Analysis of the pH dependence of folate binding and transport by rat kidney brush border membrane vesicles.

Folate reabsorption by the mammalian kidney occurs following a tight binding reaction with the renal brush border membrane. Previous studies have shown that transport of folic acid (PteGlu) by rat kidney brush border membrane vesicles occurs maximally at pH 5.6 via a saturable system that is associated with a binding component. The present studies have shown that the pH dependency of transport was due to the development of the transmembrane pH gradient (7.3 in/5.6 out), not to the acidic pH per se. The pH gradient-mediated transport was stimulated by an inwardly directed ionic gradient, either of NaCl or choline chloride. These gradients also stimulated the membrane binding of PteGlu suggesting that NaCl and choline chloride may have increased PteGlu transport by altering binding to the brush border membrane. Renal brush border membrane vesicular transport of PteGlu was not affected by induction of a relatively positive intravesicular space. Transport was inhibited by 4,4'-diisothiocyano-2,2'-disulfonic acid stilbene, an anion exchange inhibitor. The results suggest that rat kidney brush border membrane transport of PteGlu is initiated by association with a specific membrane protein, followed by transfer of folate across the membrane. The overall activity is influenced by a transmembrane pH gradient.

Effects of chronic ethanol and diet treatment on urinary folate excretion and development of folate deficiency in the rat.

Because the folate deficiency of chronic alcoholism has been proposed to result from ethanol-induced effects on metabolism or urinary excretion of folate, the present study was designed to evaluate the role of chronic ethanol-induced urinary folate loss on folate homeostasis in the rat. Male Sprague-Dawley rats were fed nutritionally sufficient liquid diets for 12 wk with or without ethanol, folate and sulfonamide. Urinary folate excretion was increased in ethanol-fed rats consuming folate-containing diets, but not in rats fed folate deficient diets. Consumption of folate-deficient diets led to a rapid decrease in urinary folate excretion, suggesting renal adaptation to conserve folate. Tissue and plasma levels of folate were mostly unaffected by ethanol ingestion in rats fed folate-containing diets. Ethanol treatment did not consistently enhance tissue folate depletion in rats fed folate-deficient diets. The results suggest that in rats consuming diets containing high levels of folate, chronic ethanol ingestion increased urinary folate excretion, but not to a sufficient magnitude to consistently affect folate homeostasis.