gamma-Glutamyl transpeptidase activity along the small intestine of sheep: potential areas of amino acid and peptide transport

First-pass uptake and oxidation of glucose by the splanchnic tissue in young goats fed soy protein-based milk diets with or without amino acid supplementation: glucose metabolism in goat kids after soy feeding

The study was designed to examine whether feeding soy protein isolate as partial replacement of casein (CN) affects glucose metabolism in young goats and whether effects may be ameliorated by supplementation of those AA known to be lower concentrated in soy than in CN. Goat kids (d 20 of age) were fed comparable milk protein diets, in which 50% of the crude protein was either CN (control, CON), soy protein isolate (SPI), or soy protein isolate supplemented with AA (SPIA) for 43 d (n=8 per group). On d 62 of age, a single bolus dose of d-[(13)C6]glucose (10mg/kg of BW) was given with the morning diet, and simultaneously, a single bolus dose of d-[6,6-(2)H2]glucose (5mg/kg of BW) was injected into a jugular vein. Blood samples were collected between -30 and +420 min relative to the tracer administration to measure the (13)C and (2)H enrichments of plasma glucose and the (13)C enrichment of blood CO2. Glucose first-pass uptake by the splanchnic tissues was calculated from the rate of appearance of differentially labeled glucose tracer in plasma. Glucose oxidation was calculated from (13)C enrichment in blood CO2. In addition, plasma concentrations of triglycerides, nonesterified fatty acids, glucose, insulin, and glucagon were measured. On d 63 of age, kids were killed and jejunal mucosa and liver samples were collected to measure lactase mRNA levels and lactase and maltase activities in the jejunum and activities of pyruvate carboxylase and phosphoenolpyruvate carboxykinase (PEPCK) in the liver. Basal plasma glucose concentration tended to be higher in the CON than the SPIA group, whereas basal insulin was higher in the CON group than the SPI and SPIA groups, and glucagon was higher in the CON than the SPIA group. Plasma glucose and insulin concentrations increased during the first hour after feeding, whereas plasma glucagon increased immediately after feeding and after 1h of feeding. First-pass uptake and glucose oxidation were not affected by diet. Maltase activities in proximal and mid jejunum and lactase activities in mid jejunum were lower in the CON than in the SPIA group. Activities of PEPCK were higher in the SPIA than in the SPI group. In conclusion, feeding milk diets with soy protein isolate seems to affect glucose status in kids, but has no effect on first-pass uptake and oxidation of glucose. The highest activities of lactase and maltase were observed after supplementation with AA. Higher PEPCK activities in the liver may point at elevated gluconeogenic activities after AA supplementation in soy-fed kids.

γ-Glutamyl Transpeptidase Inhibition Suppresses Milk Protein Synthesis in Isolated Ovine Mammary Cells

The membrane spanning enzyme gamma-glutamyl transpeptidase (gamma-GT; EC 2.3.2.2) catalyses the breakdown of the tripeptide glutathione and uses free amino acids (AA) to form gamma-glutamyl transpeptidase (GT) AA that become transported into cells and converted back into free AA. gamma-Glutamyl transpeptidase activity has been shown to be important for mammary AA uptake in rodent systems, and while gamma-GT activity is high in lactating bovine mammary tissue, the role of this enzyme in milk protein synthesis of the ruminant has not been defined. The present study shows that gamma-GT activity in the ovine mammary gland, like that of rodents, increases during pregnancy and peaks early in lactation. Acivicin, a well-known inhibitor of gamma-GT, decreased gamma-GT activity in acini isolated from the ovine mammary gland and did not have secondary toxicity effects on cell viability or the uptake of radiolabeled amino-isobutyric acid. Isolated ovine acini were incubated in the presence of radiolabeled leucine, and incorporation of label into secreted protein increased during incubation. Incubation of acini with acivicin decreased milk protein secretion by 75%, indicating that gamma-GT plays an important role in milk protein production in the ruminant. Acivicin did not inhibit secretion of specific caseins but caused a global decrease in individual proteins suggesting that gamma-GT may be responsible for providing a complement of AA for milk protein synthesis.

The distribution of peptidase activity in the small intestine of sheep

1. The activity of peptidase enzyme(s) was measured in homogenates of mucosa taken at slaughter from sites along the length of sheep small intestine. 2. The activity of the homogenate towards six dipeptides showed a similar pattern of distribution for each substrate. Activity was lowest in the proximal duodenum, rising through the jejunum to peak in the mid-ileum and declining towards the terminal ileum. The distribution of activity towards two tripeptides was similar. 3. Studies using the inhibitor p-hydroxymercuribenzoate showed that the activity towards the dipeptides was mainly localized in the cytosol fraction, while that towards the tripeptides was evenly distributed between the cytosol and the brush-border and activity towards a tetrapeptide was mainly associated with the brush-border. 4. The results are discussed in relation to the site and mechanism of protein digestion and absorption in the ruminant small intestine.

Gamma Glutamyl Transferase in domestic animals

In domestic animals, Gamma Glutamyl Transferase is mainly in the kidneys, the pancreas and the intestine; its liver activity is relatively high in cows, horses, sheep and goats and very low in dogs, cats and birds. The use of plasma reference values can help to interpret the variations of serum GGT mainly in hepatobiliary diseases of cattle, sheep, goats and cholestatic disorders of dogs. Urinary GGT is a good test of kidney toxic damage.