2.4. Extraction and assay of peptidase activity
Assay of peptidase activitywas performed based on the method
of Nieri et al. (1998). Petals were homogenized in 3mL of ice-cold
extraction buffer (50mmolL.1 Tris.HCl, pH 7.4), whereas 5mL of
extraction buffer was added for columns. The homogenates were
centrifuged at 10,000~g for 10min at 4 .C and the supernatant
was collected and kept at .70 .C until peptidase assay. Duplicates
of each sample in 1.5-mL microcentrifuge tubes containing 400L
incubation buffer (50mmolL.1 Na-acetate, pH5.0 with 0.5% azocasein,
from Fluka), and 200L crude extractwere used for the assay.
Preliminarymeasurements at various pHs showed that pH 5.0 was
close to optimal in the present assay. All sample tubes were incubated
at 37 .C for 24 h. Reactions were stopped by adding 100L
of 50% TCA to sample tubes. Sample and reference tubes were then
incubated on ice for 1 h and centrifuged at 10,000~g for 3min at
4 .C. The supernatantswere alkalinized by adding 100L of 10mol
L.1 NaOH and absorbance was then measured at 492 nm. Peptidase
activity was defined using arbitrary units; one unit is equal to
a change of 0.01 absorbance units per hour at 492 nm.