Cold pepsin digestion: a novel method to produce the Fv fragment from human immunoglobulin M.

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Abstract
A strategy for the proteolytic fragmentation of human Ig[immunoglobulin]M was developed. This method is called cold pepsin digestion because of its unique feature of achieving restricted peptic cleavages at 4.degree. and pH 4.0. Cold pepsin digestion was applied successfully to produce an Fv fragment from 14 human IgM proteins. The Fv fragment consists of the H chain variable domain (VH) and the L chain variable domain (VL) held together by strong noncovalent interaction. Thus, each Fv fragment contains 1 intact antigen-binding site and represents the minimal active fragment derivable from an antibody molecule. A series of other structurally and functionally important fragments were also isolated and characterized. Two basic digestion pathways were recognized; these mainly reflect the relative accessibility of 5 sets of major interdomain cleavage sites.