UV-B light

In recent years, advertisements have tended to sound more technological, and many now proudly claim protection against harmful UV-b light . What does this mean ... 

Vass I (1997) Adverse effects of UV-B light on the structure and function of the photosynthetic apparatus. In Pessarakli M (ed) Handbook of photosynthesis. Marcel Dekker Inc., New York, pp 931-949... 

The precursor, 7-dehydrocholesterol is converted by a non-enzymatic reaction to cholecalciferol (calciol). This reaction occurs in skin exposed to sunlight due to irradiation by UV-B light at a wavelength of about 300 nm. Cholecalciferol is transported via carrier proteins to the liver where hydroxylation at carbon-25 occurs in a reaction catalysed by a microsomal cytochrome P450 hydroxylase to form calcidiol. This compound travels to the kidney attached to specific binding proteins, where another cytochrome P450 enzyme, mitochondrial 1-a-hydroxylase, introduces a second hydroxyl group in to the molecule to form the active calcitriol. 

At certain times of the day, sunlight can be of such intensity that it has effects on the bodies of humans and other animals. Exposure to UV in sunlight can have both beneficial and detrimental side effects for humans, o UV-C is light below 290 nm, UV-B, light between 290 and 320 nm and UV-A 320 00 nm light. 

Ouwerkerk, P. B. F., Hallard, D., Verpoorte, R., and Memelink, J., Identification of UV-B light-responsive regions in the promoter of the tryptophan decarboxylase gene from Catharanthus roseus, Plant Mol. Biol., 41, 491, 1999. 

D.O. Hessen (1994). Daphnia responses to UV-B light. Arch. Hydrobiol Beih., 43, 185-195. 

This case, however, was quite unusual in that the phototoxic reaction was provoked by UV-B light, while hypericin 1 is known to absorb UV-A wave-lengths, and the morphology of the skin reaction was rather atypical. No other case of a phototoxic skin reaction on antidepressant doses of Hypericum extracts has been reported in the literature so far. 

Ozone, helped to a certain extent by diatomic ox en, filters out all of the sun s radiation in the UV-C range, but can only absorb some of the UV-B light in the 290-320 range (look at Figure 22.2). The unabsorbed radiation reaches the surface of the Earth. No component of the atmosphere absorbs UV-A radiation to any great extent, so most of this radiation reaches the Earth s surface. 

Yarush et ai. 159) used liposomes prepared from phosphatidylcholine, cholesterol hemisuccinate. and oleic acid to the skin with a DNA repair enzyme. T4 endonuclease V. Before and after exposure to UV-B light the enzyme was applied on the skin. After 6 h the animals were sacrificed and the DNA dimers counted. It appeared that post- and pretreatment with DNA repair-loaded liposomes resulted in a decrease in the number of dimers. Repealed application did not funher enhance the repair. These results. suggest that the DNA repair at least entered the cells. In a. second study. Yarosh et al. (60) studied the fate of the di-i-labellcd... 

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