Intestinal Folate Absorption

and 2%, respectively, was converted to 5-AMeFH4. In contrast, less than 4% of the folate transferred across ileal mucosa was 5-MeFH4 when the initial mucosa concentration was 20 nM. Specific activity of dihydrofolate (DHF) reductase, the enzyme responsible for converting PGA to tetrahydrofolic acid, was measured in villus homogenates and was significantly greater in the jejunum than in the ileum. 1,000 nM methotrexate (MTX), a DHF reductase inhibitor, markedly inhibited PGA conversion to 5-MeFH4 by the jejunum. Studies of transmural flux, initial rate of mucosal entry (influx) and mucosal accumulation (uptake) of folate were also performed. Although MTX did not alter the influx of PGA, MTX decreased jejunal mucosal uptake but increased transmural movement. TransThis work was presented in part at a joint meeting of the American Society of Clinical Investigation and American Federation of Clinical Research, in Atlantic City, 30 April 1972 and appeared in abstract form (1972. Clin. Res. 20: 462). Dr. Olinger was a recipient of the Norman Jolliffe Student Fellowship from the American Society of Clinical Nutrition. Dr. Bertino is a recipient of a Career Development Award of the National Cancer Institute. Address reprint requests to Dr. Binder. Received for publication 15 September 1972 and in revised form 6 April 1973. mural folate movement across ileal mucosa was greater than across jejunal mucosa although mucosal uptake was greater in the jejunum than in the ileum. These results could explain previous studies which have failed to identify conversion of PGA to 5-MeFH4 when intestinal preparations have been exposed to higher and less physiologic concentrations of PGA. Further, these studies suggest that 5-MeFH4 may be retained by the jejunal mucosa.