Immunoglobulin fragments, whether of polyclonal or
monoclonal antibodies, offer a number of advantages over the intact
immunoglobulin. The generation of immunoreactive fragments from
monoclonal antibodies (MAb) is not always a straightforward task. Both
pepsin and
papain can be used to digest MAb to a bivalent molecule with a Mr of 100,000. However,
pepsin pepsin digestions does not always result in immunoreactive fragments and a stable consistent product by
papain digestion is often difficult to obtain. MAb
B72.3 is an example of both situations. MAb
B72.3 reacts with a
glycoprotein (TAG-72) with a molecular weight greater than 10(6). MAb
B72.3 has been shown to exhibit a high degree of selective reactivity with colon, breast and ovarian
carcinomas and has been used for radioimmunodiagnosis in model systems and in clinical trials. A third
enzyme,
bromelain, in the same family of sulfhydryl
proteases as
papain, has been used to generate a fragment of MAb
B72.3, with a Mr of approximately 100,000. The
bromelain-generated fragment of MAb
B72.3 retained 100% immunoreactivity as measured in competitive solid-phase radioimmunoassays and could be generated with consistent results from one preparation to another. Both the
bromelain- and
papain-generated fragments were radiolabelled with 125I without significant loss of the MAb's reactivity to
tumor extracts. Differences were observed between the
bromelain- and
papain-generated fragment when compared in vivo. Fragmentation of MAb
B72.3 with
bromelain has yielded a superior bivalent fragment for radioimmunolocalization.