Evidence for transfer of folate compounds by a specialized erythrocyte membrane system

Transfer of folate compounds was studied in human erythrocytes. At steady state, the measured distribution ratio of 5-CH3-H4-folate in erythrocyte suspensions exceeded the ratio predicted from the chloride distribution ratio by a factor of 1.58, suggesting that human erythrocytes concentrate folate. Because folate compounds are anionic at physiologic pH, we investigated the possibility that transport occurs via the inorganic anion channel associated with the predominant integral membrane protein, band 3. Erythrocyte uptake of 5-CH3-H4-folate was decreased (60% to 80%) by several known inhibitors of anion transport--pyridoxal phosphate, dipyridamole, phlorizin, and SITS. However, unlike the inorganic anion transfer system, 5-CH3-H4-folate uptake was only slightly decreased by DIDS; was reduced 50% to 70% by the sulfhydryl reagents NEM, PMB, and pCMBS; and was not affected by the proteolytic enzymes trypsin, chymotrypsin, and pronase. These studies suggest that folate compounds are transported by a specialized carrier system, independent of the inorganic anion channel, which contains sulfhydryl and amino groups. In contrast to 5-CH3-H4-folate transfer, uptake of pteroylglutamic acid was either unaffected or somewhat increased by these membrane modifications. This result indicates that the human erythrocyte transports the reduced and oxidized forms of the vitamin by entirely separate mechanisms.