Intestinal beta-galactosidases. I. Separation and characterization of three enzymes in normal human intestine.

Previous studies based on work in the rat and preliminary experiments with human intestine have suggested that two beta-galactosidases are present in small intestine, and it is believed that only one of these enzymes is a lactase important for the digestion of dietary lactose. The high prevalence of intestinal lactase deficiency in man prompted more complete study of these enzymes. Human intestinal beta-galactosidases were studied by gel filtration on Sephadex G-200 and Biogel P-300 as well as by density gradient ultracentrifugation. Gel filtration produced partial separation into three peaks of enzyme activity, but much activity against synthetic substrates was lost. Only the trailing peak with specificity for synthetic beta-galactosides was completely separated from the other enzymes. Thus gel filtration was not a suitable preparative procedure for biochemical characterization. Density gradients separated the enzymes more completely, and they were designated according to their sedimentation rates and further characterized. Enzyme I has a molecular weight of 280,000, pH optimum of 6.0, and specificity for lactose of at least five times that for cellobiose or synthetic substrates. A second lactase, enzyme II, possesses slightly greater activity against lactose than for some synthetic substrates and is incapable of splitting cellobiose. Further, it has a lower pH optimum (4.5) and is present in two molecular species (molecular weights 156,000 and 660,000). Enzyme III shows specificity only for synthetic beta-galactosides but has a pH activity curve identical with enzyme I and a molecular weight of 80,000. Whereas human liver and kidney contain a beta-galactosidase with the same biochemical characteristics as intestinal enzyme II, enzymes I and III appear to be peculiar to intestine, and enzyme I most probably represents the lactase of importance in the mucosal digestion of dietary lactose. The following paper considers this further in terms of the biochemical change in intestinal lactase deficiency.