MITOCHONDRIAL EFFECTS
Mitochondria are the organelle and key place for understanding the main
effects of biguanides, especially of MET [20]. Understanding their
capture becomes fundamental to the understanding of their action
mechanisms and applications. The MET molecule is positively charged and
its capture by the mitochondria depends on the energetic status of this
organelle [20, 21, 24, 50]. Lower respiratory activity associated
with an increase in membrane potential induced by inhibition of complex
I, may help in the capture and accumulation of MET in the mitochondria
[21]. Therefore, the concentration may be 100 to 300 times higher
within this organelle at a potential between 120 and 150mV, compared to
cytosol [20, 50]. Moreover, compared to the extracellular medium,
this concentration can be 1000 times higher in the mitochondria [20,
23]. Therefore, milli-molar scales concentrations of MET are
achievable, despite micro-molar concentrations of cytoplasm [23]. It
is worth noting that its uptake is favored by the action of complex I
due to the increase in potential, however, the drug inhibits this
complex, decreasing its subsequent uptake [20, 23].
The I complex of the electron transport chain was the first identified
target of MET. It is known to have two forms: A (activated) and D
(deactivated) [21]. The exact location of the biguanide connection
still arouses debate. According to Pecinova et al. (2017), biguanides do
not bind to any specific site of dehydrogenesis in the respiratory chain
[50]. However, Bridges et al. (2014) concluded that MET is a
reversible non-competitive ubiquinone binding site inhibitor. In
addition, this compound exhibits greater inhibition in the non-catalytic
state of the complex (D) [20]. The possible site of MET in this
complex is an amphipathic region at the interface of the hydrophilic and
membrane regions, where redox processes initiate proton translocation.
MET would keep the complex in an open loop conformation, state D [20,
24]. Matsuzaki and Humphries (2015) observed that 25 mM of MET was
able to inhibit 12.1% of the activity of complex I in state A, and
about 40% in state D induced by heating, while the same concentration
inhibited 44.6% of the activity of complex III-IV, and 59.0% of the
activity of NADH oxidase (combination of activities of I-III-IV).
Bridges et al. (2014) approached that MET presents a CI50 of 19.4 mM
(±1.4) over the bovine heart mitochondrial complex I (Bos taurus)
[20]. PHEN at 200 µM is able to inhibit 10% and 35-40% of complex
I activity in states A and D, respectively [21]. The main effect of
the inhibition of complex I related to MET is the activation of AMPK and
its pathways [24, 25]. AMPK favors a cellular catabolic state,
regulating several metabolic factors that lead to the restoration of the
energy balance [23].
Some effects of MET, including the redox state of the cell, may vary
depending on the time or concentration used. While short-term
administration has caused little inhibition of complex I activity,
long-term is associated with benefits such as reduced oxidative stress
and increased antioxidant defenses, resulting in lower incidence of
organism damage [58]. Regarding the concentration, a change in the
redox state of the mitochondria was observed comparing concentrations of
100 µM and 500 µM. In 500 µM, as observed for retenone, the mitochondria
was in a lower NADH/NAD+ state; while in 100 µM, in a more oxidized
state [72]. Other effects are described in Chart 2.