Kinetic analysis of the effect of luminal pH on transport of folic acid in the small intestine.
Transport of folic acid in the small intestine is markedly affected by luminal pH and is optimal at pH 5.5-6.5. To clarify the nature of this pH sensitivity, we measured transport of [3H] folic acid into rat jejunum at pH 5.5 and 7.4, at folic acid concentrations of 1 to 30 microM, using the influx chamber method. At this range of concentrations, uptake of folic acid exhibited Michaelis-Menten kinetics. In order to determine the kinetic parameter which is pH sensitive, we have fitted a modified Michaelis-Menten equation to the pooled data at the two pH conditions such as they share a common Vmax but distinct Km values. This model fitted closely the experimental data (F = 0.693), yielding Km values (+/- SE) of 24.22 (+/- 5.96) and 36.32 (+/- 8.73) microM for pH 5.5 and 7.4 respectively. The difference in the Km at the two pH conditions was statistically significant (p less than 0.01). The change in affinity for folic acid transport at the two pH conditions may reflect different membrane carriers, or may be due to protonation of a membrane carrier and/or the folic acid molecule, favored at low pH.
Department of Gastroenterology, Hadassah University Hospital, Jerusalem, Israel.
J
J
Zimmerman
Z
Z
Gilula
J
J
Selhub
I H
IH
Rosenberg
D000136:Acid-Base Equilibrium; D000818:Animals; D002462:Cell Membrane / Q000378:metabolism; D003627:Data Interpretation, Statistical; D005492:Folic Acid / Q000493:pharmacokinetics*; D007408:Intestinal Absorption; D007413:Intestinal Mucosa / Q000378:metabolism; D007421:Intestine, Small / Q000378:metabolism*; D008956:Models, Chemical; D051381:Rats
D005492:Folic Acid