I got something very unexplainable during the ADP-Glucose
Pyrophosphorylase Assay.
The samples which i analysed were collected from the cotton (1-2
flowers open) in greenhouse, including stem, hypocotyl and taproot.
All those samples were sliced to small disks and storage in -80 degree
freezer.

The extraction solution which includes 50mM HEPES,pH7.4,5mM MgCl2, 2mM
DTT (DL-Dithiothreitol), 1mL EDTA and 0.1%Triton-X is 1mL for each
sample.

The final Assay Solution which includes 50mM HEPES,pH8.0,10mM MgCl2,
2mM DTT (DL-Dithiothreitol), 5mM ADP-glucose ,0.4mM NAD ,1.8mM PPi
(Tetrasodium pyrophosphate), 2 units phosphoglucomutase , 2 units
glucose-6-phosphate dehydrogenase and 0.05mL extraction solution.

Final 0.9mL assay solution were placed in a Acryl-cuvette (Sarstedt)
before initiated by 0.1mL 18mM PPi.As soon as i mixed the PPi with the
solution together, sometimes there were percipition in the cuvette ,
heavily or little, sometimes there were no percipition in the cuvette
at all. Everything is the same but the extracted sample. I cannot
explain why this happened. If i mixed the solution in a glass test
tube, there always had percipiton in the test tube!

can anyone give me some clues to figure it out?