We have inserted a fourth protein ligand into the zinc coordination polyhedron of carbonic anhydrase II (CAII) that increases metal affinity 200-fold (Kd = 20 fM).The three-dimensional structures of threonine-199 -- nu-cleophile. The engineering of an additional protein ligand represents a general approach for increasing protein-metal affinity if the side chain can adopt a reasonable conformation and achieve inner-sphere zinc coordination. Moreover, this structure-assisted design approach may be effective in the development of high-sensitivity metal ion biosensors.One goal of metalloprotein (re)design is to develop a rationale for the de novo construction of high-affinity metal ion sites in proteins that serve particular catalytic, regulatory, or structural functions. To date, modest progress has been reported in the construction of a-helical bundles (1, 2) and antibodies (3-5) that bind transition metals with micromolar affinity. Additionally, regulatory metal-binding sites have been created in trypsin (6) and glycogen phosphorylase (7) and characterized by x-ray crystallography. We currently report the design and x-ray crystallographic structure determination of a carbonic anhydrase II (CAII) variant with femtomolar zinc affinity; enhanced affinity is achieved by insertion of an additional protein ligand into the metal coordination polyhedron. This strategy was guided by previous work with threonine-199 --cysteine (T199C) CAII, in which a 4-fold enhancement of zinc affinity was achieved; here, the engineered cysteine side chain displaces zinc-bound solvent to form a H3C-Zn2+ site (8, 9).The zinc-binding site of CAII is an outstanding paradigm for structure-assisted design experiments since this metalloprotein is easily manipulated at the genetic level, is highly overexpressed in Escherichia coli, and is readily crystallized for high-resolution x-ray crystallographic structure determinationThe publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.(8-13). The 1.54-A resolution crystal structure of CAII (14) Inspired by the zinc-binding behavior of the T199C CAII variant, modeling studies suggested that substitution of amino acids with longer side chains at position 199 would further enhance zinc binding. Therefore, we replaced T199 with the potential metal ligand residues of aspartate, glutamate, and histidine. The three-dimensional structures of threonine-199 -_ aspartate (T199D) and threonine-199 --glutamate (T199E) CAlls reveal the successful addition of a fourth protein ligand to zinc, and metal affinity is enhanced 200-fold for the T199E CAII variant. However, the structure of threonine-199 -> histidine (T199H) CAII reveals that the engineered histidine residue does not coordinate to zinc and metal affinity is compromised 20-fold. Not surprisingly, the catalytic activity of all three CAII variants is decreased by more than three orders of magnitu...

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An integrated approach to existing methods of extracting biodistribution data, pharmacokinetics and radiation absorbed dose estimates from serial scintigraphic images is described. This approach employs a single computer-generated user interface to reformat planar scans into a standard file type, align conjugate (anterior and posterior) images, draw regions of interest (ROIs) over selected organs and lesions and generate count data for anterior and posterior views and calculated geometric means. Using standard correction methods, the fraction injected activity is obtained for all ROIs and total body. This methodology has been applied to the analysis of indium-III-labelled breast-cancer-directed antibodies and technetium-90m-labelled CEA-specific antibody fragments in non-small-cell lung cancer. It is anticipated that this approach will be useful for evaluating the dosimetry of other radiolabelled monoclonal antibodies, as well as other radiopharmaceuticals.

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