Demonstration of high-affinity folate binding activity associated with the brush border membranes of rat kidney.

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Abstract
Folate binding activity of high affinity was identified in the particulate fractions of rat kidney homogenates. This binding activity cofractionated with alkaline phosphatase and maltase, 2 brush border membranes markers. With an enriched preparation of brush border membranes, freed of endogenous folate by acid treatment, the binding of [3H]folate was saturable (Kb = 4.2 .times. 10-11 M) and rapid. Binding was optimal at pH 6.4-7.7. At neutral pH, competition for binding with [3H]folic acid required 1.45 equivalents of pteroylheptaglutamate, 6.25 equivalents of N5-methyltetrahydrofolate, 29 equivalents of methotrexate and 125 equivalents of N5-formyltetrahydrofolate. At alkaline pH, N5-methyltetrahydrofolate was as effective a competitor as folic acid. Since renal tubular reabsorption of folate includes an initial tight binding step, the binding activity associated with the brush border membranes may participate in this process.

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