Opiate activity and transepithelial passage of intact beta-casomorphins in rabbit ileum

Comparative Effects of Milk Containing A1 versus A2 β-Casein on Health, Growth and β-Casomorphin-7 Level in Plasma of Neonatal Dairy Calves

Simple Summary

Bovine milk generally contains two types of the milk protein β-casein, A1 and A2. Enzymatic digestion of the A1 type yields the opioid peptide β-casomorphin-7, which is suggested to adversely affect human and animal health. This study aimed to compare the effects of milk containing either homozygote A1 or A2 β-casein on health and growth parameters in 47 dairy calves during the first three weeks of life. Additionally, we studied, for the first time, the levels of intact β-casomorphin-7 in plasma of calves fed milk of alternative β-casein genotypes. Milk feeding of “A2-milk” led to a lower milk intake and a looser fecal consistency (higher prevalence of diarrhea) compared to “A1-milk”. Nevertheless, weight gains and end weights of calves of both feeding groups were similar, which might be caused by the associated higher protein content of milk containing the A2 variant. Intact β-casomorphin-7 was detected in plasma after A1- and A2-milk consumption, but was almost 5 times higher for A1-calves. In summary, A2-milk minimized the cleavage of the opioid peptide β-casomorphin-7 and might have advantages in the development of pre-weaned dairy calves.

Abstract

Research has shown that digestion of A1 β-casein (β-CN) affects gastrointestinal motility and opioid activity through the release of the peptide β-casomorphin-7 (β-CM7). In the case of the A2 variant, the cleavage of β-CM7 does not occur or occurs at a very low rate. Therefore, the aim of the study was to compare the effects of milk containing either homozygote A1 or A2 β-CN on health and growth parameters of dairy calves. Forty-seven neonatal calves (24 females, 23 males) of the breeds German Holstein (GH, n = 9), German Simmental (GS, n = 33) and their crossing (GH × GS, n = 5) were used in a 21-day feeding study. Fecal score (FS), respiratory frequency (RF), and rectal body temperature (BT) were recorded daily, whereas body weight was measured at birth and at day 21 to estimate the average daily weight gain (ADG). Additionally, blood was collected from calves three times during the experimental period and, for the first time, the respective plasma samples were analyzed for intact β-CM7. Consumption of A2-milk led to a lower daily milk intake (dMI) (p < 0.05). Furthermore, fecal consistency was softer for calves fed A2-milk (p < 0.05). Although 44% of A2-calves had diarrhea or revealed a tendency towards it (FS ≥ 3), A1-calves had a prevalence of 21%. Calves with a FS of 4 were offered an electrolyte solution and received a dietary food supplement for the stabilization of the fluid and electrolyte balance. Nevertheless, similar ADG and end weights (EW) of calves fed A1- or A2-milk (p > 0.05) indicate that A2-milk may compensate higher diarrhea rates and lower dMI due to the associated higher protein content. This is the first report of intact β-CM7 in plasma of calves fed milk of either A1 or A2 β-CN. Evidence from this study suggests that due to the change in the amino-acid sequence, A2-milk might be able to prevent or, at least, to minimize the cleavage of β-CM7 in calves.

Digestion of Protein in Premature and Term Infants

Premature birth rates and premature infant morbidity remain discouragingly high. Improving nourishment for these infants is the key for accelerating their development and decreasing disease risk. Dietary protein is essential for growth and development of infants. Studies on protein nourishment for premature infants have focused on protein requirements for catch-up growth, nitrogen balance, and digestive protease concentrations and activities. However, little is known about the processes and products of protein digestion in the premature infant. This review briefly summarizes the protein requirements of term and preterm infants, and the protein content of milk from women delivering preterm and at term. An in-depth review is presented of the current knowledge of term and preterm infant dietary protein digestion, including human milk protease and anti-protease concentrations; neonatal intestinal pH, and enzyme activities and concentrations; and protein fermentation by intestinal bacteria. The advantages and disadvantages of incomplete protein digestion as well as factors that increase resistance to proteolysis of particular proteins are discussed. In order to better understand protein digestion in preterm and term infants, future studies should examine protein and peptide fragment products of digestion in saliva, gastric, intestinal and fecal samples, as well as the effects of the gut micro biome on protein degradation. The confluence of new mass spectrometry technology and new bioinformatics programs will now allow thorough identification of the array of peptides produced in the infant as they are digested.

Regulation of L-valine absorption by opioids interacting with mu-receptors in rabbit ileum

In intact tissue, [D-Ala2,MePhe4,Gly-ol5]enkephalin (10(-5) M; mu-ligand), diminished short-circuit current (Isc) and increased water, Na+ and Cl- net fluxes in vitro under open circuit conditions; it also inhibited L-valine absorption and L-valine-dependent variations of short-circuit current (delta Isc,val). Naloxone (10(-6) M) antagonized these effects. In the absence of the muscularis and myenteric plexus this enkephalin or morphine (mu-ligand) reduced Isc and delta Isc,val. These enkephalin effects occurred at different times. Different concentrations of enkephalin were tested for their effects on delta Isc,val. [D-Ala2,D-Leu5]enkephalin (mainly a delta-ligand) significantly decreased Isc but not delta Isc,val. The reduction of L-valine absorption does not depend on the effects on basal ion transport. Interaction of opioids with mu-receptors located in the submucosal plexus and/or in the epithelial cell accounts for this reduction. This enkephalin effect seems to be at least partially under the control of the myenteric plexus.

Oral absorption of peptides: influence of pH and inhibitors on the intestinal hydrolysis of leu-enkephalin and analogues

Leu-enkephalin (YGGFL) and several analogues were chosen as model peptides for the study of peptide absorption and hydrolysis in the rat jejunum. An HPLC assay was adapted to detect YGGFL or the analogues and metabolites. Peptide hydrolysis was studied in the rat jejunum using a single-pass perfusion method. Extensive hydrolysis of YGGFL was observed in the rat jejunum and approaches to reduce its metabolism were studied. The brush border enzymes are a major site of enkephalin hydrolysis. Lumenal peptidases were secondary to the brush border enzymes in hydrolyzing the enkephalins in this system. In the in situ perfusion system, YGGFL is hydrolyzed primarily to Tyr and GGFL by the brush border aminopeptidase and to YGG and FL by brush border endopeptidase. Lowering the jejunal pH below 5.0 significantly reduces aminopeptidase activity and, to a lesser extent, endopeptidase activity. An aminopeptidase inhibitor, amastatin, produced more pronounced inhibitory effects at higher pH and the endopeptidase inhibitors, tripeptides YGG and GGF, are effective even below pH 5.0. Coperfusion of YGGFL with a combination of aminopeptidase and endopeptidase inhibitors, e.g., amastatin and YGG, is more effective in inhibiting hydrolysis since both metabolic pathways are inhibited. Leu-D(Ala)2-enkephalin, while showing enhanced stability against aminopeptidase hydrolysis, is hydrolyzed at the Gly-Phe bond by the endopeptidase. Its hydrolysis is not affected by pH changes or amastatin but is decreased by YGG. The YGGFL wall permeability was estimated and is not a limiting factor for oral absorption.