Short Peptides as Selective Chelating Agents for Lead Detoxification

Abstract

Lead (Pb) is the most distributed toxic metal associated with significant negative impacts on human health and the ecological system. As a result, no blood Pb level (BLL) is deemed safe for humans, and any quantity is considered toxic. One-third of the children worldwide are poisoned with Pb, and half of the adults in the United States of America (USA) have elevated BLL. Furthermore, approximately one million deaths yearly are caused directly by Pb. The clinically approved chelating agents to treat Pb poisoning suffer from several disadvantages, mainly due to their poor metal selectivity. Thus, they are dispensed at a very high BLL of 45 μg dL-1, leaving the majority of the cases without medical interventions. Nature evolved short peptides and proteins to tackle the issue of metal toxicity. Inspired by such systems and encouraged by the exceptional properties of peptides, we designed and developed two families of cyclic tetrapeptides and a family of linear tripeptides to detoxify Pb2+ ions, the most abundant and toxic oxidative state of Pb. The first family was designed based on head-to-tail cyclization of all-α linear tetrapeptides. Despite many attempts, we were unable to synthesize the desired cyclic products due to the massive ring strain. Upon increasing the ring size by replacing two α-amino acids with β-amino acids in the second family, the desired products were formed exclusively. Assessing the Pb detoxification capabilities of the studied compounds based on recovering Pb- poisoned bacterial culture (E. coli DH5α) and Pb-poisoned human cells (HT-29 cell line) revealed the potential of two candidates to surpass the state-of-the-art drugs in Pb detoxification with distinguished Pb binding affinities and selectivity. Furthermore, studying the capacity of one of these peptides in recovering Pb-exposed mice disclosed its outstanding in vivo detoxification ability. We also investigated the ability of immobilized cyclic tetrapeptides to filter out and remediate Pb2+ ions. Two peptides that exhibited outstanding results in Pb detoxification were linked to a photo- cleavable Tentagel resin as the solid support. These devices showed high efficiency to capture Pb2+ ions from contaminated media with high metal selectivity while being also recyclable. Lastly, we examined the Pb-detoxification capabilities of three analogs of glutathione (GSH), in which cysteine and γ-glutamic acid residues in GSH were replaced with the non-canonical amino acids β-mercapto-aspartic acid and γ-mercapto-glutamic acid, respectively. This study concluded the significance of non-canonical amino acids in enhancing the Pb-binding affinity and selectivity of such analogs compared to the natural peptide, exhibited by their improved abilities to recover Pb-contaminated human cells. Overall, this thesis presents the successful harnessing of short peptides to eradicate and detoxify Pb2+ ions and opens a new avenue towards developing ideal Pb chelating and remediating agents to fill the urgent need for proper technologies.