Biochemical changes in the jejunal mucosa of dogs with naturally occurring exocrine pancreatic insufficiency

Abstract

SUMMARY The roles of extracellular and intracellular mechanisms in the degradation of brush border proteins have been investigated by studying the small intestinal mucosa of dogs with naturally occurring exocrine pancreatic insufficiency. Peroral jejunal biopsies were homogenised and the organelles separated by isopycnic centrifugation on continuous sucrose density gradients. The distributions of marker enzymes for the principal subcellular organelles were determined in the gradients and related to the specific activities in the homogenates. There were increased activities of the brush border carbohydrases zinc-resistant x-glucosidase, maltase and sucrase in the pancreatic insufficient animals, but no change in lactase activity. The activity of y-glutamyl transferase was also higher in the affected group; the activities of two other brush border enzymes, alkaline phosphatase and leucyl-3-naphthylamidase, however, were unaltered. These findings with an increase in the modal density of the brush border from 1 20 to 1'22 are consistent with an enhanced glycoprotein content of the microvillus membrane. There were also rises in the activities of lysosomal enzymes. N-Acetyl-3-glucosaminidase activity was increased in the soluble fractions and the percentage latent enzyme activity was reduced, findings indicative of an increased fragility of the lysosomal membrane. There were no marked alterations in the activities or density gradient distributions of marker enzymes for the other organelles, stressing the specificity of the changes in the brush borders and lysosomes. These findings are compatible with the degradation of certain exposed brush border proteins by pancreatic proteases and suggest that when this is defective, intracellular degradative mechanisms may be stimulated.The enterocyte is a highly specialised cell involved in the terminal processing of nutrients and their transference from the lumen of the small intestine. Although the life span of this cell is relatively short, the protein content of the enterocyte is not static, proteins turning over at heterogeneous rates by a continuous process of synthesis and degradation (Alpers and Kinzie, 1973). A balance between these processes maintains the steady state level of individual proteins. Free ribosomes are responsible for the synthesis of intracellular proteins, whereas those destined for export or insertion into membranes are synthesised on the rough endoplasmic reticulum (Dallner et al., 1966).

Brush Border Enzymes in Evaluation of Acute Small Bowel Rejection

“…The large oligosaccharidases are removed from the surface of the enterocytes by pancreatic enzymes [11]. Increased sucrase and maltase, but not lactase, activity has been observed in dogs [12] and in humans [13] with pancreatic insufficiency. The slight diminution seen after the fifth day may depend on the lack of substrate.…”

Small intestinal absorption of amino acids and a dipeptide in pancreatic insufficiency.

“…A previous in vitro study had suggested that jejunal malabsorption of neutral amino acids might be because of a defect intrinsic to cystic fibrosis.4 In our study the defect in phenylalanine absorption was also present in patients with other forms of pancreatic insufficiency suggesting perhaps that pancreatic insufficiency per se was involved. There is evidence that pancreatic proteolytic enzymes22 23 and mucosal lysosomal hydrolases24 play a role in the turnover of brush border proteins, and thus may modulate brush border function. It is tempting to speculate that the present data could be explained on the basis of a reduction in the number of brush border transport sites, which also functioned less efficiently.…”

Biochemical changes in the jejunal mucosa of dogs with a naturally occurring enteropathy associated with bacterial overgrowth.

“…Results are expressed in the form of frequencydensity histograms the averaging of distributions being performed by computer.27 Marker enzymes for the principal subcellular organelles were assayed as described previously 26 28 Maltase, sucrase, and lactase were assayed29 in portions of biopsies collected in deionised water and disrupted in a Dounce homogeniser (Kontes Glass Co, Vineland, New Jersey, USA) with 20 strokes of a type B pestle. Protein was determined according to Schacterle and Pollack3" with bovine serum albumin (Armour Pharmaceutical Co, Chicago, USA) as standard.…”