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Activities of digestive enzymes (maltase, alkaline phosphatase, amino peptidase M, and glycyl-L-leucine dipeptidase) in small and large intestine, liver, and kidney were studied in rats of different ages kept in normal (8) and low (3) amounts of pups per litter. The low-protein diet for 10 days at once after weaning was found to change the mass of the organs and their digestive enzyme activities in all studied rat groups. The revealed changes were more prominent in rats kept under conditions of breast-overfeeding. In adult animals of this group, distribution of the alkaline phosphatase activity along the small intestine differed from that in control rats. The obtained results seem to confirm the fact that any disturbance of the nutrition quality in early ontogenesis leads to disturbance of the "metabolic programming of enzyme systems" of digestive and non-digestive organs.

Restriction of protein in nutrition of rat pups weaned at different terms has been found to produce changes in activities of digestive enzymes (maltase, alkaline phosphatase, aminopeptidase M, and glycyl-L-leucine dipeptidase) in the small and large intestine both at once after cessation of nutrition with low-protein diet for 10 days and 4 months later. In adult animals after the earlier or later weaning there are observed not only a decrease or increase of the enzyme activities, but also a different type of distribution of the alkaline phosphatase activity along the small intestine, which is more pronounced in the lately weaned rats. Thus, disturbance of metabolic programming of enzyme systems of the small and large intestine due to a change of quality of nutrition in early ontogenesis depends on terms of weaning of animals.

Per os administration of Vilon (Lys-Glu) or Epithalon (Ala-Glu-Asp-Gly) to aged Wistar rats for 1 month significantly increased activity of membrane enzymes maltase and alkaline phosphatase in epithelial layer of the small intestine. In addition, Vilon significantly increased activity of cytosolic glycyl-L-leucine dipeptidase in the stromal and seromuscular layers of the small intestine in comparison with the control rats not treated with this agent. These findings suggest improvement of trophic and barrier functions of the small intestine and corroborate the hypothesis on the existence of not only epithelial, but also subepithelial enzymatic barrier supporting the enzyme system in the small intestine, especially in aged animals.

With ageing the membrane/soluble form ratio of membrane-boundenzymes increases especially in the ileum and that of peptidases changes rather insignificantly in both portions of the rat small intestine. The soluble forms are supposed to take part in assimilation of food substrates penetrating enterocytes in an unsplit form due to an enhanced permeability of the membranes, particularly those of the ileum.

Proceeding from the newly obtained evidence on the distribution of digestive enzymes in mucosal, submucosal and muscular layers of the small intestine in rats, a hypothesis has been formulated: in addition to the enterocyte enzyme complex forming the epithelial barrier, there seems to exist a postepithelial barrier formed by the enzymes localized in subepithelial structures. The efficiency of this second barrier was manifested by the study of the peptide hydrolysis and transport using everted intact and de-epithelialized intestinal sacs.

The large intestine of human and dog contains considerable activities of various aminopeptidases and genuine dipeptidases. Furthermore, a dipeptidase is described, which is independant in respect to the C-terminal configuration of the peptide. This dipeptidase is an intrinsic membrane protein. The physiological significance of these enzymes is discussed.

The role of the hydrophobic part in the functioning of some amphipathic enterocyte enzymes has been studied in various animal species (fish, mammals, insects). Kinetic (Km, Vmax, pHopt) and regulatory properties of membrane, detergent and protease forms of carbohydrases, alkaline phosphatase and alanine aminopeptidase were compared. All the species under study showed significant differences among these forms of enzymes. The data obtained demonstrate the important role of the hydrophobic anchor in forming both catalytic and regulatory properties of enzymes.

Alanine aminopeptidases from the mucous membranes of the small intestines of dog, rabbit and trout were purified as detergent forms. The enzymes obtained were characterized with respect to substrate specificity, effector behaviour, Michaelis constants, molecular weight and temperature dependence. In comparison to data of the protease forms of the alanine aminopeptidases, differences could be demonstrated.

Alanine aminopeptidases from the mucous membranes of the small intestines of dog, rabbit and trout were concentrated 200 fold in the case of dog and rabbit, and 15 fold in the case of trout. The enzymes were analyzed with respect to substrate specificity, effector behaviour, optimum pH value, Michaelis constants and molecular weight. The values obtained for these enzymes are not consistent in all points with those found for alanine aminopeptidases isolated from the mucous membrane of human jejunum and ileum.