Protein-bound 3,4-dihydroxyphenylalanine (PB-DOPA), a long-lived, redox-active product of
protein oxidation, is capable of functioning as both a pro- and anti-oxidant. A number of
in vitro and in vivo studies have demonstrated a toxic, non-toxic or even beneficial effect of
free DOPA, however little investigation has examined the physiological activity of PB-DOPA.
Furthermore, as free DOPA is currently the major treatment available for Parkinson?s disease,
most studies have focused on the effect of DOPA within neurological cells or tissues,
although the presence of PB-DOPA in other locations, for example within atherosclerotic
plaques, suggests that broader research is needed to fully understand the physiological effects
of both free and PB-DOPA.
The hypothesis presented in this thesis is that under physiological conditions, when little
redox active transition metal is available, PB-DOPA can function as a redox signalling
molecule, triggering an enhancement of cellular antioxidant defences, with a potentially
specific role in the regulation of defences targeted against protein oxidation. Physiological
levels of PB-DOPA are very low, however the level on individual proteins can change to a
proportionally large degree during oxidative stress, an appropriate property for a signalling
molecule. In addition, remarkably elevated levels occur in some pathologies, including
atherosclerosis. As an initial and commonly formed product of protein oxidation, PB-DOPA
is well placed for a signalling role, promoting a significant up-regulation of antioxidant
defences in the early stages of oxidative stress, before extensive damage has occurred. As an
initiator of antioxidant defences, PB-DOPA would be potentially useful as a therapeutic for
the treatment of diseases involving oxidative stress or the accumulation of oxidative damage.
The main objective of this thesis was, therefore, to examine the effect of PB-DOPA on the
cellular antioxidant defence system using monocytic and macrophage-like cells, key cells
involved in the formation of atherosclerotic plaques. The incorporation of free DOPA into
protein during protein synthesis, a process previously shown to occur both in vitro and in vivo,
was used to generate PB-DOPA. Neither free nor PB-DOPA were found to be toxic to
monocytic or macrophage-like cells in culture, but rather were both capable of protecting
these cells from oxidative stress. Free DOPA was shown to be capable of directly scavenging
radicals, a process that was thought to be in part responsible for the protection induced during
oxidative stress. The presence of free and PB-DOPA up-regulated the activity of catalase and
NAD(P)H:quinone oxidoreductase, two enzymatic antioxidants, however the activity of
superoxide dismutase and the concentration of oxidised and reduced glutathione were not
affected. Whilst it was thought that PB-DOPA would have a specific effect on the activity of
antioxidant defences targeted against protein oxidation, proteolysis and bulk chaperone
activity were not affected by a combination of free and PB-DOPA. Oxidatively-induced
protein aggregation, however, was inhibited by the presence of free and PB-DOPA,
suggesting that a more specific chaperone regulation may be taking place.
The regulation of gene and protein expression was thought to be one possible mechanism by
which PB-DOPA could function as a signalling molecule. To test this hypothesis, the effect of
free and PB-DOPA on transcription factor activation and protein expression were investigated.
Free and PB-DOPA did not induce the expression or activation of Nrf2, AP-1 or NFJB, three
transcription factors thought to be involved in the expressional regulation of genes involved in
the antioxidant defence system. However, the expression of a number of proteins, including
antioxidants, chaperones and proteins involved in cell cycle progression, were regulated in
monocytic and macrophage-like cells following the administration of free DOPA under
conditions that resulted in either a high or low level of PB-DOPA generation. The regulated
proteins differed between the two conditions, suggesting that the level of PB-DOPA may be a
key factor in determining the specific defences targeted.
The results presented in this thesis support the hypothesis that PB-DOPA can function as a
signalling molecule, triggering an enhancement of cellular antioxidant defences, with a
specific role in the regulation of the chaperone system, a key defence targeted against protein
oxidation. This thesis may provide the basis for the potential use of free or PB-DOPA as a
therapeutic for diseases known to involve oxidative stress or oxidative damage, however more
research will be required to determine if the effects demonstrated in this thesis are also
capable of occurring in vivo.
| Identifer | oai:union.ndltd.org:ADTP/202623 |
| Date | January 2008 |
| Creators | Nelson, Michelle Amy, n/a |
| Publisher | University of Canberra. n/a |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | ), Copyright Michelle Amy Nelson |
Page generated in 0.0018 seconds