Abstract
Summary The mitochondrion, often referred to as the energetic “powerhouse” of the eukaryotic cell, is a vital component for all mammalian life. The oxygen that is breathed in from the environment is ultimately consumed in mitochondria and in turn mitochondria harness energy used to maintain metabolic and biological function. The health of an individual is intimately related to the health of his or her mitochondria and yet much about the mitochondria is unknown.
Research investigating mitochondrial function and its relationship to both health and disease remains at the forefront of a multitude of different scientific fields. Despite the significance of mitochondrial physiology in maintaining both physical and mental health, however, there is paucity of literature that actually examines mitochondrial respiration directly in vivo or in intact tissue, ex vivo, providing dynamic assessments of electron transfer efficacy and substrate influence on respiration. This thesis will review the origin and evolution of the mitochondrion, summarize metabolic pathways fundamental to mitochondrial function, introduce and discuss oxidative phosphorylation together with the structure and function of the electron transport chain, and present 7 studies specific to the investigation of mitochondrial function. The collection of studies included in this thesis all focus on the study of mitochondrial function through systematic analysis of electron transport system utility.
The results collected in seven independent studies support the following:
1. Human skeletal muscle mitochondria are capable, independent from cytosolic lactate dehydrogenase (LDH), of metabolizing lactate. Respiration is stimulated with the additions of malate + lactate + ADP + NAD+ in a skeletal muscle preparation that perforates the sarcolemma, facilitating the loss of all soluble cytosolic components including cytosolic LDH. Lactate stimulated respiration despite the loss of cytosolic LDH suggests the existence of a mitochondrial-specific lactate oxidation complex. With the application of specific four different substrate titration protocols, the site of this complex was determined to most likely exist in the mitochondrial intermembrane space.
2. Skeletal muscle predominantly expressing a fast-twitch, glycolytic phenotype is predisposed to age-associated alterations in respiratory chain function. Slow-twitch and more oxidative skeletal muscle does not share this predisposition with aging and appears protected from the progressive lose with senescence. Specific alterations include an amplified respiratory capacity through mitochondrial respiratory complexes I and III with an attendant loss of electron coupling capacity at complex I, both suggestive of a greater oxidant production.
3. Mitochondrial respiratory capacity and control are similar between human and mouse skeletal muscle. The similarities in function, however, are largely dependent on skeletal muscle type with human and mouse quadricep expressing the greatest similarities. One difference that exists across all mouse skeletal muscle with that of human is the phosphorylative restraint of electron transport. Murine muscle electron transport capacity is not limited by the phosphorylative system of mitochondrial complex V, ATP Synthase, whereas human skeletal muscle demonstrates a certain degree of restraint.
4. Eleven days of exposure to high-altitude does not markedly modify integrated measures of mitochondrial functional capacity in human skeletal muscle despite significant decrements in the concentrations of certain enzymes involved in the tricarboxylic acid cycle and oxidative phosphorylation. Though respiratory capacities and efficiency are largely unaffected throughout an eleven-day sojourn to high altitude, mass-specific maximal state 3 respiration, oxidative phosphorylation capacity, does demonstrate a tendency to diminish over time.
5. One month of exposure to high-altitude, however, reduces respiratory capacity in human skeletal muscle while the efficiency of electron transport improves. Specifically, electron transport capacity specific to complex I-, complex II-, and maximal state-3 oxidative phosphorylation capacity all diminished independent from any measureable loss in mitochondrial content. Leak control coupling, respiratory control ratio, and oligomycin-induced leak respiration, all measures of mitochondrial efficiency, improved with hypoxic exposure.
6. Integrative functional differences in mitochondrial function are apparent across groups of individuals that differ in aerobic capacities. Respiration capacities representative of fat oxidation, maximal oxidative phosphorylation, and electron transport system capacity all correspondingly improve with aerobic capacity. These observations are apparent even when controlling for differences in mitochondrial content in the skeletal muscle. Though differences in respiratory capacity are apparent, electron-coupling control for fat oxidation does not differ.
7. Maximal state-3 respiratory capacity, oxidative phosphorylation capacity, is the strongest predictor of endurance performance across a group of highly trained athletes. Mitochondrial content, however, which was similar across all subjects, had no predictive value of performance. Overall exercise performance, including incremental exercise performance in addition to endurance capacity, are best predicted by the measures of maximal oxygen consumption, total hemoglobin mass, maximal state-3 respiration, and electron transport system capacity of the skeletal muscle.
In brief, respiratory capacity and substrate control are dependent on both quantitative as well as qualitative mitochondrial characteristics. The mammalian reliance on this dynamic organelle, capable of utilizing many different substrates through multiple pathways, is fairly consistent across species. The capacity for oxygen utilization is extremely plastic, and modified through various modes of metabolic stress (i.e. hypoxia and exercise) to optimize the benefit of the host organism. Zusammenfassung Mitochondrien, oft als auch Kraftwerke der eukaryotischen Zelle bezeichnet, sind lebenswichtige Komponenten für alle Säugetiere. Der aus der Umwelt aufgenommene Sauerstoff wird von den Mitochondrien zur Energiegewinung für metabolische und biologische Funktionen eingesetzt. Die Gesundheit des Menschen hängt unmittelbar mit dem Gesundheitszustand der Mitochondrien zusammen, jedoch ist noch vieles über das Mitochondrium unbekannt.
Jacobs, Robert Acton