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-r egulating enzyme ribonucleotide reductase (RR) in purified Escherichia coil and mouse R2 protein preparations was studied by EPR spectroscop y. The inhibitor-induced inactivation of the catalytic subunit protein R2 was measured at 77 degrees K by observing the decrease of the typi cal EPR signal from the functionally essential protein-linked tyrosyl free radical. p-Methoxy-, p-ethoxy-, p-propoxy; and p-allyloxyphenol w ere about 2 orders of magnitude more effective in inhibiting mouse R2, compared with E. coil R2. Among the p-alkoxyphenols studied, p-propox yphenol was the most effective inhibitor of mouse R2 (IC50, 0.7 mu M) and p-methoxyphenol was the least effective (IC50, 11 mu M); p-ethoxy- and p-allyloxyphenol were intermediate. The observed half-maximal inh ibition values characterize p-alkoxyphenols as a new class of strong i nhibitors of the R2 protein of mammalian RR. p-Propoxy-, p-ethoxy-, an d p-allyloxyphenol could be considered as new candidates for anticance r drugs. A special cellular inhibition assay of RR in proliferating tu mor cells, in which the tyrosyl radical of R2 at natural concentration was monitored by EPR, showed that the four para-substituted alkoxyphe nols also inhibited the enzyme with high efficiency in tumor cells (IC 50, between 0.5 mu M and 5 mu M). Our results with inactivation of pro tein R2 of RR imply that the cytostatic effect of p-alkoxyphenols on m elanoma cells, which has been hitherto explained by inhibition of tyro sinase [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 additio nal target that could be used for future cancer chemotherapy.