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.

Vitamin A has diverse biological functions and is essential for human survival. STRA6 is the high-affinity membrane receptor for plasma retinol binding protein (RBP), the principle and specific carrier of vitamin A (retinol) in the blood. It was previously shown that STRA6 couples to lecithin retinol acyltransferase (LRAT) and cellular retinol binding protein I (CRBP-I), but poorly to CRBP-II, for retinol uptake from holo-RBP. STRA6 catalyzes both retinol release from holo-RBP, which is responsible for its retinol uptake activity, and the loading of free retinol into apo-RBP, which can cause retinol efflux. Although STRA6-catalyzed retinol efflux into apo-RBP can theoretically deplete cells of retinoid, it is unclear to what extent this efflux happens and in what context. We show here that STRA6 can couple strongly to both CRBP-I and CRBP-II for retinol efflux to apo-RBP. Strikingly, pure apo-RBP can cause almost complete depletion of retinol taken up by CRBP-I in a STRA6-dependent manner. However, if STRA6 encounters both holo-RBP and apo-RBP (as in blood), holo-RBP blocks STRA6-mediated retinol efflux by competing with apo-RBP's binding to STRA6 and by counteracting retinol efflux with influx. We also found that STRA6 catalyzes efficient retinol exchange between intracellular CRBP-I and extracellular RBP, even in the presence of holo-RBP. STRA6's retinol exchange activity may serve to refresh the intracellular retinoid pool. This exchange is also a previously unknown function of CRBP-I and distinguishes CRBP-I from LRAT.

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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