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| Term | Abbreviation | Description |
|---|---|---|
| Inorganic phosphate | Pi | Inorgnic phosphate (Pi) is a salt of phosphoric acid. In solution near physiological pH, the species HPO42- and H2PO4- dominate. See also: Phosphate carrier (Pic). |
| Isocitrate |  isocitrate, C6H5O7-3, is a tricarboxylic acid trianion, intermediate of the TCA cycle, obtained by isomerization of citrate. The process is catalyzed by aconitase, forming the enzyme-bound intermediate cis-aconitate. | |
| Jmax | Jmax | Jmax is the maximum pathway flux (e.g. oxygen flux) obtained at saturating substrate concentration. Jmax is a function of metabolic state. In hyperbolic ADP or oxygen kinetics, Jmax is calculated by extrapolation of the hyperbolic function, with good agreement between the calculated and directly measured fluxes, when substrate levels are >20 times the c50 or p50. |
| Malate | M |  Malic acid, C4H6O5, occurs under physiological conditions as the anion malate2-, M, with pKa1 = 3.40 and pKa2 = 5.20. L-Malate is formed from fumarate in the TCA cycle in the mitochondrial matrix, where it is the substrate of malate dehydrogenase oxidized to oxaloacetate. Malate is also formed in the cytosol. It cannot permeate through the lipid bilayer of membranes and hence requires a carrier (dicarboxylate carrier, tricarboxylate carrier and 2-oxoglutarate carrier). Malate alone cannot support respiration of mt-preparations from most tissues, since oxaloacetate accumulates in the absence of pyruvate or glutamate. Malate is a type N substrate (N) required for the FAO-pathway. In the presence of anaplerotic pathways (e.g., mitochondrial malic enzyme, mtME) the capacity of the FAO-pathway can be overestimated due to a contribution of NADH-linked respiration, F(N) (see SUIT-002). |
| Malate transport | Carriers for malate: | |
| Metabolic control variable | X | A metabolic control variable X causes the transition between a background state Y (background rate YX) and a reference state Z (reference rate ZX). X may be a stimulator or activator of flux, inducing the step change from background to reference steady state (Y to Z). Alternatively, X may be an inhibitor of flux, absent in the reference state but present in the background state (step change from Z to Y). |
| Methylmalonic acid | Mma | Methylmalonic acid (Mma) is a common intermediate in many catabolic processes. In methylmalonic acidemia mitochondrial dysfunction can be observed, related to accumulation of Mma and associated with neurological symptoms. |
| MitoKit-CII |  Cell permeable prodrugs, composed of MitoKit-CII/Succinate-nv and MitoKit-CII/Malonate-nv, stimulates (Snv) or inhibits (Mnanv) mitochondrial respiration in CI-deficient human blood cells, fibroblasts and heart fibres, acting on Complex II of the electron transfer system. | |
| NADH | NADH | NAD+ and NADH: see Nicotinamide adenine dinucleotide. |
| NS-pathway control state | NS, CI&II |  NS-pathway control is exerted in the NS-linked substrate state (flux in the NS-linked substrate state, NS; or Complex I&II, CI&II-linked substrate state). NS-OXPHOS capacity provides an estimate of physiologically relevant maximum mitochondrial respiratory capacity. NS is induced in mt-preparations by addition of NADH-generating substrates (N-pathway control state in combination with succinate (Succinate pathway; S). Whereas NS expresses substrate control in terms of substrate types (N and S), CI&II defines the same concept in terms of convergent electron transfer to the Q-junction (pathway control). NS is the abbreviation for the combination of NADH-linked substrates (N) and succinate (S). This physiological substrate combination is required for partial reconstitution of TCA cycle function and convergent electron-input into the Q-junction, to compensate for metabolite depletion into the incubation medium. NS in combination exerts an additive effect of convergent electron flow in most types of mitochondria. |
| Nicotinamide adenine dinucleotide | NADH | Nicotinamide adenine dinucleotide, NAD+ and NADH (pyridine nucleotide coenzymes, NAD and NADP), is an oxidation-reduction coenzyme (redox cofactor; compare FADH2). In the NADH electron transfer-pathway state fuelled by type N-substrates, mt-matrix dehydrogenases generate NADH, the substrate of Complex I (CI). The reduced N-substrate RH2 is oxidized and NAD+ is reduced to NADH,:::: RH2 + NAD+ → NADH + H+ + RThe mt-NADH pool integrates the activity of the TCA cycle and various matrix dehydrogenases upstream of CI, and thus forms a junction or funnel of electron transfer to CI, the N-junction (compare F-junction, Q-junction). NAD+ and NADH are not permeable through the mt-inner membrane, mtIM. Therefore, an increase of mitochondrial respiration after the addition of NADH may indicate an alteration of the mtIM integrity. Cytosolic NADH is effectively made available for mitochondrial respiration through the malate-aspartate shuttle or glycerophosphate dehydrogenase Complex. |
| Octanoate | Oca | Octanoate (octanoic acid). C8H16O2 Common name: Caprylic acid. |
| Octanoylcarnitine | Oct | Octanoylcarnitine is a medium-chain fatty acid (octanoic acid: eight-carbon saturated fatty acid) covalently linked to carnitine, frequently applied as a substrate for fatty acid oxidation (FAO) in mitochondrial preparations. |
| Oxaloacetate | Oa |  Oxaloacetic acid, C4H4O5, occurs under physiological conditions as the anion oxaloacetate2-, Oa. Oxaloacetate is formed from malate by MDH. Oa reacts with acetyl-CoA through citrate synthase to form citrate, or with glutamate through transaminase to form oxoglutarate and aspartate. Oa transport is restricted across the inner mt-membrane of various tissues. Oa is a potent inhibitor of succinate dehydrogenase. |
| Oxoglutarate | Og |  2-Oxoglutaric acid or alpha-ketoglutaric acid, C5H6O5, occurs under physiological conditions as the anion 2-Oxoglutarate2-, Og. 2-Oxoglutarate (alpha-ketoglutarate) is formed from isocitrate as a product of isocitrate dehydrogenase (IDH) in the TCA cycle, and is a substrate of oxoglutarate dehydrogenase (OgDH). The 2-oxoglutarate carrier exchanges malate2- for 2-oxoglutarate2- as part of the malate-aspartate shuttle. In the cytosol, oxoglutarate+aspartate are transaminated to form oxaloacetate+glutamate. Cytosolic malate dehydrogenase converts oxaloacetate+NADH to malate. |
| Oxygen | O2 |  Molecular oxygen, O2 or dioxygen, has two atoms of oxygen, O, which is the chemical element with atomic number 8. The relative molecular mass of O2, Mr,O2, is 32 (or 31.9988). The element O has 8 protons, 8 neutrons and 8 electrons. In the figure, the two electrons in the first electron shell are not shown. Of the six electrons in the outer shell (blue bullets), one electron from each of the two atoms is shared in O2 forming the covalent bond, and one electron in each atom is unpaired. |
| P50 | p50 | p50 is the oxygen partial pressure at which (a) respiratory flux is 50% of maximum oxygen flux, Jmax, at saturating oxygen levels. The oxygen affinity is indirectly proportional to the p50. The p50 depends on metabolic state and rate. (b) p50 is the oxygen partial pressure at which oxygen binding (on myoglobin, haemoglobin) is 50%, or desaturation is 50%. |
| Palmitate | Paa | Palmitate is a term for the salts and esters of palmitic acid (CH3(CH2)14COOH). Palmitic acid is the first fatty acid produced during fatty acid synthesis and the precursor to longer fatty acids. Palmitate negatively feeds back on acetyl-CoA carboxylase (ACC), which is responsible for converting acetyl-CoA to malonyl-CoA, which in turn is used to add to the growing acyl chain, thus preventing further palmitate generation. In order to dissolve the water-insoluble sodium palmitate, BSA is needed to form the water-soluble compound called palmitate:BSA. |
| Palmitoyl-CoA | Pa-CoA | Palmitoyl-CoA is a coenzyme A derivative of palmitate formed by acyl-CoA synthase. In contrast to medium- and short-chain acyl-CoA, palmitoyl-CoA cannot freely diffuse into the mitochondrial matrix. Formation of palmitoylcarnitine by CPTI is necessary prior to transfer into mitochondria for further fatty acid oxidation (β-oxidation). To study Fatty acid oxidation using Palmitoyl-CoA, Carnitine and low amount of malate is needed on mitochondrial preparations. |
| Palmitoylcarnitine | Pal | Palmitoylcarnitine is an ester derivative of carnitine (long-chain acylcarnitine) involved in the metabolism of fatty acids. Within the cell, palmitoylcarnitine is transported into the mitochondria to deliver palmitate for fatty acid oxidation and energy production. |
