George Pairas

Structural features of angiotensin-I converting enzyme catalytic sites: conformational studies in solution, homology models and comparison with other zinc metallopeptidases.

Angiotensin-I Converting Enzyme (ACE) is a Zinc Metallopeptidase of which the three-dimensional structure was unknown until recently, when the X-ray structure of testis isoform (C-terminal domain of somatic) was determined. ACE plays an important role in the regulation of blood pressure due to its action in the frame of the Renin-Angiotensin System. Efforts for the specific inhibition of the catalytic function of this enzyme have been made on the basis of the X-ray structures of other enzymes with analogous efficacy in the hydrolytic cleavage of peptide substrate terminal fragments. Angiotensin-I Converting Enzyme bears the sequence and topology characteristics of the well-known gluzincins, a sub-family of zincins metallopeptidases and these similarities are exploited in order to reveal common structural elements among these enzymes. 3D homology models are also built using the X-ray structure of Thermolysin as template and peptide models that represent the amino acid sequence of the ACEs two catalytic, zinc-containing sites are designed and synthesized. Conformational analysis of the zinc-free and zinc-bound peptides through high resolution 1H NMR Spectroscopy provides new insights into the solution structure of ACE catalytic centers. Structural properties of these peptides could provide valuable information towards the design and preparation of new potent ACE inhibitors.

Enzymatic stability, solution structure, and antiproliferative effect on prostate cancer cells of leuprolide and new gonadotropin‐releasing hormone peptide analogs

Analogs of GnRH, including [DLeu6, desGly1o]-GnRH-NHEt (leuprolide, commercial product), have been widely used in oncology to induce reversible chemical castration. Several studies have provided evidence that, besides their pituitary effects, GnRH analogs may exert direct antiproliferative effects on tumor cells. To study the effect of modifications in positions 4 and 6 of leuprolide on prostate cancer cell proliferation, we synthesized 12 new leuprolide analogs. All GnRH analogs lacked the carboxy-terminal Gly10-amide of GnRH, and an ethylamide residue was added to Pro9. Gly6 was substituted by DLys, Nepsilon-modified DLys, Glu, and DGlu. To improve the enzymatic stability, NMeSer was incorporated in position 4, and the rate of hydrolysis by alpha-chymotrypsin and subtilisin was investigated. Our results demonstrate that this incorporation increases enzymatic stability in all analogs of GnRH, whereas the antiproliferative effect on PC3 and LNCaP prostate cancer cells is similar to that of leuprolide. Conformational studies were performed to elucidate structural changes occurring on substitution of native residues and to study structure-activity relationship for these analogs. The solution models of [DLeu6, desGly10]-GnRH-NHEt (leuprolide), [NMeSer4, DGlu6, desGly10]-GnRH-NHEt, [Glu6, desGly10]-GnRH-NHEt, and [DGIu6, desGly10]-GnRH-NHEt peptides were determined through two-dimensional nuclear magnetic resonance spectroscopy in dimethylsulfoxide. Nuclear magnetic resonance data provide experimental evidence for the U-turn-like structure appeared in all four analogs, which could be characterized as beta-hairpin conformation. The most stable analog [NMeSer4, DGlu6, desGly10]-GnRH-NHEt against proteolytic cleavage forms a second extra backbone turn observed for residues 1-4.

Structural studies and cytotoxicity assays of “aggregation‐prone” IAPP8–16 and its non‐amyloidogenic variants suggest its important role in fibrillogenesis and cytotoxicity of human amylin

Amyloid deposits to the islets of Langerhans are responsible for the gradual loss of pancreatic β‐cells leading to type II diabetes mellitus. Human mature islet amyloid polypeptide (hIAPP), a 37‐residue pancreatic hormone, has been identified as the primary component of amyloid fibrils forming these deposits. Several individual segments along the entire sequence length of hIAPP have been nominated as regions with increased amyloidogenic potential, such as regions 8–20, 20–29, and 30–37. A smaller fragment of the 8–20 region, spanning residues 8–16 of hIAPP has been associated with the formation of early transient α‐helical dimers that promote fibrillogenesis and also as a core part of hIAPP amyloid fibrils. Utilizing our aggregation propensity prediction tools AmylPred and AmylPred2, we have identified the high aggregation propensity of the 8–16 segment of hIAPP. A peptide analog corresponding to this segment was chemically synthesized and its amyloidogenic properties were validated using electron microscopy, X‐ray fiber diffraction, ATR FT‐IR spectroscopy, and polarized microscopy. Additionally, two peptides introducing point mutations L12R and L12P, respectively, to the 8–16 segment, were chemically synthesized. Both mutations disrupt the α‐helical properties of the 8–16 region and lower its amyloidogenic potential, which was confirmed experimentally. Finally, cytotoxicity assays indicate that the 8–16 segment of hIAPP shows enhanced cytotoxicity, which is relieved by the L12R mutation but not by the L12P mutation. Our results indicate that the chameleon properties and the high aggregation propensity of the 8–16 region may significantly contribute to the formation of amyloid fibrils and the overall cytotoxic effect of hIAPP.

Cytogenetic and antineoplastic effects of modified steroidal alkylators.

INTRODUCTION The aim of this study was to design new potentially antineoplastic agents by combining nitrogen mustard with steroidal skeleton, in an effort to improve specificity and simultaneously to reduce systemic toxicity. The steroidal part is aimed to act as a biological platform enabling the alkylating moiety to approach its site of action by altering its physicochemical properties. MATERIALS AND METHODS The compounds tested have, as alkylating agents, either p-N,N-bis(2-chloroethyl)aminophenyl-butyrate or p-N,N-bis(2-chloroethyl)aminophenyl-acetate esterified with a modified steroidal nucleus. The four newly synthesized compounds were compared on a molar basis, regarding their ability to induce sister chromatid exchanges and modify proliferation rate indices in cultured human lymphocytes. Life span of BDF1 mice inoculated with L1210 leukemia was also estimated (antileukemic activity). RESULTS A compound having p-N,N-bis(2-chloroethyl)aminophenyl-acetate as the alkylator and two ketone groups in the steroidal part demonstrated the highest statistically significant enhancement of sister chromatid exchanges and suppression of proliferation rate indices, and also caused significant antineoplastic activity. The other compounds proved less active. CONCLUSION These results suggest that cytogenetic and antileukemic activity of alkylating steroidal esters depends on the configuration of the whole molecule and the appropriate combination of the alkylator with the steroidal molecule.

Synthesis and cytogenetic studies of structure--biological activity relationship of esters of Hecogenin and aza-homo-Hecogenin with N,N-bis(2-chloroethyl)aminocinnamic acid isomers.

The esters of Hecogenin and aza-homo-Hecogenin with N,N-bis(2-chloroethyl)aminocinnamic acid isomers have been prepared and their cytogenetic studies of structure-biological activity relationship were evaluated. The cytogenetic effects (sister chromatid exchanges (SCEs) induction and proliferation rate indices (PRIs) depression) by o-, m- and p-[N,N-bis(2-chloroethyl)amino] cinnamic acid were also investigated. Among the above compounds tested, those of the m-[N,N-bis(2-chloroethyl)amino] cinnamic acid and of the o-[N,N-bis(2-chloroethyl)amino] cinnamic acid ester of aza-homo-Hecogenin were more active in comparison to the others.

H-Bond: Τhe Chemistry-Biology H-Bridge

Abstract H‐bonding, as a non covalent stabilizing interaction of diverse nature, has a central role in the structure, function and dynamics of chemical and biological processes, pivotal to molecular recognition and eventually to drug design. Types of conventional and non conventional (H−H, dihydrogen, H‐ π, CH‐ π, anti‐ , proton coordination and H−S) H‐bonding interactions are discussed as well as features emerging from their interplay, such as cooperativity (σ‐ and π‐) effects and allostery. Its utility in many applications is described. Catalysis, proton and electron transfer processes in various materials or supramolecular architectures of preorganized hosts for guest binding, are front‐line technology. The H‐bond–related concept of proton transfer (PT) addresses energy issues or deciphering the mechanism of many natural and synthetic processes. PT is also of paramount importance in the functions of cells and is assisted by large complex proteins embedded in membranes. Both intermolecular and intramolecular PT in H‐bonded systems has received attention, theoretically and experimentally, using prototype molecules. It is found in rearrangement reactions, protein functions, and enzyme reactions or across proton channels and pumps. Investigations on the competition between intra‐ and intermolecular H bonding are discussed. Of particular interest is the H‐bond furcation, a common phenomenon in protein‐ligand binding. Multiple H‐bonding (H‐bond furcation) is observed in supramolecular structures.

NMR conformational properties of an Anthrax Lethal Factor domain studied by multiple amino acid-selective labeling

NMR-based structural biology urgently needs cost- and time-effective methods to assist both in the process of acquiring high-resolution NMR spectra and their subsequent analysis. Especially for bigger proteins (>20 kDa) selective labeling is a frequently used means of sequence-specific assignment. In this work we present the successful overexpression of a polypeptide of 233 residues, corresponding to the structured part of the N-terminal domain of Anthrax Lethal Factor, using Escherichia coli expression system. The polypeptide was subsequently isolated in pure, soluble form and analyzed structurally by solution NMR spectroscopy. Due to the non-satisfying quality and resolution of the spectra of this 27 kDa protein, an almost complete backbone assignment became feasible only by the combination of uniform and novel amino acid-selective labeling schemes. Moreover, amino acid-type selective triple-resonance NMR experiments proved to be very helpful.

Structure-activity studies of lGnRH-III through rational amino acid substitution and NMR conformational studies.

Lamprey gonadotropin‐releasing hormone type III (lGnRH‐III) is an isoform of GnRH isolated from the sea lamprey (Petromyzon marinus) with negligible endocrine activity in mammalian systems. Data concerning the superior direct anticancer activity of lGnRH‐III have been published, raising questions on the structure–activity relationship. We synthesized 21 lGnRH‐III analogs with rational amino acid substitutions and studied their effect on PC3 and LNCaP prostate cancer cell proliferation. Our results question the importance of the acidic charge of Asp6 for the antiproliferative activity and indicate the significance of the stereochemistry of Trp in positions 3 and 7. Furthermore, conjugation of an acetyl‐group to the side chain of Lys8 or side chain cyclization of amino acids 1‐8 increased the antiproliferative activity of lGnRH‐III demonstrating that the proposed salt bridge between Asp6 and Lys8 is not crucial. Conformational studies of lGnRH‐III were performed through NMR spectroscopy, and the solution structure of GnRH‐I was solved. In solution, lGnRH‐III adopts an extended backbone conformation in contrast to the well‐defined β‐turn conformation of GnRH‐I.

Further studies on the antineoplastic activity of homo-aza-steroidal esters.

The modified steroidal alkylating agents, 17 beta-hydroxy-3-aza-A-homo-4 alpha-androsten-4- one(p-[bis(2-chloroethyl)amino]phenyl)butyrate(1),3 alpha-hydroxy- 13,17-seco-5 alpha- butyrate(2),3 beta-hydroxy-13,17-seco-5-androsten-17-oic- 13,17-lactam(p-[bis-(2-chloroethyl)amino]phenyl)butyrate(3) and and (p-[bis(2-chloroethyl)amino]phenyl)butyric acid(4) have been tested against L1210, P388, Ehrlich ascites tumors (EAT), Lewis lung (LL) carcinoma and adenocarcinoma CA-755. Of four compounds evaluated in L1210 leukemia, none displayed antileukemic activity. Almost all of the four compounds were more or less active against P388 leukemia. Compound 2 possesses a slight antitumor activity in EAT, while only compound 1 appears to be active in LL carcinoma. The antitumor activity of the three modified steroidal esters on adenocarcinoma CA-755 seems to be interesting.

Synthesis of a New nor-Aza-Steroidal Ester of p-N,N-bis-(2-Chloroethyl)aminophenylbutyric Acid and in vitro Study of Its Mutagenicity and Clastogenicity

A new nor-aza-steroidal ester of chlorambucil has been synthesized. The study of the mitotic index in CHO and HeLa cells treated with this compound showed that it may be a cytostatic drug. It was also found that treatment of CHO cells with a dose as low as 5 micrograms/ml induces a large number of sister chromatid exchanges. A great number of abnormal metaphases has been observed when CHO cells were treated with the compound at a dose of 25 micrograms/ml. When the compound was tested in the Ames/Salmonella microsome assay, it was found to be mutagenic in strains TA100 and TA1535, both with and without metabolic activation.