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
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
A single Beta-carotene molecule.
Beta-carotene can be split by enzymes in the small intestinal lining (see diagram
Beta-carotene is "cleaved" into 2 parts
by intestinal enzymes.
Once cleaved, the result is 2 molecules
Menadione is another example of a provitamin. This synthetic and fat-soluble
provitamin can be converted in the body to the active form of
Menadione is also known as vitamin K3 and menaphthone.
Molecule common to the different forms of
Derivatives of cholesterol are activated by sunlight
as they pass in circulation near the skin surface and are thus considered provitamin D.
The amino acid tryptophan can be converted to