Segmental distribution of soluble neutral maltase activity in suckling rat intestine levels vary independently of membrane-bound maltase and lysosomal enzymes during development

Abstract

In the distal intestine of the suckling rat a significant proportion of the activity of a number of brush border enzymes is found in the soluble compartment of mucosal homogenates. To improve understanding of the origin of these enzymes the segmental distribution of membrane bound and soluble neutral ‘membrane-typical’ maltase was studied before and during the weaning period, and before and after injection with hydrocortisone acetate at 13 days of age. Changes in soluble and membrane-bound neutral maltase activities were also compared with cathepsin B 1 and acid β-galactosidase activity from day 11 to day 22. Levels of soluble enzyme were highest in the distal intestine in the early and late weaning period and before and after induction of maltase activity by hydrocortisone. In contrast the membrane-bound enzyme was uniformly distributed along the intestine at 13 days of age, but increased predominantly in the mid intestine at weaning and after hydrocortisone. Since the segmental location of soluble and induced membrane activity differs, the soluble enzyme is apparently not accumulated as an immediate precursor of the membrane-bound form. Soluble activity declined after weaning but the onset of the decline was later than the onset of the decline in lysosomal activity. Since the disappearance of the soluble enzyme is not coincident with that of the lysosomal markers, it seems unlikely that its distribution is dependent upon the endocytic-lysosomal apparatus of the ileal cell.