A recent study conducted by a team of researchers from Columbia University Medical Center has captured images of one of the smallest proteins to be observed with a microscopic eye. The protein called STRA6, or “Stimulated by retinoic acid 6”, is responsible for absorbing vitamin A into cells. Vitamin A is vital for embryonic development, particularly placenta and fetus. It also helps make light receptors in our eyes.
This latest discovery is quite significant as scientists didn’t know how vitamin A was absorbed in the cells before this discovery. Most molecular chemicals transport interacts directly with the chemicals they are transporting, but STRA6 only acts with an intermediary protein that carries the essential vitamin into the bloodstream.
After viewing the protein and getting familiar with STRA6’s structure, the researchers will have a better insight into the world of intracellular transportation. They will be able to figure out how other related transport systems work.
A new type of camera technology was a key element to getting the images of STRA6. When paired with an electron microscope, the camera allows biologists to see tiny, never-before-seen structural details of the inner machinery of our cells. “We can now get near atomic resolution because the new camera is much faster and allows us to take a movie of the molecules,” says Oliver Clarke, PhD, an associate research scientist in the Hendrickson lab at Columbia University Medical Center. “Even under the electron microscope, the molecules are moving around by a tiny amount, but when you take a picture of something moving, it comes out blurry. With such a movie, we can align the frames of the movie to generate a sharper image.”
The researchers also approximately calculated 70,000 individual images of STRA6 and generated a three-dimensional model of the protein molecule, which was used to construct a highly intricate atomic model which represented even the smallest details.
Although STRA6 enables vitamin A to pass through to enter the cell, there is no channel in STRA6 like most other transporter molecules. Instead, vitamin A enters the top of STRA6 but then appears poised to exit through a side window that opens directly into the cell membrane, not the cell interior.
Though this needs to be verified, the mechanism may be a way to protect cells from too much vitamin A. “Vitamin A is actually somewhat toxic,” says Dr Mancia. “Trapping vitamin A inside the membrane may keep control of the amount inside the cell.” The new model of STRA6 advances the understanding of a critical cellular function and may help researchers understand how other, still mysterious cellular components, work.
Using this newfound knowledge scientists can monitor how vitamin A through supplements is absorbed into the system, if at all. People with vitamin A deficiency can be diagnosed better. The prospect of individuals with a higher susceptibility to excessive vitamin A absorption can be healed by engineering the STRA6 molecules in the body so that their absorption rate can be slowed down.
Since excessive vitamin A in the body can cause many problems such as dizziness, nausea, headaches, coma and even death. High intakes of preformed vitamin A in pregnant women can also cause birth defects in their babies. Women who might be pregnant should not take high doses of vitamin A supplements.
Consuming high amounts of beta-carotene or other forms of provitamin A can turn the skin yellow-orange, but apart from an aesthetical downfall the condition is harmless. High intakes of beta-carotene do not cause birth defects or the other more serious effects caused by getting too much preformed vitamin A.
Since two forms of vitamin A are available in the human diet: preformed vitamin A (retinol and its esterified form, retinyl ester) and provitamin A carotenoid. It is extremely important that people should be more aware of its significance. Preformed vitamin A is found in animal based dietary products, including dairy products, fish, and meat (especially liver).
By far the most important provitamin A carotenoid is beta-carotene; other provitamin A carotenoids are alpha-carotene and beta-cryptoxanthin. The body converts these plant pigments into vitamin A.
Both provitamin A and preformed vitamin A must be metabolized intracellularly to retinal and retinoic acid, which are the active forms of vitamin A, to support the vitamin’s important biological functions. Other carotenoids found in food, such as lycopene, lutein and zeaxanthin, are not converted into vitamin A.