Abstract
The physiological function of mitochondria requires the transport of Pi across the inner mitochondrial membrane for 3 main metabolic reactions: (1) Uptake of phosphate together with ADP for the synthesis of ATP within the matrix; (2) Exchange of phosphate against dicarboxylates for metabolite fluxes between cytosol and matrix; (3) Uptake and release of phosphate together with Ca* for calcium homeostasis in the cytoplasm [I]. For these functions 4 separate phosphate transport systems have been described in mitochondria which could only be differentiated by their sensitivity against SH-group ~ibitors: (1) The electro~leutr~ phosphate~proton symporter is inhibited by MalNEt and low concentrations of p-chloromercuribenzoate and mersalyl [2-4]; (2) The electroneutral dicarboxylate antiporter, exchanging dicarboxylates or phosphate against each other, is inhibited by p-chloromercuribenzoate, mersalyl and the dicarboxylate analogon butyimalonate, but not by MalNEt [S-7]; (3) The electrogenic phosphate uniporter is i~ibited by mersalyl [S] and with inverted inner membrane vesicles by p-chloromercuribenzoate and MalNEt [9]; (44) The electrogenic calcium/phosphate symporter was found insensitive against MalNEt and mersaylyl [lo,1 11. The latter transport system, however, does not seem to represent a separate transporter, because the ~sensiti~ty against MalNEt and ~-c~oromercu~benzoate could not be corroborated [12,13]. The simultaneous occurrence of the electroneutral phosphate/proton symporter (phosphate uptake) and the electrogenic phosphate uniporter (phosphate
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