In our discussion of vitamins, we learned that each vitamin has a unique funtion or set of functions. We also learned that vitamins must be supplied in the diet to maintain health. Provitamins on the other hand, are substances which can be used as vitamins as well. However, provitamins need to be broken down, or converted first into another substance, before they can be used as a vitamin. Therefore, provitamins are precursors to vitamins.
For example, carotene is one of four similar pigments, found in plants and having colours ranging from violet to yellow. Each of the four carotenes is preceded by a Greek letter (alpha, beta, gamma, or delta) and merely indicates the slight difference in structure. Carotenes are found in many dark green, leafy and yellow vegetables such as carrots, sweet potatoes, squash, and turnips. Carotenes are also found in yellow fruits such as oranges peaches, apricots, cantaloupes, etc.). They are fat-soluble, and can be split into 2 molecules of retinal by enzymes in the intestinal wall and the liver. In humans, B-carotene is the major provitamin (precursor) of vitamin A. B-carotene is not absorbed as well as retinol, but is in greater availability in the diet.
Beta-carotene can be used as a provitamin (precursor) of vitamin A since it can be split into 2 molecules of retinal (see diagram below). Oral Beta-carotene preparations have also been used to reduce the severity of photosensitivity in patients with certain types of porphyria. Beta-carotene has also been used as a method of artificial tanning.
Menadione is another example of a provitamin. This synthetic and fat-soluble provitamin can be converted in the body to the active form of vitamin K. Menadione is also known as vitamin K3 and menaphthone.
Derivatives of cholesterol are activated by sunlight as they pass in circulation near the skin surface and are thus considered provitamin D.
