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ABSTRACT
Skin surface lipid fractions represent the first target of ultraviolet (UV) irradiation of
the skin. Following irradiation, squalene (SQ), a peculiar polyunsaturated sebum fraction,
undergoes a rapid and time dependent oxidation and decomposition with generation of
reactive radical by-products, possible mediators of the biological effects of UV-R on
skin. In this connection, SQ decomposition can be viewed as a hallmark of epidermal
oxidative stress. Our work aimed at verifying if a relationship does exist between UV
induced SQ oxidation and protection afforded by commercial UV-B and UVA sunscreen agents
(Octyldimethyl PABA, UV-A filter; Parsol MCX, UV-B filter; Benzophenone-3, UV-B+UVA
filter; UV-B; Parsol 1789, UV-A filter). Moreover, we evaluated if the sunscreens were
able to exert a antioxidant action when subjected to oxidative sources other than UV
irradiation.
We have investigated by GC-MS methods, following different periods of UV exposure (UV-B
and UV-A ranges): the degree of inhibition of peroxidation of SQ and of a tri-unsaturated
fatty acid (a-linolenic acid, LIN), when subjected to different oxidative sources, in
three in vitro experimental models: photo-oxidation, auto-oxidation, chemical
oxidation with an initiator of lipid peroxidation (in the absence of light).
We have measured: a) the rate of SQ and/or LIN decomposition in the presence or in the
absence of sunscreens tested at concentrations and in combinations similar to those
present in cosmetic products; b) sunscreen stability under the same experimental
conditions; c) the possible inhibition exerted by filters on the formation of a stable
byproduct of LIN oxidation (i.e. azelaic acid, AZ).
Depending on concentration and time of irradiation, all sunscreens displayed the capacity
of inhibiting, to different degrees, SQ oxidation. Our results show that sunscreen
efficacy correlates well with the inhibition of SQ oxidation. The protective effect
observed on LIN oxidation was significantly correlated with the inhibition of the
production of AZ (p<0.01). As for AMVN oxidation, all filters tested exerted a weak but
significant protection on LIN, indicating that antioxidant power of sunscreens relies
mainly on their capability of absorbing UV-R, but may in sone cases due to a true
antioxidant action.
Independently of the mechanism of
action, the biological effects of irradiation by-products of sunscreens such as
Octyldimethyl PABA and Parsol 1789, should be carefully investigated, considering the wide
use of these molecules. |
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