Measurements of enzymatic activity : Measurements of enzymatic activity were made using the potentiometric method of Wilbur and Anderson 37 , with modifications: 1. Carbonic anhydrase and CO 2 concentrating mechanisms in microalgae and cyanobacteria. FEMS Microbiol. Aizawa, K. Kinetic studies on the active species of inorganic carbon absorbed by cells of Dunaliella tertiolecta Plant Cell Physiol , , Axelsson, L.
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Cytosolic and extracellular buffer capacity and the activity of carbonic anhydrases contribute to shape pH changes, which can be elicited by neuronal activity, neurotransmitters and neuromodulators, metabolic processes, active cellular pH regulation, and secondary transporters carrying acid—base equivalents, and in turn these pH changes can affect neuronal functions Deitmer and Rose, ; Chesler, In other words, there is a pool of protons in rapid exchange between buffer sites and free solution, with 10 5 or more protons being buffered for each proton in solution.
At a blood pH of 7. Lactate, pyruvate, and ketone bodies are transported into and out of cells via MCTs SLC 16 , of which 14 isoforms have been described. MCT2, the high-affinity carrier, is mainly found in neurons, and MCT4, the low-affinity, high-capacity carrier, has been reported for glial cells in the brain. The lactate shuttle hypothesis suggests that lactate is produced and exported by glial cells, in particular astrocytes, under normoxic conditions, and taken up by neurons for further metabolization Pellerin and Magistretti, The ANLSH infers that astrocytes help to supply energetic substrates for neurons to meet their energy requirements, especially during enhanced neuronal activity.
There is substantial evidence, both in vitro and in vivo , that lactate indeed can substitute for glucose to maintain neuronal functions, such as e. During energy deprivation, the addition of monocarboxylates has been shown to restore synaptic function and to be neuroprotective in vivo , in acute rodent brain slices, isolated optic nerve and neuronal cultures Izumi et al.
The finding that glucose is preferentially taken up by astrocytes and at higher rates than by neighboring neurons Barros et al. A transport metabolon has been defined as a supramolecular complex of sequential metabolic enzymes and cellular structural elements in which metabolites are passed from one active site to another without complete equilibration with the bulk cellular fluids Srere, All of these interactions have in common that CA-mediated augmentation of transport activity requires the catalytic activity of the different CA isoforms.
Augmentation of MCT activity by extracellular CAs has also been found in the brain: By inhibition of extracellular CA activity with benzolamide and an antiserum against CAIV, respectively, Svichar and Chesler could show a significant reduction in lactate-induced intracellular acidification in rat hippocampal pyramidal neurons and in cultured astrocytes.
When CO 2 increases in the cell, e. CAIV has recently been shown to display intracellular activity in addition, which would further contribute to high intracellular CA activity Schneider et al. Furthermore, both extra- and intracellular CA isoforms, as e. Disruption of transport metabolon function, as suggested to occur after CAIV mutation, can interfere with photoreceptor maintenance and pH regulation in the retina Yang et al.
Regulation of metabolism in organisms is not only complex, but also involves a large number of enzymes and membrane transporters, which may form networks to enhance their efficacy. Lactate, as a metabolic intermediate from glucose or glycogen breakdown, appears to play a major role as energetic substrate shuttled between cells and tissues, both under hypoxic and normoxic conditions. Carbonic anhydrases mediate between different states of metabolic acidosis, induced by glycolysis and oxidative phosphorylation, and play a relay function in coupling pH regulation and metabolism.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Mitochondrial carbonic anhydrase VA CAVA catalyzes the hydration of carbon dioxide to produce proton and bicarbonate which is primarily expressed in the mitochondrial matrix of liver, and involved in numerous physiological processes including lipogenesis, insulin secretion from pancreatic cells, ureagenesis, gluconeogenesis, and neuronal transmission. To understand the effect of pH on the structure, function, and stability of CAVA, we employed spectroscopic techniques such as circular dichroism, fluorescence, and absorbance measurements in wide range of pH from pH 2.
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