Net efflux of citrate in Penicillium simplicissimum is mediated by a transport protein.
Penicillium simplicissimum excreted citrate, isocitrate, and succinate when grown in a strongly buffered medium [1 M Mes (pH 6) or 1 M Hepes (pH 7.3)]. Growth in a weakly buffered medium did not lead to citrate excretion despite a similar intracellular citrate concentration. When nongrowing, citrate-excreting hyphae were aerated in a glucose solution, the following steady-state intracellular concentrations of organic acids were measured: succinate (25 mM); citrate, isocitrate, malate, and fumarate (all less than 5 mM). After 2 h of incubation, the extracellular concentrations of these acids were [micromol (g dry wt.)-1]: isocitrate [100], citrate [60], succinate [30], and malate, fumarate, and alpha-ketoglutarate [<5]. The excretion of citrate was due neither to an unspecific change in the permeability of the plasma membrane nor to simple diffusion of undissociated citric acid. The involvement of a transport protein in citrate excretion was indicated because N-ethylmaleimide and sodium azide inhibited citrate excretion strongly despite an unchanged outward-directed citrate gradient. Arguments are given why efflux via a citrate uptake carrier is not considered probable. These results indicate that citrate is excreted by P. simplicissimum via a transport protein that probably specifically mediates the efflux of citrate.
Gallmetzer
M
M
Institute of Microbiology, Universität Innsbruck, Technikerstrasse 25, A-6020 Innsbruck, Austria.
Müller
B
B
Burgstaller
W
W
D001386:Azides / Q000494:pharmacology; D002352:Carrier Proteins / Q000494:pharmacology*; D002455:Cell Division / Q000187:drug effects; D002951:Citrates / Q000037:antagonists & inhibitors / Q000378:metabolism* / Q000493:pharmacokinetics*; D003470:Culture Media / Q000494:pharmacology; D003593:Cytoplasm / Q000737:chemistry / Q000187:drug effects / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005033:Ethylmaleimide / Q000494:pharmacology; D005110:Extracellular Space / Q000737:chemistry / Q000187:drug effects / Q000378:metabolism; D005650:Fumarates / Q000378:metabolism; D005656:Fungal Proteins / Q000378:metabolism; D005947:Glucose / Q000494:pharmacology; D007523:Isocitrates / Q000037:antagonists & inhibitors / Q000378:metabolism; D007656:Ketoglutaric Acids / Q000378:metabolism; D008293:Malates / Q000378:metabolism; D009584:Nitrogen / Q000494:pharmacology; D010407:Penicillium / Q000737:chemistry / Q000187:drug effects / Q000378:metabolism*; D013386:Succinates / Q000037:antagonists & inhibitors / Q000378:metabolism; D013997:Time Factors
D001386:Azides; D002352:Carrier Proteins; D002951:Citrates; D003470:Culture Media; D004791:Enzyme Inhibitors; D005650:Fumarates; D005656:Fungal Proteins; D007523:Isocitrates; D007656:Ketoglutaric Acids; D008293:Malates; D013386:Succinates; D005947:Glucose; D009584:Nitrogen; D005033:Ethylmaleimide