XXII. BIOCHEMICKÝ ZJAZD - Jesseniova lekárska fakulta
XXII. BIOCHEMICKÝ ZJAZD - Jesseniova lekárska fakulta
XXII. BIOCHEMICKÝ ZJAZD - Jesseniova lekárska fakulta
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Lectures<br />
FREE raDICAL SITUATION IN PIGMENT CELLS<br />
Jan Borovanský, Adéla Lipšová and Jiří Vachtenheim<br />
Institute of Biochemistry & Experimental Oncology, 1 st Faculty of Medicine,<br />
Charles University, Prague<br />
Biochemical specificity of pigment cells consists in their capacity to synthesize specific<br />
metabolic products – cytoprotective eumelanins and cytotoxic phaeomelanins in the<br />
process of melanogenesis. Melanogenesis represents a potential threat for the pigment<br />
cell because the intermediates belong to cytotoxic species – quinones, semiquinones and<br />
the synthesis of melanins is accompanied by the production of superoxide anions and<br />
H 2<br />
O 2<br />
. For that reason melanogenesis is strictly compartmentalized to melanosomes. The<br />
free radical situation is quite complex because superoxide anions are tyrosinase substrate<br />
and melanin polymer behaves as a pseudosuperoxide dismutase producing H 2<br />
O 2<br />
. In 1991<br />
we demonstrated the presence of aberrant melanosomes with membrane defects (with<br />
subsequent leakage of cytotoxic species) as a common phenomenon in melanoma cells.<br />
Pigment cells are protected by scavenging mechanisms (a) intramelanosomal binding of<br />
cytotoxic species to proteins which can be illustrated by our finding of protein-bound dopa<br />
in melanosomal proteins; b) conversion of quinones into adducts with cysteine and GSH<br />
into cysteinyldopa which is excreted via urine; c) prevention of diphenol conversion into<br />
quinones by COMT. If the capacity of scavenging mechanisms is overcome, pathological<br />
reactions ensue which can be exploited in melanoma therapy: a) Trojan horse approach =<br />
administration of tyrosine and DOPA analogues that are converted by tyrosinase specifically<br />
in pigment cells to cytotoxic molecules; b) inhibition of scavenging mechanisms = using<br />
COMT inhibitors we were able to inhibit proliferation of melanoma cells in vitro but not<br />
in vivo. Tumour proliferation is often free radical burden for the host. To our surprise the<br />
growth of B16 and S91 melanomas in mice and MeLiM melanoma in minipigs was not accompanied<br />
with signs of free radical damage. For that reason we compared the activities<br />
of antioxidant enzymes in tumour cells between 7 human melanoma cell lines and human<br />
osteosarcoma, glioma, colorectal carcinoma, lung carcinoma and neuroblastoma cell lines.<br />
The comparison showed significantly higher (p=0,004) catalase activity in melanoma lines<br />
compared to nonmelanoma lines, whereas there were no significant differences as for<br />
glutathione peroxidase, SOD and γ-glutamyltransferase. The total antioxidant status (TAS)<br />
of melanoma cells was also significantly higher than in nonmelanoma cells (p=0.004).<br />
Correlation between catalase activity and TAS (R=0.909, p level = 0,00004) confirmed that<br />
the defense of melanoma was based on catalase activity. Taking into account the role of H 2<br />
O 2<br />
in cell proliferation, angiogenesis, invasion and metastasizing, apoptosis, the manipulation<br />
of catalase can be promising tool in experimental melanoma therapy.<br />
Ackowledgement: Supported by VZ MSM 21620808.<br />
<strong>XXII</strong>. Biochemistry Congress, Martin<br />
49