Conjugation of metallothionein to a murine monoclonal antibody

In vitro characterization of ligand-induced oligomerization of the S. cerevisiae G-protein coupled receptor, Ste2p

BACKGROUND

The S. cerevisiae alpha-factor receptor, Ste2p, is a G-protein coupled receptor that plays key roles in yeast signaling and mating. Oligomerization of Ste2p has previously been shown to be important for intracellular trafficking, receptor processing and endocytosis. However the role of ligand in receptor oligomerization remains enigmatic.

METHODS

Using functional recombinant forms of purified Ste2p, atomic force microscopy, dynamic light scattering and chemical crosslinking are applied to investigate the role of ligand in Ste2p oligomerization.

RESULTS

Atomic force microscopy images indicate a molecular height for recombinant Ste2p in the presence of alpha-factor nearly double that of Ste2p alone. This observation is supported by complementary dynamic light scattering measurements which indicate a ligand-induced increase in the polydispersity of the Ste2p hydrodynamic radius. Finally, chemical cross-linking of HEK293 plasma membranes presenting recombinant Ste2p indicates alpha-factor induced stabilization of the dimeric form and higher order oligomeric forms of the receptor upon SDS-PAGE analysis.

CONCLUSIONS

alpha-factor induces oligomerization of Ste2p in vitro and in membrane.

GENERAL SIGNIFICANCE

These results provide additional evidence of a possible role for ligand in mediation of Ste2p oligomerization in vivo.

Metal-binding chimeric antibodies expressed in Escherichia coli

Metallothionein, a well-characterized biological chelator of metals, has been genetically fused to the binding domain of an antibody and expressed in the periplasm of Escherichia coli. Specific delivery of 109Cd to immobilized hapten or to haptenated cells was demonstrated directly in periplasmic extracts. This approach is potentially useful for targeted radiotherapy and diagnostic imaging. We find six to seven atoms of metal per active antigen-combining site. Absence of the Fc portion of the immunoglobulin along with low immunogenicity of metallothionein-metal complexes should reduce immunologic reactions.

Recombinant antibody-metallothionein: design and evaluation for radioimmunoimaging

We have produced a chimeric antibody (Ab) in which metallothionein, a well-characterized biological chelator of metals, was genetically fused to the F(ab') domain of the S107 Ab heavy chain. Coexpression with the Ab light chain that conveys specificity for the synthetic antigen phosphocholine was achieved in plasmacytoma cells. Metal- and antigen-binding domains of the Ab-metallothionein hybrid function with normal avidity and specificity. Ab-metallothionein can be efficiently loaded with 99mTc and used to specifically bind phosphocholine-haptenated cells in vitro or to localize plasma-cell ascites tumors in mice. The approach offers potential advantages for producing radiolabeled Ab for targeted radiotherapy and diagnostic imaging.

Gram scale purification and preparation of rabbit liver zinc metallothionein

A simplified procedure for purifying gram quantities of rabbit liver metallothionein (MT) using gel filtration and anion exchange chromatography is presented. The MT purification made use of anion exchange batch elution chromatography which greatly shortened the procedure. Quantitation techniques for use with crude and purified MT are discussed. This paper also describes the preparation of large amounts of ZnMT from Cd,ZnMT.

Direct labeling of proteins with 99mTc

The direct labeling of antibodies and antibody fragments to form a highly stable bond between technetium and the sulfide groups of proteins is now well established. To optimize this reaction, the antibody protein must have sufficient reactive sulfides available to accept that technetium metal ions that are formed by the reduction of pertechnetate in the presence of a weak complexing agent. The reactive sulfide groups are provided by first reducing a small fraction of the disulfide bridges in the antibody protein or by starting with Fab' fragments, which already have reactive sulfide groups. When the antibody protein has been appropriately reduced, and the reactive sulfide groups protected by a metal ion with a lower binding affinity than technetium, such as tin or zinc, very high labeling yields of high-affinity-bonded 99mTc can be achieved. This can be accomplished without loss of immunoreactivity, measured as either affinity or immunoreactive fraction. Side reactions can produce radiochemical impurities such as low-affinity, bound 99mTc; 99mTc colloids; 99mTc peptides or antibody aggregates; or 99mTc-complexes. Also, pertechnetate ions may be an impurity if the sodium pertechnetate solution added to the reduced antibodies is not completely reduced. The specifics of minimizing these side reactions have not been extensively discussed in the prior literature; however, it is clear that appropriate reduction of the protein prior to labeling and complete removal of the reducing agent, particularly if it contains reactive sulfide groups or is toxic, are critical. One- or two-step 99mTc-labeling kits for preparing 99mTc-labeled antibody or antibody fragments are rapidly being introduced for use in clinical nuclear medicine studies. These direct labeling methods employ a common sequence of chemical reactions, although the reducing agents for both the antibody and the [99mTc]pertechnetate may vary. Different 99mTc transfer agents may be used, but all transfer agents have the common feature of quickly forming weak to moderately strong complexes with reduced technetium. Most use Sn(II) to reduce the pertechnetate, although other reducing agents can be used.

The expression of a synthetic rainbow trout metallothionein gene in E. coli

A synthetic gene for rainbow trout metallothionein was constructed and inserted into a dual origin plasmid where expression was induced by a temperature shift in a proteinase-deficient strain of Escherichia coli. The recombinant protein was purified to homogeneity, and a partial amino acid sequence was determined to confirm its identity. Its immunochemical characteristics were similar to those of native metallothionein from rainbow trout. The amounts of recombinant metallothionein produced were quantified in soluble cell extracts by ELISA. Low concentrations were detected when growth was performed either in L-broth or defined (GMM-II) medium. Supplementation of the medium with zinc or copper had no effect on the amount of metallothionein produced. By contrast, when cadmium was included in either L-broth or GMM-II medium, much higher concentrations of the protein within the cells (approx. 13 micrograms/mg soluble cell protein) were detected. This stabilisation of the protein by metal reconstitution in vivo is considered in relation to the selective uptake/exclusion of metals by the cells and its significance for the scavenging of certain precious or toxic heavy metals is discussed.

Recent developments in the radiolabeling of antibodies with iodine, indium, and technetium

The use of radiolabeled antibodies for tumor detection and therapy has provided some striking successes despite unfavorable tumor-to-normal tissue radioactivity ratios due, in part, to the accumulation of the label in normal tissues. One approach, which has been and is still under consideration to reduce unwanted background levels, is the improvement of ways in which radiolabels are attached to antibody, especially with the goal of increasing in vivo stability. Although these improvements have occurred throughout the history of this field, important developments have recently been reported in the labeling of antibodies with three radiolabels, namely radioiodine, 111In, and 99mTc. Thus, antibodies may now be labeled with radioisotopes of iodine in ways that minimize the extent of in vivo dehalogenation leading to thyroid, stomach, and gut radioactivity uptake. Newer and stronger chelates for 111In have been developed in the hope that their use would result in lower radioactivity levels in the liver. Finally, newer methods, both direct and indirect, for the attachment of 99mTc to antibodies have been developed and are now being clinically tested. Although these developments have taken place only recently and the in vivo behavior of labels attached in these ways have not yet been fully characterized, it is possible to make tentative conclusions regarding their impact. Thus, the use of stably radioiodinated antibodies appears to have resulted in modest improvements in patient images. In contrast, the use of stable chelates for labeling antibodies with 111In may have had no appreciable effect on liver radioactivity levels. The use of antibodies radiolabeled with 99mTc, especially via the newer direct labeling methods, are providing superior images in patients with low radioactivity levels in organs such as liver. However, it must still be established whether the short physical half-life of 99mTc lowers sensitivities and specificities of detection relative to other labels.