XXII. BIOCHEMICKÝ ZJAZD - Jesseniova lekárska fakulta

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