pH modulation of the kinetics of rabbit jejunal, brush-border folate transport

In jejunal brush-border membrane vesicles, an outwardly directed OH- gradient (in greater than out) stimulates DIDS-sensitive, saturable folate (F) uptake (Schron, C.M. 1985, J. Clin. Invest. 76:2030-2033), suggesting carrier-mediated folate: OH- exchange (or phenomenologically indistinguishable H+: folate cotransport). In the present study, the precise role of pH in the transport process was elucidated by examining F uptake at varying pH. For pH gradients of identical magnitude, F uptake (0.1 microM) was greater at lower (pHint/pHext: 5.5/4.5) compared with higher (6.5/5.5) pH ranges. In the absence of a pH gradient, internal F trans stimulated DIDS-sensitive 3H-folate uptake only at pH less than or equal to 6.0. Since stepwise increments in internal pH (4.5----7.5; pHext = 4.5) stimulated F uptake, an inhibitory effect of higher internal pH was excluded. In contrast, with increasing external pH (4.35----6.5; pHint = 7.8), a 50-fold decrement in F uptake was observed (H+ Km = 12.8 +/- 1.2 microM). Hill plots of these data suggest involvement of at least one H+ (OH-) at low pH (monovalent F- predominates) and at least 2 H+ (OH-) at high pH (divalent F-2 predominates). Since an inside-negative electrical potential did not affect F uptake at either pHext 4.55 or 5.8, transport of F- and F-2 is electroneutral. Kinetic parameters for F- and F-2 were calculated from uptake data at pHext 4.55 and 5.0.(ABSTRACT TRUNCATED AT 250 WORDS)