Physiological significance of the concentration of human milk glucose

Summary The concentration of glucose in human milk, [Glucose]_milk, was directly related to the volume of milk secreted not only during lactogenesis and weaning but also during full lactation. To investigate the mechanism for this observation we first established that glucose equilibrates across the apical membrane of the mammary alveolar cell, using infusion of stable isotopically labelled glucose into lactating women. Our results indicate that [Glucose]_milk can be used to measure the glucose concentration in the mammary alveolar cell, [Glucose]_cell. We then investigated the regulation of glucose transport into the mammary alveolar cell using glucose clamp methodology in fully lactating and weaning women. Maintenance of high plasma insulin concentrations for four hours under euglycemic conditions had no effect on [Glucose]_milk, demonstrating that insulin does not regulate glucose transport into the mammary gland. On the other hand, maintaining the [Glucose]_plasma at twice the fasting level resulted in a 3-fold increase in the steady state [Glucose]_milk in fully lactating women and a 5-fold increase in [Glucose]_milk in weaning women. Kinetic analysis of the data showed that the V_max for glucose transport into the mammary alveolar cell across the basolateral membrane is regulated by the level of synthetic activity in the mammary alveolar cell.Dedicated to Professor Stuart Patton on the occasion of his 70th birthday.     

The significance of tryptophan in human nutrition

Summary Aside from its role as one of the limiting essential amino acids in protein metabolism, tryptophan (TRP) serves as precursor for the synthesis of the neurotransmitters serotonin and tryptamine as well as for the synthesis of the antipellagra vitamin nicotinic acid and the epiphyseal hormone melatonin.By involvement in so manifold pathways, TRP and its metabolites regulate neurobehavioral effects such as appetite, sleeping-waking-rhythm and pain perception. TRP is the only amino acid which binds to serum albumin to a high degree. Its transport through cell membranes is competetrvely inhibited by large neutral amino acids (NAA). The TRP/NAA ratio in plasma is essential for the TRP availability and thus for the serotonin synthesis in the brain.Due to its high TRP-concentration, human milk protein provides optimal conditions for the availability of the neurotransmitter serotonin. Low protein cow's milk-based infant formulas supplemented with-lactalbumin — a whey protein fraction containing 5.8% TRP — present themselves as a new generation of formulas, with an amino acid pattern different from the currently used protein mixtures of adapted formulas, resembling that of human milk to a much higher degree.     

Zinc binding in cow''s milk and human milk

In both cow's milk and human milk, zinc was associated with proteins of high molecular weight (greater than 100 000), as judged by analysis with Sephadex G-75. Precipitation of the casein at pH 4.6 and filtration of the resultant acid whey on Sephadex G-25 led, however, to the recovery of about 90% of the zinc as a compound of low molecular weight, which was tentatively identified as zinc citrate. Over 95% of the zinc of cow's milk was sedimented with the casein micelles on ultracentrifugation. Filtration of these micellar caseins on Sephadex G-150 gave two peaks containing zinc, which corresponded to aggregates of alpha-casein-kappa-casein and of alpha-casein-beta-casein. Ultracentrifugation of human milk sedimented only approx. 40% of total zinc. Analysis of sediment and supernatant on Sephadex G-150, however, indicated that about 85% of the zinc was associated with a protein complex of molecular weight greater than 150 000. The major protein of this complex was identified as lactoferrin. A minor zinc-binding component of average molecular weight 30 000 was also observed in the supernatant. The results indicated that zinc is bound to different macromolecules in cow's and human milk. This may be a factor affecting the bioavailability to the human infant of zinc from the two milks, and it is suggested that in human milk lactoferrin may be involved in the uptake of zinc.     

The epithelial cells and cell fragments in human milk

Summary The cell fragments and epithelial cells in human milk were examined in samples obtained from 30 women: 3 of these provided sequential samples at weekly intervals for 110 days. Membrane-bound cytoplasmic fragments in the sedimentation pellet greatly outnumbered the population of intact cells in all samples. Most of the fragments were derived from secretory cells and contained numerous cisternae of rough endoplasmic reticulum, lipid droplets and Golgi vesicles containing casein micelles. Secretory epithelial cells were present in small numbers in all samples and after the 2nd month of lactation replaced the macrophage as the predominant cell type. Ductal epithelial cells represented less than 1% of the total cell population up to 8 days post-partum, but thereafter they were very rarely found. They occurred in aggregates of 2–4 cells and possessed tight junctions that circumscribed the area of cell-cell contact. All samples of milk contained squamous epithelial cells derived from the galactophores and/or the skin of the nipple. Bacteria were often attached to the surface of the squamous cells. The possible relationship between the presence of secretory epithelial cells in milk and the occurrence of milk proteins in the blood of lactating women is discussed.     

The significance of steroid metabolism in human cancer

Epidemiological and clinical evidence suggests that steroid hormones are intimately involved in the natural history of many cancers, including those of the breast, endometrium and prostate. However, it has been difficult to demonstrate that progressive changes in tumour development are related to circulating levels of steroids. This may be because further metabolism of steroids occurs locally within the tumour and its adjacent host tissue. Using the breast as an example, data has been reviewed that such local metabolism may (a) markedly change the biological potency of steroid hormones and (b) be associated with the risk, presence, pathology, stage and hormone sensitivity of cancer. The implications of these findings are discussed including the need to identify factors which regulate steroid metabolism in peripheral tissue and tumours. In this way the potential to influence the microenvironment around and within tumour cells may be realized in favour of the patient.     

The genotoxicity of alpha particles in human embryonic skin fibroblasts

Cell inactivation and induced mutation frequencies at the hypoxanthine-guanine phosphoribosyl transferase (HGPRT) locus have been measured in cultured human fibroblasts (GM10) exposed to alpha particles from 238Pu (LET at the cell surface was 100 keV/microns) and 250 kVp X rays. The survival curves resulting from exposure to alpha particles are exponential. The mean lethal dose, D0, is approximately 1.3 Gy for X rays and 0.25 Gy for alpha particles. As a function of radiation dose, mutation induction at the HGPRT locus was linear for alpha particles whereas the X-ray-induced mutation data were better fitted by a quadratic function. When mutation frequencies were plotted against the log of survival, mutation frequency at a given survival level was greater in cells exposed to alpha particles than to X rays.     

Glycobiology of human milk

Glycans are characteristic components of milk, and each species has unique patterns of specific carbohydrates. Human milk is unusually rich in glycans, with the major components being lactose and oligosaccharides, representing approximately 6.8 and 1% of the milk, respectively. Other sources of glycans in human milk include monosaccharides, mucins, glycosaminoglycans, glycoproteins, glycopeptides, and glycolipids. In human milk, the presence and patterns of these glycans vary depending upon the stage of lactation and the maternal genes and their genetic polymorphisms that control glycosyl transferases. The synthesis of milk glycans utilizes a significant portion of the metabolic energy that the mother expends when producing her milk, but other than lactose, these glycans contribute little to the nutritional needs of the infant. The data herein support several functions. 1) Many human milk glycans inhibit pathogens from binding to the intestinal mucosa. 2) Human milk glycans attenuate inflammation. 3) Glycans also directly stimulate the growth of beneficial (mutualist) bacteria of the microbiota (formerly considered commensal microflora of the intestine); these mutualists and their fermentation products can, in turn, (a) inhibit pathogens, (b) modulate signaling and inflammation, and (c) the fermentation products can be absorbed and utilized as a source of dietary calories. These functions can help direct and support intestinal postnatal growth, development, and ontogeny of colonization. The many functions of the milk glycans may synergistically protect infants from disease. Hence, human milk glycans and their homologs may serve as novel prophylactic or therapeutic agents for a diverse range of deleterious conditions.     

Peptidome analysis of human skim milk in term and preterm milk

The abundant proteins in human milk have been well characterized and are known to provide nutritional, protective, and developmental advantages to both term and preterm infants. Due to the difficulties associated with detection technology of the peptides, the expression of the peptides present in human milk is not known widely. In recent years, peptidome analysis has received increasing attention. In this report, the analysis of endogenous peptides in human milk was done by mass spectrometry. A method was also developed by our researchers, which can be used in the extraction of peptide from human milk. Analysis of the extracts by LC–MS/MS resulted in the detection of 1000–3000 Da peptide-like features. Out of these, 419 peptides were identified by MS/MS. The identified peptides were found to originate from 34 proteins, of which several have been reported. Analysis of the peptides’ cleavage sites showed that the peptides are cleaved with regulations. This may reflect the protease activity and distribution in human body, and also represent the biological state of the tissue and provide a fresh source for biomarker discovery. Isotope dimethyl labeling analysis was also used to test the effects of premature delivery on milk protein composition in this study. Differences in peptides expression between breast milk in term milk (38–41 weeks gestation) and preterm milk (28–32 weeks gestation) were investigated in this study. 41 Peptides in these two groups were found expressed differently. 23 Peptides were present at higher levels in preterm milk, and 18 were present at higher levels in term milk.