Melanins are polymorphous and multifunctional biopolymers with a relatively high concentration of freeradicals. EPR spectroscopy was used to study o-semiquinone free radicals in model eumelanins synthesizedfrom 3,4-dihydroxyphenylalanine (DOPA) and tyrosine in the presence of tyrosinase, and melanins isolatedfrom A-375 and G-361 human melanoma malignum cells exposed to two compounds: 5,7-dimethoxycoumarin(DMC) and valproic acid (VPA). Changes were determined in the concentrations of free radicals in theindividual melanins from tumour cells treated with DMC and VPA. A strong decrease in the concentrationsof free radicals characterizes melanins isolated from tumour cells treated together with DMC and VPA.Slow spin-lattice relaxation processes were noted in the melanins tested with homogeneous broadened EPRspectra. The EPR technique may be useful not only for the elucidation of free radicals in melanins fromA-375 and G-361 cells treated with VPA and DMC but it could also be applied to establish the relationshipbetween melanin type and the malignancy of melanoma malignum
Our results show that betulin and 28-O-propynoylbetulin were effective in inhibition of cell growth and induction of apoptosis in a human melanoma cell line. The addition of the propynoyl group at the C-28 hydroxyl group of betulin led to a greater proapoptotic and antiproliferative effect in comparison to unmodified betulin. These observations suggest that the obtained derivative is a potent anti-melanoma agent.
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