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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.

The effects of sodium nitrate on the activity of digestive enzymes in mucosal and submucosal layers of the small intestine as well as in liver and kidney were studied in the growing rats maintained on either standard or protein-free diets. The enzyme systems were more sensitive in the growing rats to protein deprivation than in adult animals. However, the digestive enzymes were more resistant against nitrate intoxication in the growing rats.

The fact that the enzymes splitting food nutrients, those of protein nature in particular, not only in the epithelial cells of the small bowel, but in the lower structures of the intestinal wall, i.e. in the stromal and muscle serous layers, is shown for the first time. They are integral parts of the intestinal enzymatic barrier that protects the internal medium of the body from penetration of proteins and peptides. The activities of the enzymes which are present in the epithelial, stromal, and muscle serous layers have been demonstrated to be substantially changed in disorders (immobilization in the presence of starvation, surgical interventions into the small bowel). It is suggested that examining the functions of the postepithelial enzymatic barrier along with those of the epithelial and immunological ones will provide a deeper insight into some certain disorders of the alimentary tract.

The activity of digestive enzymes was reduced both in the small intestine mucosal layer and in the postepithelial ones. The reduction was due to sodium nitrate administration and was more obvious in the rats fed with proteins. The response of the enzyme systems of the liver and kidney was heterogeneous.

Studies have been made on the formation of the spectrum of digestive enzymes in the small intestine of weanling rats which were taken from lactating female and given semisynthetic food at the age of 17, 21 and 25 days. It was shown that the rate of general development and maturation of the digestive enzymes depends on the age of animals and the mode of their breeding (isolated or in a group of the same age). These factors significantly affect also the level of the activity and proximo-distal gradient of distribution of the following intestinal enzymes: lactase, sucrase, alkaline phosphatase, alanine aminopeptidase, glycyl-L-leucine dipeptidase, as well as the mass of the mucose and protein content of the small intestine.

Studies have been made on the activity of various digestive enzymes and their distribution between the mucosal, submucosal and muscle-serosal layers of the small intestine in Wistar and Sprague-Dawley rats. Three groups of the enzymes were found, their activities being maximal either in the mucosal, or in muscular layers, or being evenly distributed between three intestinal layers.

A wide range of digestive enzymes with different cellular location (membrane, intracellular, Lysosomal) was determined in gastrointestinal organs (stomach, duodenum, jejunum, ileum, colon) as well as in undigestive organs (liver, kidney, spleen) in normal conditions and in altered functional states (fasting, refeeding). High levels of peptidase activity was noted in undigestive organs and the colon as compared to that in the small intestine. Adaptive responses were (revealed not only for a number of membrane enzymes but also for intracellular ones including those in undigestive organs.

Activities of the digestive enzymes (lactase, maltase, saccharose, di-, tri- and aminopeptidases, alkaline and acid phosphatase) in the gastrointestinal tract, the kidney and the liver in one-day-old and adult rats as well as in 10-day-old rats after injections of hydrocortisone (H) or thyroxine (T4) were determined. On the basis of the results obtained it is summarized that (1) high levels of the digestive enzyme activities are observed in the nondigestive organs in immature and adult rats; (2) H induces the enzyme activities in the small intestine more than in other organs; T4 does not influence the activities of the most of enzymes studied both in the digestive and in the nondigestive organs; (3) high activities of a number of enzymes in the colon in one-day-old rats implies its involvement in the digestion at early stages of the ontogenesis.

Alpha-amylase was present in considerable amount in the chyme in rats both at rest and during digestion, in the latter case its level being increased. Activity of intrinsic intestinal enzymes dominated in the homogenate of the mucosa. Transition from fasting to feeding and digestion increased the level of alkaline phosphatase and dipeptidase activities in the homogenate of the mucosa. The data obtained suggest the final stages of biopolymers' hydrolysis to be localized in various enterocyte structures. The surface mucus, owing to absorbed pancreatic enzymes, binding proteins and other types of physiologically active molecules, is assumed to play the role of a specific molecular filter and take part in the initial stages of digestion.

Advances in the study of membrane digestion are described which relate to techniques for the separation of the apical glycocalyx and the study of the distribution of enzymes between the latter and the cell membrane. The regulatory properties of brush border enzymes have been demonstrated. Membrane digestion by pancreatic enzymes adsorbed on the mucosal surface and by enteric enzymes predominates in early development, whereas intraluminal digestion develops during the transition to definitive (adult) nutrition. Substrate and other, non-substrate factors are involved in the regulation of intraluminal and membrane digestion in ontogeny. The importance of lipid components of the diet for the maintenance of proximal-distal gradients of enzyme activity in the small intestine during the transition from milk to adult nutrition is discussed. At this period of development hydrocortisone affects both the synthesis of enzymes and their incorporation into the enterocyte membrane. The inducibility of different enzymes is not identical. The hypothesis has been proposed that stress is one of the factors inducing or repressing the synthesis of brush border enzymes. These effects are mediated through the hypothalamus, adrenals, hypophysis and thyroid. The experimental findings demonstrate that various stressors are responsible for the induction of sucrase, maltases, gamma-amylase, peptidases and alkaline phosphatase, and for the repression of lactase in suckling rats.