p-Alkoxyphenols, a new class of inhibitors of mammalian R2 ribonucleotide reductase: possible candidates for antimelanotic drugs.
The inhibition by different p-alkoxyphenol derivatives of the growth-regulating enzyme ribonucleotide reductase (RR) in purified Escherichia coli and mouse R2 protein preparations was studied by EPR spectroscopy. The inhibitor-induced inactivation of the catalytic subunit protein R2 was measured at 77 degrees K by observing the decrease of the typical EPR signal from the functionally essential protein-linked tyrosyl free radical. p-Methoxy-, p-ethoxy-, p-propoxy-, and p-allyloxyphenol were about 2 orders of magnitude more effective in inhibiting mouse R2, compared with E. coli R2. Among the p-alkoxyphenols studied, p-propoxyphenol was the most effective inhibitor of mouse R2 (IC50, 0.7 microM) and p-methoxyphenol was the least effective (IC50, 11 microM); p-ethoxy- and p-allyloxyphenol were intermediate. The observed half-maximal inhibition values characterized p-alkoxyphenols as a new class of strong inhibitors of the R2 protein of mammalian RR. p-Propoxy-, p-ethoxy-, and p-allyloxyphenol could be considered as new candidates for anticancer drugs. A special cellular inhibition assay of RR in proliferating tumor cells, in which the tyrosyl radical of R2 at natural concentration was monitored by EPR, showed that the four para-substituted alkoxyphenols also inhibited the enzyme with high efficiency in tumor cells (IC50, between 0.5 microM and 5 microM). Our results with inactivation of protein R2 of RR imply that the cytostatic effect of p-alkoxyphenols on melanoma cells, which has been hitherto explained by inhibition of tyrosinase [Melanoma Res. 2:295-304 (1992)], may be caused at least partly by inhibition of RR. Protein R2 of RR may be considered as an additional target that could be used for future cancer chemotherapy.
Pötsch
S
S
Max Delbrück Center of Molecular Medicine, Berlin-Buch, Germany.
Drechsler
H
H
Liermann
B
B
Gräslund
A
A
Lassmann
G
G
D000818:Animals; D002286:Carcinoma, Ehrlich Tumor; D004578:Electron Spin Resonance Spectroscopy; D004926:Escherichia coli / Q000201:enzymology; D006131:Growth Inhibitors; D066298:In Vitro Techniques; D008545:Melanoma / Q000188:drug therapy; D051379:Mice; D012264:Ribonucleotide Reductases / Q000037:antagonists & inhibitors* / Q000737:chemistry
D006131:Growth Inhibitors; D012264:Ribonucleotide Reductases