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Digestion of antibody using pepsin - no F(ab)2 was found (Aug/24/2008 )

I want to remove the Fc part of Antibody(IgG against S. aureus) by digestion using pepsin, in order to get the F(ab)2 fragment.
After 24 h incubation at 37 oC, I checked the digested products using SDS-PAGE (8%)
To my disappointment, neither F(ab)2 or Fc fragments were found on the gel. In fact, no bands appeared on the gel.
I repeated this experiment twice. But no bands were found.
I'm afraid that the antibody was digested to small fragments, so they run out of the gel.
But Why? this digestion method is often used to produce F(ab)2 fragment. WHy doesn't it work for my antibody?
could anyone give me some suggestion?
Thanks a lot.

The detailed procedure:
1. Sample: 1 mg antibody (150K) in 1.2 ml PBS buffer (0.83 mg/ml, 5.56 nmol/ml). Staphylococcus aureus Monoclonal Antibody (from mouse, IgG3, Catalog No.: 15704).
2. 0.6 ml antibody solution is dialyzed against 1L 0.1M sodium acetate buffer (pH4.2, containing 0.1M NaCl) at 4 oC overnight.
3. Add 1 mg pepsin (34.62K) to the solution, and incubate at 37oC for 24h.
4. Stop the reaction by adding 0.12 ml 3M Tris–HCl (pH7.5).


take small samples during the digestion process including a control. You should be able to optimize the process and see the changes.