Preparation of F(ab')2 mu fragments from rat IgM monoclonal antibodies and their application to the enzyme immunoassay of mouse interleukin-6.

F(ab')2 fragments, herein designated as F(ab')2 mu, were prepared from rat IgM monoclonal antibodies (mAbs). The IgM was digested at a pepsin-to-IgM ratio of 1:200 (w/w) in 100 mM citrate buffer (pH 4.5) at 37 degrees C for 2 h. During digestion, the light (L) chain (27 kDa) of IgM remained undegraded, whereas the heavy (H) chain disappeared and two new bands of 44 and 48 kDa appeared. The digests were fractionated by means of hydrophobic interaction HPLC with TSKgel Phenyl-5PW. The fraction containing F(ab')2 mu was homogeneous and the recovery of antigen-binding activity was 41-52%. The molecular mass of F(ab')2 mu was estimated to be 147-153 kDa, and we concluded that the fragment was composed of two truncated H chains and two intact L chains. F(ab')2 mu was used in an enzyme immunoassay of mouse interleukin-6 and the interaction of IgM with non-specific proteins was greatly reduced, when it was converted to F(ab')2 mu fragments.