Transport of Tricarboxylic Acid Cycle Intermediates by Membrane Vesicles from Renal Brush Border

The uptake of citrate and α -ketoglutarate by membrane vesicles from rabbit renal brush border was studied by a rapid filtration technique. Both compounds exhibited transport characteristics similar to those seen for the sodium-dependent cotransport systems previously described for sugars and amino acids in brush border membranes. The estimated sodium-dependent Vmaxand Kmwere 17 nmol per mg of protein per min and 0.18 mM for citrate and 17 nmol per mg of protein per min and 1.0 mM for α -ketoglutarate. The initial rate of citrate transport was 5 times that of sugars and amino acids under comparable conditions. Uptake rates of 0.1 mM citrate and α -ketoglutarate were inhibited by >90% by 10 mM succinate, malate, fumarate, or oxaloacetate, indicating the presence in the brush border membrane of a transport system highly specialized for the renal conservation of intermediates of the tricarboxylic acid cycle.