Monosaccharide inhibition of human intestinal lactose

Lactase has been partially purified from human intestinal mucosa. Lactase was inhibited by glucose, galactose, and fructose with the use of a radioisotopic assay; sucrase was inhibited by glucose and fructose; and maltase was inhibited by glucose alone. However, the K_i for glucose was the same (about 30 mM) for all 3 enzymes. No obvious structural requirements could be determined for inhibition of lactase, but naturally occurring sugars were the best inhibitors. inhibition by sugars of all disaccharidases was pH dependent, with a pH optimum of 6. It was suggested that inhibition of lactase by sugars might be physiologically important and provide one mechanism for the production of lactose intolerance.