Parenteral nutrition (PN) is a method of feeding those incapable of absorbing nutrients from the gastrointestinal tract. All required nutrients are combined in one "big bag". Consequently, many chemical interactions are possible between components. Ascorbic acid (AA) is ubiquitous to both animal and plant kingdoms. Although its biochemistry is not fully understood, dietary deficiency is detrimental to well being, with the most extreme condition being scurvy. AA is water-soluble and frequent intake is therefore required to maintain nutritional status. AA is possibly the most reactive additive in PN mixtures, readily reacting with dissolved oxygen, initially producing dehydroascorbic acid (OHAA). OHAA retains the biological activity of AA. It was the purpose of this study to further knowledge regarding stability of AA and OHAA in PN mixtures, informing pharmaceutical practice to improve safety and efficacy of PN. A stability-indicating HPLC method was optimised for the study of AA and OHAA in PN mixtures. A study of the kinetics of OHAA degradation was undertaken to provide data that could be used to predict OHAA stability. Results obtained indicated a first order reaction. In direct contrast to AA degradation, trace elements did not catalyse OHAA degradation. A further product of AA degradation is oxalic acid (OA) which is potentially toxic. A HPLC method for the determination of OA in PN mixtures was developed and validated, although minimum quantification limits were relatively high (~10J.Lg/ml).The method was used to assess OA appearance in stored PN mixtures, with results indicating that concentrations remained below 10J.Lg/ml even after 35 days storage. The final aspect of this research was to investigate the most likely components of a PN mixture which may "protect" AA from oxidation. a-tocopherol photo-oxidises and therefore may compete with AA for oxygen. As light catalyses the reaction it is possible oxygen reacts more rapidly with a-tocopherol compared with AA. Results indicated 0.- Tocopherol did not oxidise in preference to AA and therefore offered no "protection". Cysteine is a reducing agent included in some amino acid preparations. The average dissolved oxygen content of standard adult PN mixtures was determined, from which the amount of cysteine required to react with dissolved oxygen was calculated. AA instability in PN mixtures was compared with and without cysteine. Results indicated that adding cysteine to PN mixtures 24 hours before addition of AA, resulted in retention of >95% AA. Results obtained from this study have furthered knowledge of the AA degradation profile, its kinetics and the potential influence of other components in PN mixtures. In particular potential strategies for minimising AA degradation are identified therefore ensuring patients receive quantities approaching those prescribed.

Vitamin C (ascorbate and dehydroascorbic acid) is vital for plants and found throughout the plant cell including in the apoplast. The structure of ascorbate was determined eighty years ago; however, many of its degradation pathways remain unclear. Numerous degradation products of ascorbate have been reported to occur in the apoplast but many still remained unidentified. Ascorbate is well known as an antioxidant, and acts to quench reactive oxygen species (ROS), such as hydrogen peroxide and ozone in the plant apoplast. The immediate oxidation product of ascorbate is dehydroascorbic acid (DHA), which may be quickly hydrolysed to diketogulonic acid (DKG). The further reactions of radiolabelled and non-radiolabelled DHA and DKG with various ROS have been investigated. Differences were observed in the products formed from the various ROS, allowing a unique fingerprint of oxidation products to be described for each ROS. Equally, different compounds were produced depending on the starting substrate; for example cyclic oxalyl threonate was only observed in the reactions of DHA and not DKG. A major oxidation product of DHA is OxT. A novel enzyme activity involving the transfer of the oxalyl group from OxT to an acceptor substrate such as a sugar has been detected. This enzyme activity could have potential cell wall modification roles, in the formation of oxalate cross-linkages between cell wall components. This would provide a novel role for ascorbate derivatives in cell growth. Vitamin C is also a vital component of the human diet, and most dietary ascorbate comes from plants such as salads. The degradation of ascorbate during post-harvest processing and storage of salad leaves has been investigated. Spinach leaves were found to be particularly prone to losing ascorbate during the industrial washing process. The use of radiolabelled ascorbate has allowed the determination that the major degradation product formed from ascorbate during spinach washing was oxalate.