Metal ions and complexes that hydrolyze peptides and proteins have become increasingly important in recent years. These reagents have shown great promise for use in a variety of applications including protein sequencing and proteomics. When metal-assisted hydrolytic cleavage is accomplished under nondenaturing conditions of temperature and pH, their use can be extended to include the study of protein function and solution structure, the generation of semisynthetic proteins, the proteolytic cleavage of bioengineered fusion proteins, and therapeutics. Yet, because of the extreme stability of the peptide amide bond, hydrolytically active metals are limited in number and there is now great interest in the development of new, more efficient reagents. In this review, we provide a description of relevant, early work with metal ions and complexes that have been used to hydrolyze unactivated peptide amide bonds in peptides and proteins. More importantly, we present an overview of recent contributions that have been made toward the development of synthetic metalloproteases that catalyze hydrolysis under near physiological conditions of temperature and pH.Dedicated to Professor Koji Nakanishi on the occasion of his 80 th birthday.

Five opsin cDNA clones were isolated from a goldfish retina cDNA library and sequenced. On the basis of homology to previously characterized visual pigments, one clone was identified as goldfish rod opsin and a second as a goldfish red cone opsin. Two rhodopsin-like clones were found to be similar to the chicken green opsin, a pigment which shares properties with both rod and cone pigments. A fifth clone was equally homologous to human blue cone opsin and human rod opsin. In order to characterize the spectral properties of the encoded pigments, the five clones were expressed in tissue culture cells and the apoproteins reconstituted with 11-cis-retinal. The wavelength of maximal absorption for goldfish rhodopsin is 492 nm and for the fifth pigment, identified as the goldfish blue pigment, 441 nm. Pigments encoded by the two rhodopsin-like clones absorb at 505 and 511 nm and are likely to correspond to the goldfish green pigment previously characterized by microspectrophotometry. The putative red cone opsin cDNA may encode a pigment that is a polymorphic variant of goldfish red since it absorbs maximally at 525 nm.

4,13-Diaza-18-crown-6 substantially increases the rate of zirconium(IV) hydrolysis of unactivated peptide amide bonds under near-physiological conditions of temperature and pH. In the presence of this azacrown ether, ZrCl(4) efficiently hydrolyses both neutral and negatively charged peptides (pH 7.0-7.3, 37-60 degrees C).

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The biological role of selenium is a subject of intense current interest, and the antioxidant activity of selenoenzymes is now known to be dependent upon redox cycling of selenium within their active sites. Exogenously supplied or metabolically generated organoselenium compounds, capable of propagating a selenium redox cycle, might therefore supplement natural cellular defenses against the oxidizing agents generated during metabolism. We now report evidence that selenium redox cycling can enhance the protective effects of organoselenium compounds against oxidant-induced DNA damage. Phenylaminoethyl selenides were found to protect plasmid DNA from peroxynitrite-mediated damage by scavenging this powerful cellular oxidant and forming phenylaminoethyl selenoxides as the sole selenium-containing products. The redox properties of these organoselenoxide compounds were investigated, and the first redox potentials of selenoxides in the literature are reported here. Rate constants were determined for the reactions of the selenoxides with cellular reductants such as glutathione (GSH). These kinetic data were then used in a MatLab simulation, which showed the feasibility of selenium redox cycling by GSH in the presence of the cellular oxidant, peroxynitrite. Experiments were then carried out in which peroxynitrite-mediated plasmid DNA nick formation in the presence or absence of organoselenium compounds and GSH was monitored. The results demonstrate that GSH-mediated redox cycling of selenium enhances the protective effects of phenylaminoethyl selenides against peroxynitrite-induced DNA damage.