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Abstract
The effect of DOPA and glutathione (GSH) on enzyme systems for 5-S-cysteinyl-DOPA (5SCD) genesis in murine melanoma cells cultured in tyrosine- and
cystine-free medium were studied. DOPA at its optimum concentration (10−5 M) when added alone did not alter tyrosinase, glutathione-S-transferase or γ-glutamyl transpeptidase activities. In the presence of GSH at its
optimum concentration (10−5 M), DOPA loading did not cause any significant changes in tyrosinase or glutathione-S-transferase (GST) activities. This indicates that the higher 5SCD levels observed
in the medium because of DOPA loading in the GSH dependent system results from increased
substrate availability rather than the increased enzyme activity. An acute drop in
5SCD at DOPA concentrations above 10−5 M observed in the GSH dependent system may be due to the inhibition of tyrosinase
at high substrate concentrations (10−4 M). Conversely, in the presence of DOPA, when GSH was increased, the resultant higher
production of 5SCD could be explained by the increased activity of GST. When added
alone, GSH (10−5 M) caused a significant increase in GST (≈125%) and γ-GTP (≈50%) activities. A drop
in 5SCD in the medium when GSH was added beyond its optimum concentration (10−5 M) in the DOPA-dependent system could be due to competitive inhibition of γ-GTP by
GSH.
The data demonstrate that 5SCD genesis may be enhanced due to the accumulation of
cytotoxic melanin precursors such as ![Math Eq]()
quinone. The relative quantities of GSH at the sites of DOPA quinone formation and
the levels of its metabolising enzymes can influence the type of product formed. Although
this explains the importance of GST in elevated 5SCD production, the inverse relationship
of γ-GTP with 5SCD indicates a complex regulatory influence of melanin precursors
on this enzyme in melanoma cells.
quinone. The relative quantities of GSH at the sites of DOPA quinone formation and
the levels of its metabolising enzymes can influence the type of product formed. Although
this explains the importance of GST in elevated 5SCD production, the inverse relationship
of γ-GTP with 5SCD indicates a complex regulatory influence of melanin precursors
on this enzyme in melanoma cells.Keywords
Abbreviations:
GSH (glutathione), 5SCD (5-S-cysteinyl-DOPA), GST (glutathione-S-transferase), γ-GTP (γ-glutamyl transpeptidase), DQ (DOPA quinone), cys-DOPA (cysteinyl-DOPA), glu-DOPA (glutathionyl-DOPA), GR (glutathione reductase), MEM (minimum essential medium), FCS (fetal calf serum), PBS (phosphate buffered saline), PPB (potassium phosphate buffer), CDNB (1 chloro-2,4 dinitro benzene), ΔE (changes in absorbance)To read this article in full you will need to make a payment
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Article info
Publication history
Accepted:
May 23,
1991
Received:
September 25,
1990
Identification
Copyright
© 1991 Published by Elsevier Inc.
