Citrate transport into barley mesophyll vacuoles ? comparison with malate-uptake activity

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Abstract
Citrate uptake into barley (Hordeum vulgare L.) mesophyll vacuoles was found to be saturable with a K m of about 200 μM. Uptake appears to occur via the citrate3- form, as indicated by concentration-dependent uptake studies at different pHs. Free citrate and not the Mg-citrate complex was taken up by the vacuoles, even though slow transport of the Mg complex could not be excluded. Citrate transport into vacuoles was competitively inhibited by malate (K i=0.68 mM). Various organic acids and protein-modifying agents affected the uptake of malate and citrate to a similar extent. These results indicate that both organic acids cross the tonoplast by means of the same carrier. Accumulation of citrate was ATP-dependent and could be inhibited by ionophores. Bovine serum albumin strongly stimulated citrate uptake, but other proteins tested did not show a similar stimulatory effect.