Substituted hemins as probes for structure-function relationships in horseradish peroxidase.

Low temperature visible spectra of Compounds I from peroxidases reconstituted with protohemin, 2-formyl-4-vinyldeuterohemin, 2-vinyl-4-formyldeuterohemin, 2,4-dimethyldeuterohemin, and 2,4-diacetyldeuterohemin reveal that these Fe(IV) porphyrin pi-cation radicals take the 2A2u of peroxidase-type electronic ground state. Compound I of deuterohemin horseradish peroxidase, however, takes the 2A1u or catalase type pi-cation radical electronic ground state. Since deuterohemin horseradish peroxidase possesses no catalase activity, the structure of the peroxidase apoprotein (other than those factors which might influence the Compound I pi-cation radical ground state) is concluded to play the major role in determining the reactivity of Compound I toward hydrogen donors. Studies on peroxidases substituted with the hemins 2-formyl-4-vinyldeuterohemin, 2-vinyl-4-formyldeuterohemin, 2,4-dimethyldeuterohemin, and mesohemin reveal that isoelectronic hemins differentially interact with the peroxidase apoprotein. The hemin 2- and 4-substituents are therefore concluded to interact sterically with the horseradish peroxidase apoprotein. While a variety of 2- and 4-substituted hemins were observed to bind rapidly with apo horseradish peroxidase to form active substituted enzymes, small changes in the substituents in the 6- and 7-positions had drastic effects on the rates of binding to apoperoxidase and the activities of the reconstituted enzymes. Even addition of a single methylene to form butyrate instead of propionate side chains drastically altered the rate of binding of the hemin and the activity of the substituted enzyme. It therefore appears that while the 2-, 4-, 6-, and 7-substituents of the hemins in horseradish peroxidase all interact with the protein, the polypeptide chain possesses more conformational flexibility in the area which binds the 2- and 4-substituents.

Evaluation of formamidine sulfinic acid and other reducing agents for use in the preparation of Tc-99m labeled radiopharmaceuticals.

Various reducing agents have been evaluated for their potential usefulness in the preparation of 99mTc labeled radiopharmaceuticals for use in nuclear medicine. Adequate labeling of various radiopharmaceuticals was accomplished using formamidine sulfinic acid. Nitrogen-purging of solutions is not required, which is an advantage for in-house preparation. Tagging requires heating, however, so heat-labile material cannot be used. Various compounds that could not be labeled when stannous chloride was used, could be tagged with 99mTc when formanidine sulfinic acid was used as the reducing agent.

99mTc-bioquin-7CA, a potential new hepatobiliary scanning agent.

The complex 99mTc-Sn-8-hydroxyquinoline-7-carboxylate (99mTc-bioquin-7CA) was investigated in animals as a potential hepatobiliary scanning agent. Following intravenous injection, the complex was found to localize in the liver rapidly and in the gallbladder in 5--15 min; after 15 min it was excreted into the biliary tract with a high degree of specificity. Urinary excretion was negligible, reducing scan interference from the kidneys. Distribution studies in mice compared well withe localization of activity observed in rabbit scans. Preparation of the complex was rapid and simple; stannous ion was used as the reducing agent.