A Second Enzyme Defect in Acquired Lactase Deficiency: Lack of Small-Intestinal Phlorizin-Hydrolase

Abstract Adult acquired lactase deficiency is accompanied by the lack of small‐intestinal phlorizin‐hydrolase activity, an enzyme which had not been previously investigated in man. The two activities correlate closely in the human biopsy specimens examined. These observations, together with others previously made on the rat lactase‐phlorizin‐hydrolase complex, point to related biological regulation mechanism(s) for the two enzymes (or active sites). Phlorizin‐hydrolase activity in the human small intestine is due to more than one catalytic site. Lactase itself is responsible for only one quarter of the total phlorizin‐hydrolase activity.