HIV-protease assay kits (Bachem Feinchemikalien AG, Bubendorf,
Switzerland) were used. Twenty five microliter of HIV-protease
assay buffer (50 mM NaOAc, pH 4.9) containing 2.5 lg of the
substrate [His-Lys-Ala-Arg-Val-Leu-(pNO2-Phe)-Glu-Ala-Nle-Ser-
NH2] was mixed with 2 ll of a compound solution (using DMSO
as a solvent), then 8 ll of recHIV-protease (0.01 mg/ml) was added.
The reaction mixture was incubated for 30 min at 37 C and then
terminated by addition of 3.0 ll of 10% trifluoroacetic acid (TFA).
The hydrolysate (pNO2-Phe-Glu-Ala-Nle-Ser-NH2) and the remaining
substrate were quantitatively analyzed by reversed-phase
HPLC. HPLC conditions: column, COSMOSIL Packed 5C18-MS-II,
4.6 150 mm NACALAI TESQUE.INC. Kyoto, Japan; solvent, gradient
CH3CN: H2O (containing 0.1% TFA) (20–40%); flow rate,
1.0 ml/min; detector, UV 208 nm. The substrate and the hydrolysate
were eluted at 8.1 and 3.5 min, respectively. The HIV-protease
inhibitory activity of a compound was calculated as follows:
% inhibition ¼ ðAcontrol AsampleÞ 100=Acontrol
where A is relative peak area of the hydrolysate.
Oleanolic acid was used as a positive control, its IC50 being
24.8 lg/ml.