Mitochondrial Toxicity Flashcards
physiological function of ATP
supplying majority of ATP!!
Functions common to most mitochondria
ATP synthesis terminal oxidationof pyruvate beta oxidation of fatty acids oxidation of acetyl CoA fatty acid, protein and carbohydrate oxidations
functions of some mitochondria
oxidation of branched amino chains, sulfate
nitrogen homeostatis, urea formation
activation of vit D3
mitochondrial plasticity purpose
optimize energy production relative to demand
physiological signals that could induce change
nutritional variations, different work loads, oxygen availability, developmental state
example of plasticity with: nutritional value
urea cycle enzymes are increased by high protein diets and starvation
example of plasticity with: different work load
volume density of mitonchondria in skeletal muscle change in association with aerobic activity so that ATP production and energy requirements are coordinated
example of plasticity with: oxygen availability
mitochondrial enzymes decrease during chronic hypoxia
responses to physiological signals are typically
reversible
Chemiosmotic Theory
describes the coupling of metabolic energy in the mitochondria
says that energy transducing membranes contain a proton pump
inner mitochondrial membrane contains
solute transport systems that function to allow the energy available from electron transport to be captured in the form of an electrochemical gradient which drives ATP synthesis
chemiosmotic proton pump model
H+ are pumped to the cytosol (which is the postive side)
MAIN POINT of Chemiosmotic theory
the primary H+ pump generates such a high gradient of H+ that it forces the secondary pump to reverse and synthesize ATP from ADP and P
the quantitative thermodynamic measure of this H+ gradient is
the proton electrochemical gradient
the proton electrochemical gradient has 2 components
- concentration difference of H+ across the membrane (delta pH)
- difference in electrical potential between the 2 aqeuous phases separated by the membrane (delta trident)
the electrochemical gradient is typically converted into units of and is referred to as (delta p)
electrical potential, mV
protonmotive force
use of protonmotive force is
essential for virtually every aspect of mitochondrial function
electron transfer chain comprises a sequence of electron carriers with three separate regions where
redox energy can be conserved in the synthesis of ATP
rate of respiration is controlled by
the demand for ATP
coupling between respiration and ATP synthesis can be disrupted by
uncouplers - they abolish respiratory control and allow mitochondria to catalyze a rapid ATP hydrolysis
oligomycin (an antibiotic) inhibits both the synthsis and
uncoupler-stimulated hydrolysis of ATP
the energy from respiration can be coupled not only to the synthesis of ATP but also to
the accumulation of Calcium and the reduction of NAD to NADP
this can all be driven by the hydrolysis of ATP in anaerobic mitochondria
Four Basic Postulates of Mitchell’s Chemiosmotic Theory
- respiratory ETC should translocate protons
- the ATP synthase should function as a reversible proton-translocating ATPase
- energy-transducing membranes should have a low effective proton conductance
- energy-transducing membranes should posses specific exchange carriers to permit metabolites to permeate and osmotic stability to be maintained in the presence of high membrane potential
Redox reactions are not restricted to the
ETC
the tendency of the redox couple to donate electrons is quantified by
forming an electrical cell from 2 half-cells
the KEY POINT for mid-point potentials is
redox couples with more negative Em7 values are more likely to donate electrons to redox couples with more positive Em7 values
Mitochondrial Respiratory Chains: complex I
NADH-UQ oxidoreductase
Mitochondrial Respiratory Chains: complex II
succinate dehydrogenase
Mitochondrial Respiratory Chains: complex III
bc1 complex; UQ-cyt c oxidoreductase
Mitochondrial Respiratory Chains: complex IV
cytochrome c oxidase
Ion and metabolite transport in Mitochondria
mitochondria require a continual interchange of metabolites and end-products with the cellular cytosol
- at the same time the inner membrane must maintain a high protonmotive force for ATP synthesis
Key Transport Processes: Monovalent cations
high negative membrane potential can lead to a 1000X accumulation of monovalent cations if transport occurred by a uniport mechanism - mitochondria possess a transporter that can exchange either Na or K FOR H
Key Transport Processes: Calcium
there are 2 mitochondrial Ca transporters
- membrane potential-dependent uniporter
- Ca/2H or Ca/2Na antiporter
perturbations in cellular Ca homeostasis may be important in many forms of chemically induced toxicity - the concentration of Ca is a critical factor in the regulation of
many metabolic prosses like regulation of activities of mitochondrial dehydrogenases
six processes function in the regulation of intracellular Ca homeostasis
- electroneutral Na/Ca exchange
- Mg-dependent Ca-ATPase
- endoplasmic reticulum: uptake by Mg-dependent Ca-ATPase
- uptake-driven by transmembrane potential generated across inner membrane during coupled respiration
- efflux-electroneutral Ca/H exchange
- calcium-binding proteins (like calmodulin)
Ca cycling is a major mechanism by which
toxins exert their deleterious effects in cells
all mitochondria possess the
adenine nucleotide translocase, phophate carrier and pyruvate carrier
key findings on metabolic conditions of H2O2 generation: 1. with malate + glutamate as respiratory substrates H2O2 production is
inhibited by rotenone
key findings on metabolic conditions of H2O2 generation: 2. when succinate is used as respiratory substrate & electron flow is blocked by antimycin, mitochondria exhibit high rates
of H2O2 production
key findings on metabolic conditions of H2O2 generation: 3. fatty acids and fatty acyl-CoA also support high rates of H2O2 production in the presence of
antimycin A
mitochonrial generator of H2O2 is either a component of the respiratory chain or a chemical that is at equilibrium with is since
H2O2 production is maximal in HIGHLY REDUCED states
and is minimal in oxidized states like state 3
functional consequences of mitochondrial oxidative stress
mitochondria contain 3 major types of redox active components:
- electron carriers of the respiratory chain
- protein sulfhydryl groups
- matrix GSH
Toxicological relevance
mitochondria contain a large number of critical SH groups that must be in the reduced form for appropriate enzyme activity
Mitochondrial Permeability transition: the transition is readily reversible and occurs when
Ca loading is followed or preceded by addition of a second agent
Mitochondrial Permeability transition: perturbation of a phospholipid acylation-deacylation cycle is seen as a
central event leading to the transition
Mitochondrial Permeability transition: calcium ions are hypothesized to increase activity of this cycle by
stimulating the mitochondrial phospholipase A2
Mitochondrial Permeability transition: the inducing agent is thought to inhibit phospholipid re-acylation as result
phospholipase A2 reaction products accumulate and crease membrane permeability
Mitochondrial Permeability transition: permeability transition can occur both with and without
matrix swelling
inducing agents
sulfhydryl reagents, peroxides, intermediary metabolites, heavy metals
protective agents include:
thiols & other reductants
phospholipase A2 inhibitors
calcium channel blocking agents
cyclosporin A
Genome and Toxicity and Disease
carcinogens, mitochondrial DNA (mtDNA) is also a target
several factors may contribute to selective adduct formation with mtDNA
- mtDNA lacks histone proteins which are associated with nDNA so mtDNA may be more accessible to electrophilic metabolites of xenobiotics
- the diverse and efficient DNA repair systems present in the nucleus are generally lacking in mitochondria
- enzymes that bioactivate xenobiotics to reactive electrophiles are present in the mitochondria
mitochondrial DNA repair and cell injury: mutation rate of mtDNA in mammals is reported to be 5-10X that of
nDNA
mitochondrial DNA repair and cell injury: mtDNA mutations may lead to decreased
respiratory capacity and an increase in release of ROI’s
mitochondrial DNA repair and cell injury: the relatively small amounts of mtDNA as compared with nDNA make the
study of repair processes difficult
mitochondrial genetics have several features that are unique and different from that of classical Mendelian genetics
- cytoplasmic location and high copy number
- mtDNA is maternally transmitted
- mixed intracellular populations of mutant and normal mtDNAs segregate during both meiotic and mitotic replication
- systemic OXPHOS defects show tissue-specific expression as a result of the different OXPHOS requirements of human tissues
- energetic capacities decline with age- probably because accumulation of mtDNA with age
- mtDNA has a high mutation rate partly due to lack of efficient repair systems
