Tryptic digestion of the serum immunoglobulin of the flounder, Platichthys flesus

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Abstract
Purified serum immunoglobulin (Ig) from the flounder has been fragmented by treatment with trypsin at 56°C. The major product of the digestion is a protein which resists further degradation over a two-hour period. This fragment constitutes approximately 40% of the Ig molecule and on fully reducing SDS electrophoresis is composed of a single polypeptide chain of apparent molecular weight 33 kD. This is larger than the light chains and therefore indicates that the fragment is derived only from the heavy chains of the Ig molecule. If this fragment is composed of eight polypeptide chains, consistent with the tetrameric nature of the flounder Ig, then its molecular weight is 264 kD. Although its electrophoretic mobility on 4% non-reducing SDS gels suggests a molecular weight of 180 kD, densitometric data support the 264 kD value. The kinetics of the digestion indicate that the first event is the liberation of a 50 kD fragment which is a dimer of two 25 kD polypeptides and which may be a Fabµ fragment.

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