Vitamin D2
Vitamin D2 and its origin/discovery in living organisms.
Prehistoric Perspective
“Life forms began to evolve in the oceans over 1 billion years ago. They took advantage of sunlight and used it as an energy source to generate carbohydrates. Curiously some of the earliest phytoplankton including Emiliania huxleyi (which is a coccolithophore, i.e., has a calcium carbonate exoskeleton) which has existed unchanged in the Sargasso Sea (Atlantic Ocean) for more than 500 million years when exposed to sunlight not only photosynthesized glucose but also produced vitamin D2 (Fig. 1).(1) This phytoplankton produces a large amount of ergosterol that when exposed to sunlight absorbs ultraviolet B (UVB) radiation and undergoes a photolysis reaction to form previtamin D2. Once formed this thermodynamically unstable isomer is transformed into vitamin D2. Similarly yeast and fungi also contain high amounts of ergosterol and when exposed to sunlight produce vitamin D2.(1-4) Although the functions of ergosterol and vitamin D2 are unknown in these primitive unicellular photosynthesizing factories there are at least three possible functions that have been proposed. Ergosterol can efficiently absorb UVB radiation, which would make it an ideal natural sunscreen to protect UVB sensitive macromolecules in the organism including its proteins, RNA and DNA.
After absorbing UVB radiation previtamin D2 is produced. Its absorption spectrum with a wavelength maximum at 260 nm overlaps the UV absorption spectrum for both DNA and RNA and thus would be able to protect DNA and RNA from photodamage (Fig. 2).(2) When previtamin D2 is exposed to UVB radiation it is converted to tachysterol2 which has a UV absorption spectrum with a wavelength maximum at 282 nm which overlaps the UV absorption spectrum for amino acids in proteins that have conjugated double bonds including tryptophan and tyrosine (Fig. 1 and and22).(1-4) Thus early in evolution as organisms began to utilize solar energy for photosynthesis they needed a sun protection factor to absorb solar UVB radiation to minimize damage to UVB sensitive molecules. Ergosterol, previtamin D2, and its photoproducts could have acted as an ideal UVB sunscreen since they could absorb UVB radiation and dissipate its energy by the rearrangement of the double bonds.
It has also been speculated that if ergosterol was principally present in the plasma membrane and contained within the lipid bilayer that this rigid planar structure after exposure to solar UVB radiation would be transformed into a more flexible vitamin D2 molecule that would likely be released into the extracellular space. This process could alter membrane permeability and possibly open up a pore to permit the entrance and exit of ions including calcium. This could be the connection for why vertebrates including humans have depended on sun exposure for the maintenance of their calcium metabolism.”[1]
How is D2 different from D3?