Inhibitory potencies of compounds were determined according
to an established procedure using a commercially available aromatase
test kit from BD Gentest.33 This fluorescence-based assay measures
the rate at which recombinant human aromatase
(baculovirus/insect cell-expressed) converts the substrate 7-methoxy-
trifluoromethylcoumarin (MFC) into a fluorescent product
(kex = 409 nm, kem = 530 nm) in a NADPH regenerating system.
Briefly, concentrated stock solutions of test compounds were prepared
in acetonitrile. 100 lL samples containing serial dilutions
of test compounds (dilution factor of 3 between samples) and
cofactor mixture (0.4 U/mL glucose-6-phosphate dehydrogenase;
16.2 lM NADP+; 825 lM MgCl2; 825 lM glucose-6-phosphate;
50 lMcitrate buffer, pH 7.5) were prepared in a 96 well plate. After
incubating the plate for 10 min at 37 C, 100 lL of an aromatase/
P450 reductase/substrate solution (105 lg protein/mL enzyme;
50 lM MFC; 20 mM phosphate buffer, pH 7.4) were added to
each well. The plate was covered and incubated for 30 min at
37 C. 75 lL of 0.5 MTris base were then added to stop the reaction
and the fluorescence of the formed de-methylated MFC was measured
with a plate reader (SpectraMax Gemini, Molecular Devices).
Fluorescence intensities, which were proportional to the
amount of reaction product generated by aromatase, were graphed
as a function of inhibitor concentration and then fit to a 3-parameter
logistic function. Inhibitory potencies were expressed in terms
of the IC50 value, the inhibitor concentration necessary to reduce
the enzyme activity by half. Each experiment was performed at
least in triplicate.
In cases in which the intrinsic fluorescence of a test compound
overlapped with the fluorescence of the reaction product, the
substrate MFC was substituted by dibenzylfluorescein (DBF) which
was converted by aromatase to fluorescein (kex = 485 nm;
kem = 530 nm).41 The protocol was the same as the one above, except
for a longer incubation time (2 h) and the use of a different
stop reagent (2 M NaOH).