2.7. Evaluation of protein stabilizing effects of hard-to-crystallize
sugar matrices
The protein-stabilizing effects of hard-to-crystallize sugar
(sucrose) matrices as well as sucrose alone were evaluated using
ALP as a model protein. Sucrose and the potential crystallization
inhibitor were dissolved at final concentrations of 50 mg/mL and
1 mg/mL, respectively, in a 10 mM diethanolamine buffer (pH
9.5). An aqueous solution containing the heat-treated sucrose, as
described above, was also prepared. Using these solutions, the
stock solution of ALP (1.2 mg/mL ALP, containing 50% glycerol)
was diluted to a final concentration of 72 ng/mL. Each 100 lL aliquot
of the resulting solution was freeze-dried in the same manner
as described above. The freeze-dried samples were stored in a vacuum
desiccator over silica gel at 75 C for 1 or 3 h. The samples
were then withdrawn and reconstituted in distilled water to measure
the remaining ALP activity. The remaining ALP activity was
measured in the same procedures as was used in our previous
study (Imamura et al., 2014). In brief, 100 lL aliquot of the reconstituted
ALP solution was mixed with 2.9 mL of the reaction mixture
containing 1 M diethanolamine, 0.25 mM MgCl2, and
100 mM p-NPP (pH 10.5). Immediately thereafter, the increase in
absorbance at 405 nm was monitored at 37 C. The storage stabilities
of ALP in the hard-to-crystallize matrices as well as in pure
sucrose matrix were evaluated from the initial increasing rate of
the 405-nm absorbance.
All analyses were performed in duplicate or more for each
sample.