Affinity labeling of the active sites of antibodies and myeloma proteins

A study on the trastuzumab conjugation at tyrosine using diazonium salts

Herein, we report on the conjugation of trastuzumab with 2,5-difluorobenzene diazonium tetrafluoroborate.

Iodoazidobenzylpindolol, a photoaffinity probe for the beta-adrenergic receptor

A high-affinity pindolol derivative, (+/-)-1-(indol-4-yloxy)-3-[1-(p-azido-m-iodophenyl)-2-isobutylamine]-2-propanol (IABP), has been prepared; it contains an iodide and an azide functional group and acts as a photoaffinity label for the beta-adrenergic receptor. When [125I]IABP (specific activity 1300 Ci/mmol) was photolyzed with crude duck erythrocyte membrane preparations, which contain beta-adrenergic receptor binding sites highly specific labeling of two polypeptides was observed upon electrophoresis on sodium dodecyl sulfate/polyacrylamide gels. These two polypeptides, A (Mr approximately 45,000) and B (Mr approximately 48,500), and B (Mr approximately 48,500), were photolabeled in a ratio of approximately 4:1 A/B. Binding of [125I]IABP and covalent derivatization of the beta receptor was at least 70% specific. The data indicate that [125I]IABP is a very effective compound for identification of polypeptides containing the beta-receptor binding site even in crude membrane preparations. The beta receptor of the duck erythrocyte plasma membrane may be composed of more than one subunit.

Photoaffinity labeling of the ouabain-binding site on (Na+ plus K+) adenosinetriphosphatase

An ethyl diazomalonyl derivative of cymarin was synthesized in order to photoaffinity label the cardiac glycoside-binding site on (Na(+) + K(+)) adenosinetriphosphate (EC 3.6.1.3). When a noncovalent complex of the enzyme and this cardiac glycoside derivative was photolyzed, a covalent bond was formed between the ligand and the larger of the two polypeptide subunits of the enzyme. Several control experiments demonstrate that this photochemical reaction occured while the ligand was bound to the site at which it inhibits the enzyme activity. Another specific inhibitor, tentatively identified as the ethyl chloromalonyl derivative of cymarin, produced similar photoaffinity labeling of the larger subunit, demonstrating that the photolytic dissociation of the diazo group may not be responsible for the photochemical reaction. Since the cardiac glycoside-binding site, which is accessible from the outside surface of the plasma membrane, and the phosphorylation site, which is accessible from the inside surface, are both on the larger polypeptide subunit of (Na(+) + K(+)) adenosinetriphosphatase, this polypeptide has sequences exposed to both sides of the membrane.

Three-dimensional structure of the Fab' fragment of a human immunoglobulin at 2,8-A resolution

The structure of the Fab' fragment of a human myeloma immunoglobulin was determined by x-ray crystallographic analysis at 2.8-A resolution. The Fourier map of the electron density was correlated with the aminoacid sequence to obtain a three-dimensional model. Four globular subunits, which correspond to the homology regions of the light and heavy chains, are arranged in a tetrahedral configuration. These subunits closely resemble each other, sharing a basic pattern of polypeptide chain folding. In each subunit, long sequences of tightly packed, hydrogen bonded polypeptide chain run parallel to the major axis of the subunit. No helical conformation can be seen. Different patterns of interchain disulfide linkage and unusual intrachain disulfide bonds that have been observed in other immunoglobulins can be explained with this model. The regions of hypervariable sequences in the light and heavy chains occur at one end of the molecule, in close spatial proximity.