Ph.D., Faculty of Science, University of the Witwatersrand, 2011 / This thesis describes the application pteridine chemistry in various aspects of methodology development and natural product synthesis. The introductory chapter sets the scene by describing naturally occurring pteridines, their applications in biological systems, and recent synthetic strategies. Firstly, Sonogashira coupling reactions employing benzenesulfonyloxy-O-pteridine (27) and terminal alkynes to give various 6-substituted pteridines are described. This methodology allowed for the total synthesis of a natural occurring pteridine, Sepiapterin-C (46). Negishi coupling reactions involving benzenesulfonyloxy-O-pteridine (27) and various Znreagents are also reported. This methodology, representing the first Negishi coupling on a pteridine nucleus, allowed for the introduction of both aryl- and heteroaryl- substituents at the 6- position of the pteridine ring. The use of methanesulfonyloxy-O-pteridine (26) as a coupling partner is also described. Selective deprotection and hydrolysis of the formamidine protecting groups to give either the 6- substituted 2,4-diaminopterine or 2-amino-4-oxo-pteridine (pterin), is described. The synthesized structures are supported by NMR and mass spectral data and melting points where applicable. Novel compounds are verified by NMR spectroscopy, infrared and mass spectrometry.

The effect of carbonylcyanide m-chlorophenylhydrazone (CCCP) on the performance of isolated retrograde-perfused rabbit hearts was investigated in the present study. CCCP was first investigated as the blocking agent of photosynthesis. CCCP is also commonly used as the metabolic inhibitor of energy production. The animals were heparinized and anesthetized, the hearts were quickly removed and perfused with Krebs-Henseleit buffer in Langendorff's mode. Two parallel perfusion systems were used to distinguish the control and the treated heart. Two different concentrations of CCCP, low-dose (0.1 μM) and high-dose (0.5 μM), were used to study the dose-dependent relationship. The hemodynamic parameters used in the present study are aortic pressure (AOP), end diastolic pressure (EDP), peak systolic pressure (PSP), left ventricular developed pressure (LVDP), the rate of change of left ventricular pressure (positive dP/dt & negative dP/dt), and heart rate (HR). In general, CCCP impaired the mechanical performance of the heart by decreasing cardiac contractility. Positive and negative dP/dt were decreased 49.4% and 55.6%, respectively, by the low-dose of CCCP. High-doses of CCCP also decreased positive dP/dt and negative dP/dt by 81.4% and 88.9%, respectively. In addition, low-dose CCCP caused decreases of peak systolic pressure and left ventricular pressure developed by 50.4% and 61.6%. Similarly, high-dose CCCP decreased PSP and LVDP by 74.8% and 92.5%, respectively. The end diastolic pressure was increased 66.8% by low-dose CCCP. CCCP had no significant effects on aortic pressure and the heart rate. In conclusion, CCCP impaired the mechanical performance of the isolated perfused hearts as evidenced by decreasing PSP, LVDP, positive dP/dt and negative dP/dt. This degradation of myocardial performance showed a dose-dependent relationship. CCCP also caused a higher incidence rate of arrhythmia. Because CCCP uncoupled the electron transport from the ATP production in the mitochondria, the present study suggested that the development of contracture and heart failure was due to the energy depletion by CCCP.

Bacterial 3-deoxy-D-arabinoheptulosonate 7-phosphate synthase (DAHPS) is an antimicrobial target. A transition state (TS) mimic inhibitor of DAHPS, DAHP oxime, was developed in our lab, with Ki = 1.5 μM. Despite being a potent inhibitor, some of DAHP oxime’s properties could hinder its efficacy in blocking bacterial growth. One problem was that DAHP oxime bound competitively with respect to the metal cofactor, Mn2+. Identifying the origin of this competition was crucial for in vivo success of DAHPS inhibitors, as the abundance of metal ions in living cells could hinder DAHP oxime’s effectiveness. Mutant enzymes and fragment-based inhibitors demonstrated that the competition originated in interactions involving the O4 hydroxyl of DAHP oxime and residues Asp326, and Cys61. This suggested improved inhibitor designs to avoid metal competition. Another drawback was the highly negatively charged nature of DAHP oxime and other DAHPS inhibitors. The charge hindered cell membrane penetration, and therefore its effectiveness in cells. Truncating DAHP oxime to a fragment reduced its hydrophilicity and charge, while the introduction of fluorine in the oxime’s α-position increased potency. DAHPS-specific bacterial growth inhibition was obtained with this fragment. DAHPS inhibition by high DAHP oxime concentrations resulted in a 15% residual activity. This residual activity would be sufficient for bacterial survival, so two derivatives, DAHP hydrazone and DAHP O-(2-fluoroethyl) oxime that showed complete inhibition were characterized. DAHP hydrazone was successfully co-crystallized with DAHPS. Its 100-fold higher potency relative to DAHP oxime was attributed to an extra water bound in the active site. The inhibitor bound in all four of the enzyme’s subunits. However, the observed subunit asymmetry showed that the subunits communicate with each other and that there is a significant energetic penalty to enforcing full subunit symmetry. / Thesis / Doctor of Philosophy (PhD) / Antibiotic resistance is imposing a growing disease burden on society that the United Nations calls “…a global crisis that we cannot ignore”. Many traditional antibiotics have now lost their efficacy as bacteria increasingly develop mechanisms of resistance. The development of new treatment options is therefore crucial. The first step towards this goal is the characterization of new antimicrobial targets. DAHPS is an enzyme that is exclusively expressed by bacteria, fungi and plants. Its absence in mammalian cells is expected to reduce side effects in humans. DAHPS inhibitors have previously been developed, but none of them has shown sufficient effectiveness in bacterial cell culture. Possible reasons for this failure were, among others, reduced inhibition in the presence of metal ions, low cell membrane penetration and incomplete enzyme inhibition. This thesis examines DAHPS’s properties and provides solutions on how to successfully inhibit this enzyme, to fight antibiotic resistance with new treatment options.

The biological activities of some homeosterically related analogs of dihydrofolic acid have been examined involving pyrimido[4,5-b][l,4]oxazine (8-oxapteridine) derivatives. It is anticipated that these compounds might interfere with the essential intermediary metabolic functions of the vitamin and thus serve as potential chemotherapeutic agents. Preliminary toxicity studies in microbial assay systems were disappointing; however, inhibitory effects were demonstrated in cell free enzyme systems. A comparison of the structure/activity relationships was determined using two folic acid coenzyme systems, dihydrofolate reductase and thymidylate synthetase. The 2-amino-4-hydroxy-6-(substituted)-8-oxapteridines were generally more effective inhibitors than the corresponding 2,4-diamino analogs. The relative biological activity of a series of 2-amino-4-hydroxy-6-ω-phenylalkyl derivatives were examined, and the most active derivative was the 6-phenylethyl analog which appears to function as a mixed-type inhibitor involving partially competitive and partially non-competitive inhibition.

Thesis (Ph. D.)--University of Washington, 2006. / Vita. Includes bibliographical references (leaves 121-140).

The renin-angiotensin system plays a central role in the maintenance of blood pressure. Angiotensin II, the main effector of this system, results from the action of angiotensin-converting enzyme (ACE) on angiotensin I. Angiotensin II, maintains vasomotor tone via its vasoconstrictor action, and also increases salt and water retention by stimulating the release of aldosterone. ACE inhibitors, such as captopril, enalapril and lisinopril, are highly effective in the treatment of hypertension and congestive cardiac failure. Previous studies have suggested that angiotensin converting enzyme (ACE) production may be enhanced during pharmacological inhibition of the enzyme. Little is known, however about the mechanism of this induction. After demonstrating increases in circulating ACE protein in cardiac failure patients receiving the ACE inhibitor captopril, a rat model was used to study this effect. A sensitive enzyme linked immunosorbent assay for rat ACE was developed and a partial cDNA for rat ACE cloned to enable examination of ACE mRNA and protein expression during enzyme inhibition with enalapril. Rat lung ACE mRNA increased by 156% (p<0.05) and ACE protein doubled within 3 hours of administering a single dose of enalapril. Testicular ACE mRNA also increased by 300% (p<0.05) within 2 hours and returned to pretreatment levels by 6 hours. The angiotensin II antagonist saralasin similarly caused a significant (p<0.0001) 800% enhancement of mRNA expression. Aldosterone pretreatment of rats prior to enalapril administration was found to abolish this mRNA induction. These findings indicate that increased ACE expression during inhibition results from reduced levels of angiotensin II with consequent reduced stimulation of the angiotensin 11 receptor and its effects, such as aldosterone release. This suggests that ACE levels are regulated by a negative feedback loop involving the distal components of the renin-angiotensin system, namely angiotensin II and aldosterone. In situ hybridisation and immunohistochemical techniques were employed to localise the site of this inductive response in rat tissue sections. It was found that lung macrophages were markedly induced to produce ACE, as was ACE in seminiferous tubules. ACE induction was also noted in the expected sites of renal tubular epithelium and glomerular tissue. Interestingly, ACE expression was also enhanced in cardiac valves. In these studies it has been conclusively demonstrated that new ACE expression is induced by enzyme inhibitor therapy. A variety of techniques have been developed that will allow futher study of ACE in rat tissues.

No description available.

Call number: LD2668 .T4 1986 C58 / Master of Science / Biochemistry and Molecular Biophysics Interdepartmental Program

published_or_final_version / Anatomy / Doctoral / Doctor of Philosophy

Surimi is a seafood product which is used to manufacture restructured products such as artificial crab and lobster. Surimi is produced from fish fillets by washing to remove sarcoplasmic proteins and increase the concentration of myofibrillar proteins, and mixing with cryoprotectants. A valuable attribute of surimi is its ability to form an elastic gel, the gel network being formed by the myofibrillar proteins of fish muscle. It is generally accepted that the quality of surimi gels is influenced by the activity of endogenous protease which acts on the myofibrillar proteins. The proteases in Pacific whiting surimi (Merluccius productus) are particularly problematic due to their high catalytic activity on muscle myosin. The addition of whey protein concentrate (WPC) to Pacific whiting surimi has been shown to enhance the gel strength of the corresponding products produced from this surimi. The mechanism through which WPC enhances the gel strength of Pacific whiting surimi has not been determined, but it has been suggested that WPC acts to inhibit surimi autoproteolysis. The objective of this study was to determine whether the incorporation of WPC into Pacific whiting surimi inhibits autoproteolysis and/or protects the myosin fraction from proteolytic degradation. The effect of supplementing surimi with WPC, beef plasma protein (BPP) and bovine serum albumin (BSA) on its apparent autoproteolysis activity was determined. Three WPC preparations were tested, WPC 34, 34% protein; WPC 80, 80% protein; and WPC 95, 95% protein. Each of the additives was incorporated at the 1, 2, 3 or 4% level. Proteolysis of surimi and supplemented surimi samples was allowed to occur at 55°C. Proteolytic reaction mixtures were terminated by the addition of trichloroacetic acid (TCA). Proteolytic activity was estimated by measuring the difference in TCA-soluble peptides present in reaction mixtures of paired (identical) samples, one having been incubated at 55°C while the paired sample was kept on ice. Peptides were quantified by the bicinchoninic acid, Lowry, dye-binding and trinitrobenzenesulfonic acid methods. Results based on the different peptide assays were compared in order to asses the reliance of results on specific assay methods. BPP was found to have the most inhibitory activity in the autoproteolysis assays, followed by the WPC preparations and then BSA. Autoproteolysis was completely inhibited by the incorporation of 1% BPP, 3% WPC 80 and 2% WPC 95. The extent of inhibition by the WPC preparations was related to their protein content, the higher the protein content the greater the extent of inhibition per unit weight added to surimi. BSA was not an inhibitor of autoproteolysis under the conditions used in this study. The relative extents of inhibition observed for the different additives were independent of the method used to quantify the soluble peptide products. Each of the additives was also tested for their ability to protect the myosin component of surimi from proteolytic degradation. These experiments were done as described above for the autoproteolysis assays with the exception that following the incubation period a portion of the sample, either surimi or a surimi/additive mixture, was completely solubilized in detergent solution, subjected to SDS-PAGE electrophoresis and visualized by protein staining. In these experiments the additives were incorporated at the 4% level. No apparent degradation of myosin could be detected over a 60 min reaction period for surimi samples that were supplemented with BPP, WPC 80 and WPC 95. In contrast, surimi samples incubated without additive clearly showed a loss of myosin after 15 min reaction period. Some myosin degradation was apparent following the 60 min incubation period for the WPC 34-supplemented surimi. A further experiment was conducted to determine the mechanism through which WPC protects myosin and inhibits autoproteolysis. In this experiment WPC 95 and BPP were separately incubated at 55°C with a crude fish protease preparation, i.e. the reaction mixture approximates that used in the autoproteolysis assays except that it contains no surimi. The results indicate that BPP and WPC 95 behave in a similar manner. However, the results were inconclusive with regard to explaining the additive's mechanism of action. Plausible mechanisms which are consistent with the results are discussed. / Graduation date: 1994