Development of Luminal Protein Digestion

Background:There is limited information in the literature on the capacity of the preterm infant to digest human and bovine milk proteins. We therefore studied in vivo the luminal phase of the hydrolysis of proteins in human milk, human milk fortifier, and preterm formula in preterm rhesus monkeys and in infant rhesus monkeys at 6 weeks and 7 months of age.Methods:Protein hydrolysis was followed by polyacrylamide gradient gel electrophoresis and electroimmunoassay. The serum level of absorbed unhydrolyzed human α‐lactalbumin was measured by a radioimmunoassay method. Trypsin and elastase activities in duodenal contents were measured before and after the meal.Results:In 6‐week‐old monkeys, the enzyme activities decreased by 50% postprandially, whereas they increased in 7‐month‐old monkeys. In preterm and in 6‐week‐old monkeys, hydrolysis of human and bovine whey proteins was slow, and in 6‐week‐old monkeys, 30‐50% of the proteins could still be detected immunochemically in duodenal contents after 60 min. At these ages, serum levels of absorbed α‐lactalbumin were high. At 7 months of age, no or small (lactoferrin and bovine serum albumin) amounts of the proteins could be detected in duodenal contents after 15 min. At this age α‐lactalbumin was not measurable in serum.Conclusion:The low capacity to digest whey proteins in suckling monkeys may depend upon an immaturity of the exocrine pancreas to respond to secretogogues.

Digestion of Proteins in Human Milk, Human Milk Fortifier, and Preterm Formula in Infant Rhesus Monkeys

Lindberg,

Engberg,

Jakobsson

et al. 1997

Luminal Stability of Insulin‐Like Growth Factors I and II in Developing Rat Gastrointestinal Tract

Rao,

Philipps,

Williams

et al. 1998

Background:Insulin‐like growth factors (IGF)‐I and ‐II are present in milk of a number of mammalian species. The stability of IGF‐I and ‐II in the intestinal lumen was investigated by measuring the proteolytic degradation of125 I‐labeled IGF‐I and IGF‐II by rat (suckling and adult) intestinal luminal flushings in vitro.Methods:Degradation of 125I‐labeled IGF‐I and IGF‐II was assessed by measuring the generation of acid‐soluble radioactivity and the reduction of the amounts of peak activity (gel filtration). Degradation was confirmed by measuring the loss of immunoreactivity and receptor activity.Results:Incubation of 125I‐IGF‐I with midjejunal luminal flushings from 12‐day‐old suckling rats generated acid‐soluble radioactivity in a time‐ and dose‐(flushing) dependent manner, whereas incubation of 125I‐IGF‐II produced only minor amounts of acid‐soluble radioactivity. Degradation activity in luminal flushings from adult rat intestine was several times greater than that in luminal flushings from suckling rats. Degradation of125 I‐IGF‐II was several times lower than that of 125I‐IGF‐I in the intestinal luminal flushings from suckling and adult rats. The rate of decrease in immunoprecipitable 125I‐IGF‐I was considerably lower than the rate of decrease in receptor‐active radioactivity. Except for immunoreactivity, analyses of acid‐precipitable, peak‐A, and receptor‐active radioactivities demonstrate that IGF‐II is relatively more stable than IGF‐I in luminal flushings of suckling rat duodenum, jejunum, and midjejunum.Conclusions:These results show that the stability of IGF in the gastrointestinal lumen depends on the age of the animal and the segment of the gastrointestinal tract, as well as on the peptide isoform.

Effect of Milk on Somatostatin Degradation in Suckling Rat Jejunum In Vivo

Rao,

Davis,

Williams

et al. 1999

Self Cite

Background:Somatostatin‐14 is present in breast milk, and intact somatostatin‐14 has been recovered from gastric lumen of infants. Studies have shown that somatostatin‐14 is metabolized in the intestinal luminal contents in vitro, which could be prevented by the presence of breast milk. In this study, the effect of milk on stability of somatostatin‐14 in suckling rat jejunum in vivo was examined.Methods:125I‐Somatostatin‐14[Tyr 11] was administered to the isolated jejunal loops in anesthetized suckling rats in the absence or presence of milk, fractions of milk, or known protease‐peptidase inhibitors. Structural integrity of 125I‐somatostatin‐14[Tyr 11] recovered from tissues at different intervals was analyzed by gel filtration and high‐performance liquid chromatography.Results:Radioactivity rapidly disappeared from the jejunal lumen with a 50% clearance achieved by 1.2 minutes. Gel filtration and high‐performance liquid chromatography analyses showed that 125I‐somatostatin‐14[Tyr 11] was rapidly degraded into smaller fragments. At 1 minute, jejunal luminal radioactivity was eluted in a major peak with retention time of 42.4 minutes, along with other minor peaks (retention time, 5.6, 8.0, 10.4, and 14.4 minutes); only a trace amount of intact 125I‐somatostatin‐14[Tyr 11] (retention time, 44.8 minutes) was present. Coadministration of rat's milk or its soluble fraction increased the level of intact 125I‐somatostatin‐14[Tyr 11] in the jejunal lumen and jejunal tissue. Presence of rat's milk‐casein or peptidase inhibitors (bestatin, phosphoramidon, or Bowman‐Birk inhibitor), however, failed to increase the level of intact 125I‐somatostatin‐15[Tyr 11].Conclusion:These results suggest that somatostatin‐14 is rapidly degraded in the jejunal lumen of suckling rats, and that milk‐borne peptidase inhibitors prevent this somatostatin‐14 degradation.