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1 From the Department of Nutrition, the University of California, Davis.
2 Supported in part by a grant from Wyeth Nutritionals International. The rabbit anti-human haptocorrin was a generous gift from K Kobayashi, Hokkaido University, Hokkaido, Japan. The enteropathogenic Escherichia coli O127 strain 2348/69 was a generous gift from Jim Nataro, Center for Vaccine Development, University of Maryland School of Medicine, Baltimore. 3 Reprints not available. Address correspondence to B Lönnerdal, Department of Nutrition, University of California, Davis, One Shields Avenue, Davis, CA 95616. E-mail: bllonnerdal{at}ucdavis.edu.
ABSTRACT
Background: Limited information exists on the biological role of a vitamin B-12–binding protein, haptocorrin, in human milk. The expression of haptocorrin by human mammary epithelial cells and its presence in human milk suggest a potential physiologic function in breastfed infants.
Objective: We investigated the extent to which haptocorrin could withstand proteolytic degradation and exert antimicrobial activity under in vitro conditions designed to simulate the gastrointestinal tract of breastfed infants.
Design: An in vitro model that simulates infant gastric and intestinal digestion was developed. The structural stability of porcine haptocorrin after exposure to digestive enzymes (pepsin and pancreatin) was determined by sodium dodecyl sulfate–polyacrylamide gel electrophoresis, Western blot analysis, column chromatography, and matrix-assisted laser desorption ionization–time of flight mass spectrometry (MALDI-TOF MS). The antimicrobial activity of haptocorrin was determined by incubating haptocorrin with enteropathogenic Escherichia coli O127 strain 2348/69 and monitoring bacterial growth.
Results: The structural analysis of haptocorrin exposed to enzymes did not show a decrease in molecular weight, which indicated that haptocorrin can survive proteolytic degradation. Both haptocorrin exposed to digestive enzymes and undigested haptocorrin inhibited the growth of enteropathogenic E. coli and did so to a similar extent. Thus, haptocorrin in vitro not only retains its structure after exposure to proteases but also exhibits antimicrobial activity.
Conclusion: These results suggest that haptocorrin may exert a host-defense function against pathogens in the gastrointestinal tracts of breastfed infants.
Key Words: Vitamin B-12 • cobalamin • enteropathogenic E. coli • haptocorrin • human milk proteins • human milk host-defense factors • breast milk • breastfed infants • breastfeeding
INTRODUCTION
It has been well established that human milk contains numerous proteins that benefit the newborn infant. Proteins in human milk not only provide amino acids but also bind to and facilitate the absorption of nutrients, stimulate the growth and development of the intestinal epithelium, and aid in the digestion of other nutrients (1). In addition, breast milk has been shown to contain antimicrobial and antiviral components. The antimicrobial agents include lactoferrin (2), lysozyme (3), secretory immunoglobulin A (4), kappa-casein (5), and oligosaccharides and glycoconjugates (6, 7). The antiviral agents in human milk include mucins from the milk fat globule membrane (8) and lactoferrin (9). It was suggested that the presence of these defense components in human milk contribute to the lower incidence and shorter duration of infections in breastfed infants than in their formula-fed counterparts (10).
For the breastfed infant, dietary vitamin B-12 is supplied by human milk exclusively bound to a vitamin B-12–binding protein, haptocorrin (11). Haptocorrin has a molecular weight of 68 kDa and is heavily glycosylated; 34% of its molecular weight consists of carbohydrates (12). There is a high binding capacity for vitamin B-12 in human milk; concentrations of vitamin B-12-unsaturated haptocorrin (apo-haptocorrin) are much higher than are those of vitamin B-12-saturated haptocorrin (holo-haptocorrin) (11, 13–15). Apart from its high affinity for vitamin B-12, the exact function of haptocorrin is not well understood. Because human milk haptocorrin is expressed by human mammary epithelial cells and is not passively transferred from the maternal circulation (16), we speculate that haptocorrin has some physiologic function in the neonatal gastrointestinal tract. Previous studies have proposed a bacteriostatic function for haptocorrin, but direct experiments with pathogenic bacteria have not been performed (17–19).
In order for human milk haptocorrin to exert a physiologic function in the gastrointestinal tracts of breastfed infants, haptocorrin must first be able to survive the passage through the intestine. In the present study, we simulated in vitro the conditions of the neonatal stomach and intestine and determined the structural stability of haptocorrin against the proteolytic activity of the digestive enzymes. Because human haptocorrin is not commercially available, porcine haptocorrin was used; it is similar to human haptocorrin with regard to amino acid composition, amino-terminal sequence, antigenic properties, and carbohydrate composition. Porcine haptocorrin has been well characterized for use in several studies (14, 20–25). We also assessed its ability to inhibit the growth of a strain of Escherichia coli known to cause diarrhea in infants. We theorized that if haptocorrin exhibited this function in vitro, it could have a similar effect in the gastrointestinal tracts of breastfed infants.
MATERIALS AND METHODS
Sample preparation
Porcine haptocorrin (21 800 U/mg protein, as specified; 1 U will bind 1 x 10-9 g vitamin B-12; Sigma Chemical Co, St Louis) was resuspended in 1 mL phosphate-buffered saline (PBS: 120 mmol NaCl/L, 2.7 mmol KCl/L, 10 mmol NaPO4/L, pH 7.6). Porcine haptocorrin was used in these experiments because of its biochemical and immunologic similarity to human haptocorrin (14, 20, 22, 23, 25), which is not commercially available, and because the amount of haptocorrin required for conducting the experiments in this study greatly exceeded the amount that can be isolated by our purification methods (16). The resuspended haptocorrin was divided into two 0.5-mL aliquots (0.46 mg/mL) representing apo-haptocorrin and holo-haptocorrin. The holo-haptocorrin was formed by incubating apo-haptocorrin with 20 µL of a 1 mg/mL solution of cyanocobalamin for 1 h at room temperature. Free vitamin B-12 was removed by application of the sample to a PD-10 desalting column (Amersham Pharmacia, Piscataway, NJ). Reaction volumes for apo-haptocorrin and holo-haptocorrin were brought up to 1 mL with the addition of either PBS or human milk. Human milk was collected at 2 mo postpartum from a healthy donor. For the PBS control reaction, human serum albumin (1 mg/mL) was resuspended and brought up to a final volume of 1 mL with PBS.
In vitro digestion of haptocorrin
The following is a modification of the procedure described by Rudloff and Lönnerdal (26). To mimic conditions in the infant stomach, apo-haptocorrin (0.23 mg/mL) and holo-haptocorrin (0.23 mg/mL) in PBS or human milk and serum albumin in PBS were adjusted to pH 3.5 with 1 mol/L HCl (9 µL for samples in PBS and 13 µL for samples in human milk). Approximately 2 µL 2% pepsin in 0.001 mol/L HCl (wt:vol, 3100 U/mg solid, Sigma Chemical Co) was added to all the samples, which were then placed in a shaking incubator for 30 min at 37 °C. The enzyme-to-substrate ratio (wt:wt) was between 1:20 and 1:30, depending on the protein content of the samples. Then, to simulate conditions in the infant intestine, the pH of the samples was increased gradually to 7.0 with 0.5 mol/L NaHCO3 (9 µL for samples in PBS and 13 µL for samples in human milk), which subsequently inactivates pepsin activity. Next we added 4 µL of a 0.4% pancreatin solution in 0.1 mol/L NaHCO3 (wt:vol, 8 x USP specification, Sigma Chemical Co). The amount of pancreatic enzymes added was similar to that found in the duodenal juices of human infants (27). Samples were then placed in a shaking incubator for 1 h at 37 °C. The pH of the samples was rechecked before placing the samples in a 90 °C water bath for 3 min to inactivate the enzymes. The experiments were repeated 5 times.
Sodium dodecyl sulfate–polyacrylamide gel electrophoresis
For the sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS-PAGE), apo-haptocorrin and holo-haptocorrin samples in PBS or human milk were diluted 1:1 in sample buffer (0.0625 mol tris-HCl/L (pH 6.8), 2% SDS, 25% glycerol, and 0.01% Bromophenol Blue). Approximately 20 µg protein was applied to each well of a 10–12% gradient mini-gel (BioRad, Hercules, CA). Gels were run for 40 min at 200 constant volts in tris-glycine buffer (0.025 mol tris-HCl/L, pH 8.3; 0.192 mol glycine/L, 0.1% SDS). For the detection of proteins, gels were stained overnight with Coomassie Brilliant Blue R (Sigma Chemical Co). Destaining with acetic acid:ethanol:water (10:25:65, v:v:v) was stopped when the background became clear.
Western blot analysis
After protein separation by SDS-PAGE, proteins were electroblotted onto a nitrocellulose membrane (Schleicher and Schuell, Keene, NH) in tris-glycine buffer at a constant current of 60 mA for 1 h at 4 °C. The membrane was blocked overnight at 4 °C in 4% bovine serum albumin in PBST (PBS, pH 7.4, 0.05% Tween-20) to reduce nonspecific binding of antiserum to the membrane. After the blocking step, the membrane was washed 3 times for 15 min with PBST and then incubated with rabbit anti-human haptocorrin (provided by Dr K Kobayashi, Hokkaido University, Hokkaido, Japan) at 1:10 000 dilution for 1 h at room temperature. The membrane was washed 3 times for 15 min with PBST and subsequently incubated with donkey anti-rabbit immunoglobulin G conjugated to horseradish peroxidase (Dako, Carpinteria, CA) at 1:10 000 dilution in PBST for 1 h at room temperature. The membrane was again washed 3 times for 15 min with PBST and the bound antibody was detected with an enhanced chemiluminescent system using horseradish peroxidase/hydrogen peroxide catalyzed oxidation of luminol (Amersham Pharmacia).
Column chromatography
Apo-haptocorrin and holo-haptocorrin samples in PBS before and after exposure to proteases (200 µL) were injected into a fast-protein liquid chromatography system with a Superose 12 gel filtration column (Amersham Pharmacia). Proteins were eluted with PBS at a flow rate of 0.25 mL/min. Protein peaks were monitored at 280 nm.
Matrix-assisted laser desorption ionization–time of flight mass spectrometry
Mass spectrometry analysis was performed at the Analytical Chemistry Division, Nutrition Research, Wyeth Nutritionals International (Collegeville, PA). Apo-haptocorrin and holo-haptocorrin samples in PBS before and after exposure to proteases were desalted by using a C18 silica resin Zip Tip (Millipore, Bedford, MA) and mixed with sinapinic acid matrix (1:1; Hewlett-Packard, Wilmington, DE) before crystallization on a gold-coated 10-position probe. Mass spectra were acquired by using a Hewlett-Packard G2025A linear time of flight mass spectrometer operated in positive ion mode.
Antimicrobial activity of haptocorrin
Enteropathogenic E. coli (EPEC) O127 strain 2348/69 (provided by Dr Jim Nataro, Center for Vaccine Development, University of Maryland School of Medicine, Baltimore) was cultivated overnight in 40 mL Bacto Synthetic Broth AOAC, pH 7.1 (Becton Dickinson, Sparks, MD) containing 0.1% dextrose under anaerobic conditions at 37 °C. EPEC was used because it is one of the most common causes of gastrointestinal infection with diarrhea in infants (28). Apo-haptocorrin and holo-haptocorrin samples in PBS before and after exposure to proteases (2, 5, and 10 µg/mL) were incubated in a 96-well plate (Nunc, Naperville, IL) with 107 viable EPEC per well that was obtained by adding 0.2 mL of a 1:100 dilution of the overnight culture. The concentration range of haptocorrin used in this assay correlates with the vitamin B-12-binding capacity of human milk haptocorrin (29–32). Samples were incubated for 0.5–28 h at 37 °C. Growth of EPEC was monitored at 620 nm at all time points by using a Multiskan Ascent enzyme-linked immunosorbent assay plate reader (Labsystems, Helsinki). Growth of EPEC in the presence of haptocorrin was compared with EPEC growth of the control, which was PBS without haptocorrin (0 µg/mL) before and after exposure to proteases. The experiments were analyzed in duplicate or triplicate and repeated 4 times.
The antimicrobial activity of haptocorrin was also determined by counting the number of colonies formed compared with the controls. At 10 and 21 h, EPEC culture incubated with 5 µg/mL holo-haptocorrin was removed from the wells, diluted 1:1 000 000 in sterile PBS, and plated on Trypticase soy agar plates (Becton Dickinson). The plates were then incubated at 37 °C overnight and the colonies were counted the next day.
Statistics
The data are presented as means ± SDs. The significance of differences among different concentrations of haptocorrin in terms of their effect on E. coli growth was tested by using one-way analysis of variance with Tukey’s post hoc test. Comparison of the effect of 0 and 5 µg/mL holo-haptocorrin on E. coli colony formation was performed by using Student’s t test. GraphPad PRISM version 3.02 for WINDOWS (GraphPad Software, San Diego) was used for the statistical analyses. P < 0.05 was considered statistically significant.
RESULTS
Structural stability of haptocorrin
In vitro digestion of haptocorrin was carried out in the presence of human milk, not only because this would most closely resemble what newborn infants consume, but also because some human milk proteins susceptible to proteolytic degradation (eg serum albumin, -lactalbumin, and caseins) would serve as indicators that sufficient amounts of enzymes were used. SDS-PAGE showed a 68 kDa protein which is present after in vitro digestion in the presence of human milk (Figure 1A). SDS-PAGE also showed that serum albumin (64 kDa), which co-migrates with haptocorrin, caseins (20–40 kDa), and -lactalbumin (14 kDa) are virtually absent after proteolytic digestion, which confirms that adequate amounts of enzymes were added to the reaction mixture. Positive immunoreactivity exhibited by Western blot analysis for both apo-haptocorrin and holo-haptocorrin after they were subjected to pepsin and pancreatic proteases shows that haptocorrin has the capability to resist degradation by proteolytic enzymes (Figure 1B) and that the decrease in the width of the band in the 60-kDa range on SDS-PAGE is a result of degradation of serum albumin and not haptocorrin.
FIGURE 1. . Structural stability of apo-haptocorrin (apo-HC) and holo-haptocorrin (holo-HC) in human milk before and after exposure to proteolytic enzymes: an in vitro digestion experiment was carried out to mimic the conditions of the infant stomach and small intestine. A) Sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS-PAGE; 10–12% gel) showed a 68-kDa band corresponding to haptocorrin after exposure to proteolytic enzymes. B) Western blot analysis showed positive immunoreactivity to haptocorrin, which suggested that haptocorrin was structurally intact. SDS-PAGE confirmed that sufficient quantities of enzymes were used in this experiment, because human milk -lactalbumin (14 kDa), caseins (20–40 kDa), and serum albumin (64 kDa) (which co-migrates with haptocorrin) were virtually absent after proteolytic digestion (n = 5).
To assess the extent to which haptocorrin can resist proteolytic degradation in the absence of human milk, both apo-haptocorrin and holo-haptocorrin were subjected to in vitro digestion in PBS. SDS-PAGE did not reveal a decrease in the molecular weight of either apo-haptocorrin or holo-haptocorrin after exposure to proteolytic enzymes compared with that of the unexposed haptocorrin (data not shown). Also, Western blot analysis showed positive immunoreactivity to haptocorrin incubated in PBS after exposure to proteolytic enzymes; these results were similar to the results for haptocorrin incubated in human milk. Furthermore, we observed complete degradation of the positive control, human serum albumin dissolved in PBS, signifying that sufficient amounts of enzymes were used (data not shown).
The structural integrity of holo-haptocorrin in PBS after exposure to proteases, as assessed by gel filtration column chromatography, is shown in Figure 2. The elution profile for haptocorrin exposed to proteolytic enzymes is very similar to that for the unexposed haptocorrin sample, except for an additional peak that eluted at 19 mL. SDS-PAGE revealed that this peak corresponded to enzymes that were added to the reaction mixture. Similar results were observed for apo-haptocorrin in PBS (data not shown).
FIGURE 2. . Stability of holo-haptocorrin in phosphate-buffered saline before (- - -) and after (—) exposure to proteolytic enzymes assessed by fast-protein liquid chromatography with a Superose 12 gel filtration column. Proteins were eluted with phosphate-buffered saline at a flow rate of 0.25 mL/min and peaks were monitored at 280 nm. The slight shift in the peaks eluting around 9 mL is attributable to run-to-run variability. The peak eluting around 19 mL corresponds to enzymes used in the in vitro digestion experiment. Similar results were obtained for apo-haptocorrin (not shown); n = 4. Vo, void volume; Vt, total bed volume.
To confirm that the slight shift in the first peak seen in Figure 2 was the result of run-to-run variability and was not caused by a decrease in molecular weight resulting from haptocorrin hydrolysis, matrix-assisted laser desorption ionization–time of flight mass spectrometry (MALDI-TOF MS) was performed. Mass spectra revealed a single peak with a mass-to-charge ratio (m/z) of 68 504 Da for the [M+H+] ion for haptocorrin after exposure to proteolytic enzymes, which is similar to that observed for haptocorrin before in vitro digestion (Figure 3). Peptide fragments resulting from proteolytic degradation of haptocorrin were not detected.
FIGURE 3. . Matrix-assisted laser desorption ionization–time of flight mass spectrometry (MALDI-TOF MS) of haptocorrin after exposure to proteolytic enzymes. Mass spectrometry analysis revealed a single peak with a molecular mass of 68 504 Da for the [M+H+] ion. The lack of an apparent decrease in molecular weight compared with haptocorrin before exposure to enzymes suggests that haptocorrin remained structurally intact (n = 2).
Antimicrobial actions of apo-haptocorrin and holo-haptocorrin
To measure the antimicrobial effects of apo-haptocorrin and holo-haptocorrin after exposure to proteolytic enzymes, EPEC was cultured in the presence of haptocorrin. A growth-inhibitory effect of apo-haptocorrin and holo-haptocorrin on EPEC after in vitro digestion was seen at all concentrations of haptocorrin (2, 5, and 10 µg/mL; Figure 4A and B). At 10 h, we observed significant (P < 0.001) reductions in EPEC growth of 9%, 24%, and 27% with 2, 5, and 10 µg apo-haptocorrin/mL, respectively, compared with growth of the control (0 µg apo-haptocorrin/mL). At 17 h, EPEC growth was reduced by 26%, 39%, and 38% with 2, 5, and 10 µg apo-haptocorrin/mL, respectively (Figure 4A; P < 0.001). For holo-haptocorrin at 10 h, reductions in EPEC growth of 21%, 21%, and 11% occurred with 2, 5, and 10 µg holo-haptocorrin/mL, respectively, compared with growth of the control (Figure 4B; P < 0.001). Further reductions in EPEC growth of 35%, 33%, and 27% were observed at 17 h with 2, 5, and 10 µg holo-haptocorrin/mL, respectively, compared with 0 µg holo-haptocorrin/mL (P < 0.001). Differences in the reduction in EPEC growth among the 3 holo-haptocorrin concentrations were not significant (P > 0.05). Similar inhibitory effects on EPEC growth were observed for both apo-haptocorrin and holo-haptocorrin samples not exposed to proteases (data not shown). The antimicrobial effect of haptocorrin was also observed when the EPEC mixture containing 5 µg holo-haptocorrin/mL was plated on trypticase soy agar plates. Growth of EPEC was drastically reduced, by 70%, in the presence of haptocorrin at both time points (ie, 10 and 21 h) (Figure 5).
FIGURE 4. . Growth-inhibitory effect of (A) apo-haptocorrin and (B) holo-haptocorrin on enteropathogenic Escherichia coli (EPEC) over time. Apo-haptocorrin and holo-haptocorrin samples in phosphate-buffered saline (2 µg/mL,
FIGURE 5. . Bactericidal activity of haptocorrin was determined by counting the mean (± SD) number of colonies formed compared with the control. At 10 and 21 h, enteropathogenic Escherichia coli (EPEC) culture incubated with 5 µg holo-haptocorrin/mL () was removed from the wells, diluted 1 x 106-fold, plated on trypticase soy agar plates, and incubated overnight at 37 °C. At both time points, growth of EPEC was reduced by 70% with haptocorrin present compared with the control, which was phosphate-buffered saline without haptocorrin (0 µg/mL; ), after exposure to proteases (n = 4). *Significantly different from 0 µg/mL, P < 0.001 (Student’s t test).
DISCUSSION
The antiinfectious properties of human milk are well established. Specific host-defense factors in milk, such as secretory immunoglobulin A and lactoferrin, resist proteolytic degradation and survive passage through the gastrointestinal tract of breastfed infants (33, 34), subsequently exerting their physiologic function in the infants (35, 36). The exact function of haptocorrin has not been elucidated. It has been proposed that human milk haptocorrin has the capability to withhold vitamin B-12 from pathogens that require this vitamin in the infant intestine, thereby regulating the establishment of normal intestinal microflora (17). In addition, results from animal studies suggest that haptocorrin may facilitate vitamin B-12 absorption in the piglet (37, 38). More recently, we found evidence at the cellular level that haptocorrin mediates vitamin B-12 absorption in breastfed infants during the neonatal period when the intrinsic factor system (the mechanism by which vitamin B-12 is absorbed in adults) may not be functioning to its full capacity (39). However, in order for haptocorrin to exert its biological function in the infant gut, haptocorrin needs to survive the conditions of the upper gastrointestinal tract. In the current study, porcine haptocorrin was used because it is commercially available and has extensive biochemical and immunologic similarities to human haptocorrin (14, 20–25).
In the neonatal stomach, the degree of protein hydrolysis is limited. Although hydrogen-potassium ATPase, the enzyme necessary for the secretion of hydrogen ions by the parietal cells of the stomach, is present in its functional form from 13 wk of gestation, HCl secretion in the neonate is not at the level of that in adults (40). Thus, the pH of the gastric contents in the newborn is 4.0–4.5 but decreases over time (41, 42). Therefore, in the present in vitro digestion studies, we used a pH of 3.5 rather than the pH of 1–2 that is typically found in adults. Pepsin activity has also been detected in the stomach as early as 16 wk of gestation; however, the secretory mechanism in the neonate is not as developed as that of adults (43, 44). It has been shown previously that pepsin activity in a 1-mo-old infant is only 18% of that of adults (42, 45). In addition, because the pH for maximum pepsin activity is 2, pepsin may not have been able to act at its full capacity at pH 3.5.
Furthermore, the development of a functional response by the pancreas is not complete at birth (46). In piglets, amounts of gastric and pancreatic proteases are low during the first 3–4 wk of age, but increase markedly thereafter (47). The activity of intestinal enterokinase, the enzyme that converts the proteolytic enzyme trypsinogen to trypsin (which in turn activates other zymogens), is also low at birth but increases with age (48). Enterokinase activity at birth has been shown to be 25% of that of 1-y-old infants (49). Moreover, the transit time through the infant gastrointestinal tract is rapid during the neonatal period, thereby decreasing the time that proteins are exposed to proteases (50). The structural analysis of the haptocorrin sample exposed to proteolytic enzymes by MALDI-TOF MS did not reveal a decrease in molecular weight, suggesting that haptocorrin remained intact. Results from the in vitro digestion of haptocorrin incubated in PBS or in human milk showed intact haptocorrin; therefore, it does not appear that a component of human milk contributed to its resistance against proteolytic degradation. Although in vivo experiments were not performed in this study, haptocorrin may also be resistant to proteolytic degradation in breastfed infants. Trugo and Newport (22) found that porcine milk haptocorrin, regardless of cobalamin saturation, was resistant to degradation in piglets.
Because haptocorrin survived in vitro exposure to digestive enzymes in both human milk and PBS, it is possible that some biochemical characteristic unique to haptocorrin may be contributing to its stability. Haptocorrin is a heavily glycosylated protein; carbohydrates account for 30% of its molecular weight. The carbohydrate moieties may stabilize the conformation, protect haptocorrin from proteolysis, or both. A previous study by Gordon et al (51) showed that the carbohydrate core of intrinsic factor (the vitamin B-12-binding protein involved in the absorption of dietary vitamin B-12 in the ileum of adults) may play a role in protecting the protein from hydrolysis by pancreatic proteases in the intestinal lumen. Further studies are needed to test the extent to which the glycans of haptocorrin are a factor in its resistance against proteolytic degradation.
The potential for haptocorrin to escape proteolytic degradation in vivo suggests a possible physiologic role for haptocorrin in the gastrointestinal tract of the breastfed infant. Although previous studies have proposed a bacteriostatic function for haptocorrin in the breastfed infant, direct measurements of the inhibitory activity of haptocorrin on E. coli growth have not been conducted. Gullberg (17) proposed a bacteriostatic role for haptocorrin on the basis of the high vitamin B-12-binding capacity of human milk; however, no microbiological experiments were conducted. In addition, Ford (18) used whole sow milk to examine the bacteriostatic effects of haptocorrin on E. coli native to bovine. However, because whole sow milk was used, the bacteriostatic effects specific to haptocorrin could not be dissociated from those of other milk proteins with similar antimicrobial activity. Our in vitro results showed a growth-inhibitory effect of both apo-haptocorrin and holo-haptocorrin on EPEC. Furthermore, because differences in the ability to suppress the growth of EPEC were not observed between haptocorrin that had been exposed to proteolytic enzymes and haptocorrin that had not, the results suggest that haptocorrin is functional even after exposure to proteases.
The lack of bacterial colonies formed in the presence of haptocorrin suggests a bacteriostatic effect of haptocorrin on EPEC. The decrease in absorbance at 620 nm for haptocorrin-EPEC mixtures further supports this effect (Figure 4). The exact mechanism by which haptocorrin, regardless of vitamin B-12 saturation, exerts this bacteriostatic activity on EPEC needs further investigation. Because both apo-haptocorrin and holo-haptocorrin are present in milk, we speculate that 1) haptocorrin itself may possess a unique biochemical property that exerts bacteriostatic activity (a plausible property for apo-haptocorrin and holo-haptocorrin), or 2) haptocorrin may inhibit EPEC growth by sequestering vitamin B-12, rendering it unavailable for E. coli utilization (a plausible action for apo-haptocorrin—this may explain why in vitro, the decrease in absorbance at 620 nm is not seen until 10 h).
It has been suggested previously that haptocorrin in human milk has host-defense activity, but experiments using bacteria pathogenic to humans had never been conducted. The present study shows that in vitro, haptocorrin can withstand proteolytic degradation and exert a bacteriostatic effect on a pathogenic strain of E. coli. Therefore, haptocorrin has the potential to influence bacterial growth in the gastrointestinal tract of breastfed infants. Further studies are needed to examine the antimicrobial effect of human milk haptocorrin in vivo.
ACKNOWLEDGMENTS
We gratefully thank Stephen Yachetti of Wyeth Nutritionals International, Collegeville, PA, for expert assistance with the mass spectrometry, Winyoo Chowanadisai for technical support, and Jan Peerson for statistics advice.
YA and BL were involved in the study design and writing of the manuscript. The experiments were conducted by YA. The authors had no financial or personal interest in the company that sponsored this research.
REFERENCES