Differential Binding of Folates by Rat Renal Cortex Brush Border and Basolateral Membrane Preparations

Publisher Website
Google Scholar
Abstract
The binding of radioactive 5-methyltetrahydrofolate and folic acid was greater in brush border than in basolateral membrane preparations of rat renal cortex. This appeared to be due to an increased amount of a specific folate-binding protein in the brush border membrane preparations as compared to those of the basolateral membrane. The binding was saturble and inhibited by nonradioactive folic acid and, therefore, a specific, rather than nonspecific process. The Km for folic acid binding in brush border and basolateral membrane preparations were similar and involved a single high affinity binding site. In contrast, methotrexate bound equally well to both brush border and basolateral membrane preparations. Folic acid binding was not inhibited by an equimolar amount of methotrexate. A folate binding protein could be extracted from either membrane preparation with 1% Triton X-100 and, to a lesser extent, with 0.6 M NaCl. These different extraction procedures resulted in different apparent MW for folate binding protein (> 16,000 for Triton X-100-extracted samples and 40,000 for NaCl-extracted samples). The membrane preparation pellets remaining after NaCl extraction were able to rebind tritiated folic acid and also the 40,000 DA [dalton] folate-binding protein. Membrane preparations extracted with Triton X-100 lost the ability to bind folic acid or the 40,000 Da folate-binding protein. These differences in MW and rebinding capacity may be explained by the existence of a receptor for folate-binding protein which was extracted by Triton X-100, but not by NaCl. The greater concentration of folate binding protein in the renal tubule cell brush border membrane preparations as compared to those from basolateral membranes ascribes, for the first time, a functional role for folate binding protein in the renal reabsorption of folates which is required to prevent loss of folate in the urine and perhaps in the membrane transport of folates in general.