INVESTIGADORES
DANTOLA Maria Laura
congresos y reuniones científicas
Título:
Inactivation of tyrosinase photoinduced by pterin
Autor/es:
M. LAURA DÁNTOLA; ALDANA D. GOJANOVICH; ANDRÉS H. THOMAS
Lugar:
Cordoba
Reunión:
Congreso; XI Encuentro Latinoamericano de Fotoquímica y Fotobiología (XI ELAFOT); 2012
Institución organizadora:
Universidad Nacional de Río Cuarto
Resumen:
Solar radiation induces modifications to different biomolecules and is
implicated in the generation of human skin cancers. In particular, UV-A
radiation (320-400 nm) can induce damage to DNA and other macromolecules
through photosensitized reactions [1]. This indirect action may be mediated by
endogenous or exogenous photosensitizers and can take place through different
mechanisms [2]. Although the photosensitized damage to DNA is well
characterized, much less is known about proteins and very little has been
studied on inactivation of enzymes caused by photosensitized processes.
Pterins are a family of heterocyclic compounds widespread in living
systems. Currently,
it is known that pterins are able to oxidize DNA [3] and nucleotides [4]
through photosensitizing processes. However, its action on proteins has not
been studied yet.
Vitiligo
is a skin disease that causes lack of pigmentation due to inactivation of
enzymes in the biosynthesis of melanin (melanogenesis). It has been demonstrated that in the skin of
these patients there is accumulation of pterin derivatives with high levels of
hydrogen peroxide (H2O2) [5]. Tyrosinase is a copper-containing glycoprotein that in mammals catalyzes
the first and rate-limiting step in melanin biosynthesis. Although
it is recognized that pterins may be involved in the pathophysiology of
vitiligo, no studies on the ability of these molecules to photoinduce the inactivation
of enzymes of melanogenesis have been published.
To
investigate this, aqueous solutions containing the enzyme and the
photosensitizer (pterin (Ptr)) were exposed to UV-A (lexc = 350 nm) for
different period of time (pH = 6.5, 25 °C). After irradiation
under different experimental conditions, the enzyme activity was determined.
The method used consisted in monitoring the increase in absorbance at 475 nm,
corresponding to the formation of L-Dopacroma as a function of reaction time. The
enzyme activity was calculated from the slope of the absorbance vs. time curve [6].
The obtained results
indicated that Ptr can fotoinactivate the tyrosinase, suggesting that the
reaction starts with an electron transfer from the enzyme to the triplet
excited state of the photosensitizer (3Ptr*),
thus generating
the radical cation of the enzyme (Tyr?+) and
radical anion of the photosensitizer (Ptr?−).
Tyr?+ can undergo oxidation which lead to the irreversible
inactivation of the enzyme. Ptr?− can reduce the dissolved oxygen (O2)
to generate superoxide anion (O2?-). In
light of the overall data obtained, the following scheme was proposed to
explain the mechanism of photoinactivation of tyrosinase by pterin.
References:
[1]. Cadet, J.; Douki, T. J. Invest. Dermatol. 131, 1005, 2011.
[2].
Cadet, J.; Sage, E.; Douki, T. Mutat. Res. 571, 3, 2005.
[3]. Ito K.; Kawanishi S. Biochem.
36, 1774,1997.
[4]. Petroselli, G.; Dántola, M. L.; Cabrerizo, F. M.; Capparelli, A.
L.; Lorente, C.; Oliveros, E.; Thomas A. H. J.
Am. Chem.
Soc. 130, 3001, 2008.
[5].Schallreuter, K. U.; Wood, J. M.; Pittelkow, M. R.; Gutlich,
M.; Lemke, K. R.; Rodl, W.; Swanson, N. N.; Hitzemann, K.; Ziegler I. Science, 263, 1444, 1994.
[6]. Pomerantz, S. H.; Li, J. P. C. Method
Enzimol. 17,
620. 1970.