Why some fish don’t tan
Although Homo sapiens bask for hours in the sun slathered with sunscreen to block the injurious effects of ultraviolet (UV) light, the zebrafish ( Danio rerio ) and other vertebrates species have evolved a more resourceful way to protect themselves from the sun’s harmful rays. In a recent paper, Osborn et al . describe the discovery of an endogenous biosynthetic pathway in vertebrates that produces UV-specific protective compounds mycosporine-like amino acids (MAAs) and the related gadusols.

Bacteria and algae were shown previously to produce such protective compounds, which had also been detected in fish but were believed to be acquired by the consumption of MAA- and gadusol-producing unicellular organisms. However, the team identified—in zebrafish, several other types of fish, amphibians, and birds—two genes that encode enzymes known to catalyze the synthesis of these compounds. They cloned the genes, expressed them in Escherichia coli , and incubated the recombinant proteins with the substrate sedoheptulose 7-phosphate. Subsequent identification of a catalytic product (2-epi-5-epi-valiolone) that contained a cyclohexane ring confirmed that the bacterially expressed zebrafish protein was a functioning sugar phosphate cyclase, a key enzyme in the production of gadusol. The authors then extracted gadusol directly from zebrafish, demonstrating the species’ capacity to fully synthesize the compound. Last, the authors inserted the cloned zebrafish genes into yeast expression vectors and detected and isolated gadusol from the yeast culture medium.

The new work sheds light on a biosynthetic mechanism of sun protection found in multiple species and also might present an enlightened opportunity to develop a small-molecule sunscreen.

A. R. Osborn et al ., De novo synthesis of a sunscreen compound in vertebrates. eLife 4 , e05919 (2015). [[Full Text]][1]

 [1]: http://elifesciences.org/content/4/e05919