are the two OH groups (hydroxyl groups) on the vitamin C molecule. It is the loss of the hydrogen atoms from these OHs that makes vitamin C a hydrogen donor and, therefore, a reducing agent.
Vitamin E, also known as tocopherol, is an oil-soluble vitamin antioxidant and a reducing agent. It has only one OH group on the end, which is the active part of the molecule that donates the hydrogen atom in an antioxidant reaction. Now, here is the bad thing about vitamin E; losing a hydrogen from the hydroxyl group converts the molecule of vitamin into a quinone. So what, you say, is the big deal? Quinones are highly reactive molecules associated with neoplastic disordersb. The OH group is converted to carbonyl—that is to a -C=O—which can very easily react with proteins and make them nonfunctional. Along comes vitamin C and donates an H+ to the -C=O, changing back to the more beneficial -OH group. This is a very necessary function of vitamin C as a critical part of the antioxidant defense system.
Collagen is the most abundant protein in the body; it makes up 70% of the dry weight of human skin and is a critical component of the vascular and muscular system. Vitamin C is essential to the formation of collagen. Although collagen formation is a complex process, the steps can be limited to only seven: four inside the fibroblasts and three in the cytoplasm. This process, diagrammed in Figure 2, is in a schematic form.