Folate binding and hydrolysis by pig intestinal brush-border membranes

Area of Science:

• Nutritional biochemistry
• Gastrointestinal physiology
• Molecular biology

Background:

• Intestinal absorption of dietary polyglutamyl folate is crucial for nutrient uptake.
• This process involves complex steps including hydrolysis, binding, and transport at the enterocyte brush border.

Purpose of the Study:

• To investigate the binding characteristics of folic acid (PteGlu) to the intestinal brush-border membrane.
• To differentiate folate binding from folate hydrolysis at the brush-border level.

Main Methods:

• Utilized brush-border vesicles from pig jejunal mucosa.
• Studied the binding of [3H]folic acid (PteGlu) and hydrolysis of [14C]pteroyltriglutamate (PteGlu3).
• Analyzed binding kinetics, pH dependence, and competitive inhibition by other folate forms and metal ions.

Main Results:

• Folic acid binding was saturable (Kd = 0.08 microM) and pH-dependent, with optimal binding at pH 5.2.
• Binding was competitively inhibited by PteGlu3 and 5-methyltetrahydrofolate, indicating specificity.
• Metal ions (ZnCl2, MgCl2, MnCl2) enhanced binding capacity but not affinity.
• Distinct differences in metal ion requirements and inhibitor susceptibility differentiated binding and hydrolysis.
• The binding component and hydrolytic enzyme exhibited different affinities for PteGlu.

Conclusions:

• Demonstrated specific, saturable, and pH-dependent binding of folic acid to the intestinal brush-border membrane.
• The data suggest the presence of a distinct binding component separate from the folate hydrolytic enzyme.
• This finding clarifies the initial interaction of folate with the intestinal absorptive surface.