All the preparative steps were carried out at 4 C with
continuous stirring. Skin pieces were treated with 0.1 N
NaOH to remove non-collagenous proteins at a solid to
solution ratio of 1:10 (w/v) for 3 days and washed with
distilled water till the washed water became neutral or
slightly basic pH. The alkali solution was changed every
day. Then, the sample was defatted with 10% butyl alcohol
at a solid to solvent ratio of 1:10 (w/v) for 24 h,
washed with ample amount of distilled water and lyophilized.
The lyophilized matter was extracted with 0.5 M
acetic acid for 3 days. Then, 10% of pepsin (1:10,000
units, Junsei Chemical Co. Ltd., Tokyo, Japan), according
to the lyophilized weight was added and hydrolysed
for 48 h. The viscous solution was centrifuged at
12,000 · g for 1 h at 4 C and supernatant was collected.
The supernatant was salted out by adding NaCl to a final
concentration of 0.7 M and followed by precipitation
by addition of NaCl to a final concentration of
2.3 M in 0.05 M Tris–HCl (pH 7.5). The resultant precipitate
was separated by centrifugation at 12,000 · g
for 1 h at 4 C. The precipitate was then dissolved in
0.5 M acetic acid, dialysed against 0.1 M acetic acid, distilled
water, and lyophilized.
2.4. Sodium dodecyl sulphate polyacrylamide gel