Year

2003

Degree Name

Doctor of Philosophy (PhD)

Department

Department of Biomedical Science (Metabolic Research Centre) - Faculty of Health & Behavioural Sciences

Abstract

The basal metabolic rate (BMR) or energy turnover of animals varies dramatically, being several fold higher in endotherms compared to ectotherms, and much greater, on a mass-specific basis, in smaller vertebrates compared to larger vertebrates. Despite this large variation in metabolic rate between vertebrate species, a significant and relatively constant proportion of metabolism is associated with membrane-linked energy consuming processes, regardless of the absolute level of BMR. The majority of these membrane-linked processes are mediated by membrane-bound enzymes, and in general, membranes with increased levels of polyunsaturation are associated with elevated activity in these processes and subsequently increased metabolism. It has therefore been suggested that membranes, specifically their amount and composition, may be playing a role in determining the pace (or rate) of metabolism, via an effect on the molecular activity of membrane-bound proteins. In this thesis, the relationship between membrane lipid composition and molecular activity of the sodium pump (Na(superscript +)K(superscript +)ATPase) has been assessed via comparisons of a large range of different animals. The sodium pump was chosen as a representative protein, as it is ubiquitous, has been well characterised, and accounts for a major proportion of resting energy metabolism (20%). Furthermore recent comparisons of endotherms and ectotherms, have suggested that the molecular activity of the sodium pump is highly dependent on the lipid composition of the surrounding membrane bilayer. The aims of this study were firstly to examine sodium pump molecular activity and membrane lipid composition in tissues of mammals and birds of different body size. The mammalian species examined ranged in body mass 7500-fold and had an 11-fold difference in mass-specific BMR, while the avian species ranged 3000-fold and 22-fold for body mass and mass-specific BMR respectively. These species were chosen to try and maximise metabolic differences, and represented an ideal model to examine whether variations in membrane lipid composition may have been determining the metabolic activity of these vertebrates, through an effect on membrane-bound proteins (i.e. sodium pump). The second major aim of this study was to examine sodium pump molecular activity and membrane lipid composition in tissues from two ectothermic species, the bearded dragon lizard, and the octopus. These species were chosen as the bearded dragon has been shown to have very monounsaturated membranes, which are typical of ectotherms, while membranes from the octopus tend to be very polyunsaturated. Sodium pump molecular activity has been shown to be low in a large range of ectotherms and this study was designed to examine whether the high level of polyunsaturation in octopus membranes was associated with an increased molecular activity in their sodium pumps. In the mammals and birds membrane fatty acid composition showed substantial variation, with higher unsaturation index (number of double bonds per 100 fatty acid chains) observed in heart and kidney phospholipids from the smaller species. In these tissues the most important finding was the significant and substantial allometric decline observed in the content of the highly polyunsaturated docosahexaenoic acid (22:6 n-3). Brain phospholipids from the mammals and birds however, showed no allometric trends and were highly polyunsaturated (especially 22:6 n-3) in all species. Sodium pump molecular activity was generally higher in all three tissues from the smaller mammals, while in the birds higher molecular activity was observed in the hearts of smaller birds, while no allometric trends were seen in the kidney and brain. There were no differences observed in sodium pump molecular activity between the tissues of the bearded dragon and the octopus, in spite of the fact that the octopus had very polyunsaturated membranes. A significant contributing factor however, may have been that the octopus membranes contained high levels of cholesterol, which has been shown to have an inhibitory effect on the sodium pump. To determine the relationship between sodium pump molecular activity and membrane lipid composition, linear correlation coefficients were determined between lipid parameters and sodium pump molecular activity, using a combined data set from all fifteen species and six tissues examined in the current study, plus literature values for tissues from the ectothermic cane toad. While several lipid parameters were correlated with molecular activity, the most significant relationship was observed for docosahexaenoic acid (22:6 n-3), with high concentrations of this fatty acid associated with high sodium pump molecular activity. This fatty acid provided not only the strongest correlation for the combined data set, but was also significantly correlated within the mammals, birds and ectotherms, indicating that it may potentially play an important role in metabolism. Further research is required to determine the mechanistic basis of this relationship, however the physical properties of this fatty acid may be a major contributing factor.

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