The design and synthesis of inhibitors of the enzyme poly (adenosine diphosphate ribose) polymerase and the investigation of the mechanism of action of the suspected ultimate carcinogen chloroacetaldehyde

Abstract

This thesis describes the preparation of inhibitors of the enzyme poly(adenosine diphosphate ribose)polymerase (PARP) and the mechanism of action of the carcinogen vinyl chloride. Initially the enzyme's function, purification and its known inhibitors are discussed. The enzyme's cofactor, nicotinamide adenine diphosphate (NAD+), is described in terms of its function in PARP and other enzymes, and its crystal structure and solution conformation(s). The high field proton and carbon-13 NMR spectra of this compound are fully assigned using two dimensional NMR experiments (COSY and ROESY) and selective spin decoupling. Using similar techniques the high field proton and carbon- 13 NMR spectra of the reduced analogue, NADH, are assigned. This NMR data showed that the conformation of the 1,4- dihydropyridine ring of this molecule was a dynamic equilibrium mixture of two boats. However, it was discovered that these boat conformations were unequally populated and a rationale for this phenomenon is given. The rationale of enzyme inhibitor design with respect to PARP is described and the subsequent preparation and potencies of PARP inhibitors are both described and discussed. These included the preparation of 3-methoxycyclohexane carboxamide which was synthesised to test a hypothesis of the nature of PARP inhibitors. Partially saturated substituted benzamide systems were synthesised to determine the degree of unsaturation necessary for inhibition while similar compounds were synthegised by reducing N-alkylnicotinamides. Direct NAD+ analogues were prepared in which an adenine unit was separated by a methylene chain from a 3-oxy substituted benzamide. By varying the length of the methylene chain 3-(9-(12-dodecyloxy)adenine)benzamide was shown to inhibit the enzyme. Amongst these inhibitors were the unique compounds benzoxazole-4-carboxamides which were found to be the most potent inhibitors to be synthesised during this period. A correlation of the physical properties of literature compounds with their activity is discussed along with the applicability of this technique to PARP and its inhibitors. The metabolic activation and carcinogenesis of vinyl chloride are described. The reaction of chloroacetaldehyde (a metabolite of vinyl chloride) is discussed with a detailed description of its reaction with nucleosides to form the fluorescent etheno compounds, with particular emphasis on the adeninq system. A detailed investigation of the reaction mechanism of chloroacetaldehyde with adenosine was undertaken by determining the intermediates and products of the acid catalysed decomposition of N6-(2,2-dimethoxyethyl)adenosine. An ethenoadenine deuteriation experiment along with the previous experiment gave evidence toward the reaction mechanism of chloroacetaldehyde with adenosine. In summary, the cofactor of PARP NAD+ has been fully characterised by proton and carbon-13 NMR spectroscopy; a unique conformational. property of NADH has been characterised; a new and novel class of potent inhibitors of PARP has been discovered and the reaction mechanism of chloroacetaldehyde with adenosine has been determined.