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indeed a synchronisation agent. However, further work is required to unravel how
carbon metabolism is regulated during the oscillation; specifically, how intracellu-
lar carbon flux distributions develop dynamically through each cycle.
12.5 Sulphur Metabolism
H 2 S, a potent respiratory inhibitor, reached a maximum (1.5 μ M) just prior to
minimal respiration rates (Sohn et al. 2000 ) and then decreased to 0.2 μ M before
the restoration of the high respiration state. Perturbants such as 50
μ
M glutathione,
50
M NaNO 2 or 4.5 mM acetaldehyde transiently increased H 2 S levels to more
than 6
μ
M (NH 3 ) 2 S
enabled a phase response curve to be obtained (Murray et al. 2003 ); thus the
easily oxidised, rapidly diffusing gas, H 2 S, acts as an intercellular messenger that
amplifies the respiratory oscillation. The binuclear Cu-haem reaction centre of
cytochrome c oxidase, the terminal electron transport component of the mitochon-
drial respiratory chain, is likely to be its target (Lloyd 2006b ). However, phase-
response curves indicate that it does not act by itself as a synchronising agent.
H 2 S is evolved as an intermediary metabolite of the sulphur uptake pathway
(Sohn and Kuriyama 2001a ). Pulse injection of 100
μ
M. Phase shifting of the oscillation by additions of 0.77
μ
M cysteine or methionine
altered the timing of H 2 S production and perturbed the respiratory oscillation (Sohn
and Kuriyama 2001b ). Modelling of the feedback inhibition of sulphate uptake by
cysteine suggests a major contribution to the respiratory oscillations (Wolf
et al. 2001 ; Henson 2004 ). Transcript concentration of the high affinity sulphate
permease ( SUL2 ) and all the transcripts of the sulphate assimilation pathway
showed especially high amplitude oscillation, and their peak preceded H 2 S genera-
tion by 8-10 min (Murray et al. 2007 ). Hydrogen sulphide production from
cysteine, catalysed by mitochondrial cystathione-
μ
γ
-lyase in the mitochondria of
mammalian cells, was recently demonstrated to be involved in the regulation of
energy metabolism (Fu et al. 2012 ). There is a yeast homolog of the protein
responsible ( CYS3 ); however, it is unclear if Cys3p localises to the mitochondria
and if this mechanism of H 2 S production occurs in yeast.
The network of sulphate uptake and sulphur amino acid production is shown in
Fig. 12.2 . This network is intricately interwoven with the regulation of redox state.
A direct product of this network is glutathione, and glutathione reductase is
responsible for NADPH-dependent cycling of the GSH-GSSG system that buffers
the redox state of the cell (Sohn et al. 2005a ). Oscillatory dynamics of GSH1 and
GLR1 transcript abundance, activity of glutathione reductase and the pool sizes
of cysteine and glutathione indicated that redox buffering plays a critical role
during the oscillation. Moreover, glutathione addition gave a phase-related pertur-
bation of the respiratory oscillation (Murray et al. 1999 ; Sohn et al. 2000 ). The
effects of pulse injection of thiol redox modifying agents (diethylmaleate,
N -ethylmaleimide), of inhibitors of glutathione reductase ( DL -butathionine
[ S,R ]-sulphoxamine) or of glutathione synthesis (5-nitro-2-furaldehyde) further
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