Margit Hamosh

Bioactive factors in human milk.

This article reviews the bioactive components of human milk. Special emphasis is given to immune and nonimmune protective function of major and minor nutrients in human milk. Immune modulating components, such as cytokines, nucleotides, hormones, and growth factors, are discussed. Milk enzymes with digestive function in the newborn are reviewed.

Lingual Lipase and Its Role in the Digestion of Dietary Lipid

The serous glands of rat tongue were found to contain a potent lipolytic enzyme which hydrolyzed triglyceride to mostly diglyceride and free fatty acids (FFA) at pH 4.5-5.4. Homogenates of lingual serous glands from adult rats hydrolyzed 40-70 mmol of triglyceride/g per h. The soft palate, anterior oral pharyngeal wall, and lateral oral pharyngeal glands also contained the activity, but at a much lower level. The lipolytic activity was also found in saliva collected through an esophageal cannula and in stomach contents of rats fed a fat-rich meal. The stomach contained very little activity, however, when saliva was excluded. Lipolytic activity was not found in the stomach wall or in the parotid, submandibular, and sublingual glands. The findings suggest that the lingual serous glands secrete a lipase which catalyzes in the stomach the conversion of triglyceride to partial glycerides and FFA. It is proposed that this reaction is the first step in the digestion of dietary lipid.

Effect of human milk or formula on gastric function and fat digestion in the premature infant.

The effect of diet, human milk or formula, on gastric function (lipase and pepsin activity, pH, and volume) and intragastric digestion of fat was assessed in 28 appropriate for gestational age preterm infants (gestational age, 28.9 ± 1.4, 29.1 ± 0.9, 29.5 ± 0.6 wk; birth weight, 1.00 ± 0.14 to 1.18 ± 0.07 kg). The infants were fed either human milk (n = 11), SMA Super Preemie formula (n = 9), or Similac, Special Care formula (n = 8). Fasting and postprandial activity of digestive enzymes, pH, and gastric volume (measured before or during 50 min after gavage feeding) did not differ as a function of diet among the three groups of infants. Gastric lipase output, 23.1 ± 5.1, 28.3± 6.6, and 22.5 ± 6.4 (U/kg of body weight) in human milk-, SMA SP-, or Similac SC-fed infants was comparable to the gastric lipase output of healthy adults fed a high fat diet (22.6 ± 3.0). Pepsin output was, however, significantly lower (597 ± 77, 743 ± 97, and 639± 142 U/kg of body weight) in human milk-, SMA SP-, and Similac SC-fed infants) than in healthy adults (3352 ± 753 U/kg). The hydrolysis of dietary fat was 1.7-2.5-fold higher (p < 0.01) in human milk-fed infants than in infants fed either formula. We conclude that differences in type of feeding, i.e. different fatty acid profiles(long chain or medium chain triglycerides), different emulsions (natural or artificial), and different fat particle sizes do not affect the level of activity of gastric enzymes. However, the triglyceride within milk fat globules appears to be more accessible to gastric lipase than that within formula fat particles. We suggest that the contribution of gastric lipase to overall fat digestion might be greater in the newborn (a period of pancreatic insufficiency) than in the adult.

The effect of estrogen on the lipoprotein lipase activity of rat adipose tissue.

The effect of 17beta-estradiol or progesterone administration on adipose tissue lipoprotein lipase activity was studied in male and ovariectomized female rats. Lipoprotein lipase activity was measured in acetone-ether-extracted preparations of adipose tissue with doubly labeled (14C-fatty acid, 3H-glyceryl) chylomicron triglyceride as substrate. Administration of 17beta-estradiol to male rats lowered adipose tissue lipoprotein lipase activity from 8.22 plus or minus 1.8 U/g (1 U = 1 mumol triglyceride hydrolyzed per h) to 4.96 plus or minus 0.5 U/g in the treated group. Ovariectomy increased adipose tissue lipoprotein lipase activity from 10.4 plus or minus 1.8 U/g in controls to 22.7 plus or minus 4.3 U/g. 17beta-Estradiol administration to ovariectomized rats cuased a marked fall in adipose tissue lipoprotein lipase activity: 17beta-estradiol (2.5 mug/day) lowered the enzyme activity to 9.00 plus or minus 1.2 U/g, whereas 25 mug/day further decreased lipoprotein lipase activity to 3.2 plus or minus 0.6 U/g. Blood triglyceride levels increased from 0.8 plus or minus 0.05 mumol/ml in ovariectomized rats to 1.4 plus or minus 0.09 mumol/ml in 25 mug/day 17beta-estradiol-treated rats. Progesterone administration did not affect adipose tissue lipoprotein lipase activity in either male or ovariectomized rats. Heart and lung lipoprotein lipase activity was unaffected by hormone treatment. We suggest that the rise in blood triglyceride concentrations, which accompanies high palsma estrogen levels, could be due to the marked inhibition of adipose tissue lipoprotein lipase activity.

Fat Digestion in the Newborn: CHARACTERIZATION OF LIPASE IN GASTRIC ASPIRATES OF PREMATURE AND TERM INFANTS

We have measured lipolytic activity in gastric aspirates obtained at birth in a group of 142 infants. The infants ranged in gestational age from 26 to 41 wk. Lipolytic activity, measured by the hydrolysis of long chain triglyceride ([tri-(3)H]oleate), and expressed as nanomoles FFA per milliliter gastric aspirate per minute was 333+/-66 in 55 small premature infants (gestational age 26-34 wk and body wt 750-2,000 g) and 558+/-45 in a group of 87 larger infants (gestational age 35-41 wk and body wt 2,020-4,000 g). No activity was detected in seven infants with an unusually low pH in the gastric aspirate, 2.88+/-0.44 (compared with a mean pH level of 5.59+/-0.22 in the other 135 infants). Attempts to characterize this lipase showed that it has a molecular weight of 44-48,000, pH optimum of 3.0-5.0, that FFA acceptors (albumin) stimulate activity, whereas bile salts, taurocholate and glycocholate, cause marked inhibition at concentration >3 mM. Our survey shows that enzyme activity is present as early as 26 wk of gestation, increases with gestational age, and has the same characteristics throughout gestation. The data show that the lipase in gastric aspirates differs from pancreatic lipase, but closely resembles human and rat lingual lipase. Because the lipase has a low pH optimum and does not require bile salts, it can act in the stomach where it initiates the hydrolysis of dietary fat. We suggest that intragastric lipolysis is probably of major importance in the newborn and especially in the premature infant where it compensates not only for low pancreatic lipase, but in addition, helps to overcome the temporary bile salt deficiency through the formation of amphiphilic reaction products.

Glycoproteins of the Human Milk Fat Globule in the Protection of the Breast-Fed Infant against Infections

Nonimmunological components in human milk can protect breast-fed infants against infection by microorganisms. The structural and functional characteristics of four such components are discussed. The mucin inhibits binding of S-fimbriated Escherichia coli to bucal epithelial cells; lactadherin prevents symptomatic rotavirus-induced infection; glycoaminoglycans inhibit binding of human immunodeficiency virus gp120 to its host cell CD4 receptor, and oligosaccharides provide protection against several pathogens and their toxins.

Mechanisms of Inhibition of Triacylglycerol Hydrolysis by Human Gastric Lipase

In the human stomach, gastric lipase hydrolyzes only 10 to 30% of ingested triacylglycerols because of an inhibition process induced by the long chain free fatty acids generated, which are mostly protonated at gastric pH. The aim of this work was to elucidate the mechanisms by which free fatty acids inhibit further hydrolysis.In vitro experiments examined gastric lipolysis of differently sized phospholipid-triolein emulsions by human gastric juice or purified human gastric lipase, under close to physiological conditions. The lipolysis process was further investigated by scanning electron microscopy, and gastric lipase and free fatty acid movement during lipolysis were followed by fluorescence microscopy. The results demonstrate that: 1) free fatty acids generated during lipolysis partition between the surface and core of lipid droplets with a molar phase distribution coefficient of 7.4 at pH 5.40; 2) the long chain free fatty acids have an inhibitory effect only when generated during lipolysis; 3) inhibition of gastric lipolysis can be delayed by the use of lipid emulsions composed of small-size lipid droplets; 4) the release of free fatty acids during lipolysis induces a marked increase in droplet surface area, leading to the formation of novel particles at the lipid droplet surface; and 5) the gastric lipase is trapped in these free fatty acid-rich particles during their formation. In conclusion, we propose a model in which the sequential physicochemical events occurring during gastric lipolysis lead to the inhibition of further triacylglycerol lipolysis.

Long-chain polyunsaturated fatty acids.

Long-chain polyunsaturated fatty acids (LC-PUFA) are essential for normal development. Fetal accretion of LC-PUFA occurs during the last trimester of gestation; therefore, premature infants are born with minimal LC-PUFA reserves. Recent studies indicate that the newborn can synthesize LC-PUFA from essential fatty acid precursors; however, the extent of de novo synthesis remains to be established. Postnatally, human milk provides LC-PUFA to the newborn. Maternal LC-PUFA reserves depend upon diet and can be improved by supplementation of docosahexaenoic acid and arachidonic acid during pregnancy and lactation. This in turn affects fetal LC-PUFA accretion and postnatal provision through mother’s milk. Supplementation of formula-fed preterm or full-term infants with docosahexaenoic acid and arachidonic acid leads to plasma and red blood cell LC-PUFA levels similar to those of breast-fed infants. The higher blood and presumably tissue levels of LC-PUFA following supplementation lead, however, to only temporary functional benefits.

Protective Function of Proteins and Lipids in Human Milk

Human milk provides the infant with protection against infectious diseases. This protection is conferred through several mechanisms: specific antibody targeted protection against pathogens in the infant’s environment (through milk IgA, IgG, and IgM) and broad-spectrum, nonspecific protection provided through several distinct mechanisms. These are: bactericidal effects (lactoferrin), bacteriostatic action (lactoferrin, lysozyme), lysis of microorganisms (lysozyme), antiviral effects (lactoferrin, products of milk fat digestion), antiprotozoan activity (free fatty acids produced during gastric and intestinal digestion of milk fat), and ligand action (inhibition of Helicobacter pylori adhesion to gastric mucosa by κ-casein). In addition to these protective functions of the proteins and lipids of human milk, several enzymes present in human milk might provide protection by generating components that are bactericidal (bile salt dependent lipase, peroxidase), prevent inflammatory reactions (platelet-activating factor acetylhydrolase), or protect the integrity of milk proteins (antiproteases).

Lingual and gastric lipases: species differences in the origin of prepancreatic digestive lipases and in the localization of gastric lipase

The source of the lipase(s) acting in the stomach was investigated in five animal species: rat, mouse (rodents), rabbit (lagomorphs), guinea pig (caviidae), baboon and human (primates). The activity of lingual and gastric lipases was quantitated in homogenates of lingual serous glands and of gastric mucosa, respectively, by the hydrolysis of tri[3H]oleylglycerol and is expressed in units/g (1 U = 1 mumol [3H]oleic acid released/min) per g tissue wet weight, mean +/- S.E. There were marked differences in the activity level of lingual and gastric lipases among species: mouse and rat had high levels of lingual lipase activity (250 +/- 20 and 824 +/- 224 U/g) and only traces of gastric lipase activity (4.5 +/- 0.9 and 0.04 U/g, respectively), whereas rabbit and guinea pig had no lingual lipase activity and only gastric lipase activity (78 +/- 48 and 27 +/- 7.4 U/g, respectively). In the baboon and human, gastric lipase was the predominant enzyme (109 +/- 20 U/g and 118 +/- 8.8 U/g, respectively), whereas lingual lipase activity was present in trace amounts only (0.04 U/g and 0.3 U/g, respectively). In addition to species differences in the origin of the preduodenal lipases, there were also species differences in the distribution of gastric lipase in the stomach. Thus, while in the rabbit, gastric lipase was localized exclusively in the cardia and body of the stomach, it was diffusely distributed in the entire stomach of the guinea pig and baboon. A comparison between the level of activity of lipase and pepsin (the two chief digestive enzymes secreted by the stomach), showed differences in their localization in the species studied. The difference in source (tongue vs. stomach) and site (cardia-body vs. entire stomach) of lipase secretion must be taken into account in future studies of these digestive enzymes. Although the exact contribution of lingual and gastric lipases individually to fat digestion in species which contain both enzymes cannot yet be evaluated, the markedly higher levels of gastric lipase activity in the baboon and human suggests that, in primates, gastric lipase is probably the major non-pancreatic digestive lipase.