Metabolism of lactose in the human body.
The article describes the metabolism of lactose in both normo- and hypolactasia in the human body. Attention is drawn to the differences in lactose tolerance among lactose malabsorbers.
Animals↗
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Diagnosis of hypolactasia and lactose malabsorption.
The article briefly describes the various methods that are available for diagnosing hypolactasia. Special attention is drawn to the fact that different methods are useful at different levels of the health care organization. When the test result indicates lactose malabsorption, general malabsorption should be excluded by a glucose-galactose tolerance test, for example. If the glucose-galactose tolerance test produces a normal result, it can be assumed that the patient has primary adult-type selective lactose malabsorption. The possibility of secondary lactose malabsorption must be excluded according to the principles described by Villako & Maaroos (104).
Diagnosis, Differential↗
Effect of milks inoculated with Lactobacillus acidophilus or a yogurt starter culture in lactose-maldigesting children.
Dairy products containing live bacteria that possess lactase activity are used for dietary management of lactose maldigestion. The efficacy of acidophilus milk and the effect of consuming unfermented milk that had been inoculated with yogurt bacteria have not been examined in children. We compared scores for breath H2 excretion and symptoms of 20 lactose-maldigesting children following ingestion of 250 ml of uninoculated milk with two identical milks inoculated with 10(10) cells of Lactobacillus acidophilus or with a commercial yogurt starter culture containing 10(8) cells of Lactobacillus lactis and 10(10) cells of Streptococcus thermophilus. Nine of 10 subjects who were symptomatic following ingestion of uninoculated milk experienced a reduction in symptoms following ingestion of milk inoculated with L. acidophilus, without a decline in H2 excretion. Five of 6 subjects who were symptomatic following uninoculated milk had decreased symptoms and a significant reduction in H2 excretion following milk inoculated with the yogurt culture. For lactose-maldigesting children, milks inoculated with L. acidophilus or with a yogurt culture were associated with decreased symptoms compared with those with uninoculated milk.
Adolescent↗
Lactose intolerance in East Indians of Trinidad.
The incidence of lactose intolerance and by inference intestinal lactase deficiency was investigated by means of the oral lactose tolerance test in 30 East Indians born in Trinidad but of Indian heritage. Twenty showed flat curves (67%) and twenty had cramps and diarrhoea following the lactose load (67%). In India milk consumption is less than in Trinidad. Since all subjects were of the same racial origin the findings are interpreted as suggesting that lactase deficiency among East Indians is inherited on a genetic basis.
Adolescent↗
Lactase deficiency and lactose malabsorption. A review.
The results of previous investigations of lactase deficiency and lactose malabsorption are reviewed. It showed that lactase activity and its decline in animals and humans is controlled genetically, but also that its phenotypic expression as lactose malabsorption is influenced by nongenetic factors: adaptation, biological (circadian) rhythmicity, hormones, gastrointestinal functions, and nutritional components can alter the response to lactose intake.
Biological Clocks↗
Lactose intolerance in the stumptail macaque (Macaca arctoides): case report.
A female stumptail macaque had signs of intermittant diarrhea and chronic weight loss over a 2-year period during which she was fed a commercial laboratory maintenance diet. Intolerance of this individual to lactose, a ubiquitous constituent of most commercial primate diets, was diagnosed on the basis of lactose tolerance tests and favorable clinical response to dietary carbohydrate substitution with glucose. Oral administration of varying doses of glucose to this animal consistently produced elevations in plasma glucose concentrations exceeding 20 mg/dl, while orally-administered doses of lactose did not cause significant elevations in blood glucose concentrations.
Animals↗
[Incidence of lactose intolerance in postmenopausal osteoporosis].
The incidence of lactose intolerance was determined in 31 women with idiopathic postmenopausal osteoporosis. Nine subjects exhibited a pathological rise in blood glucose (less than 20 mg/100 ml) after receiving 50 g lactose. The absorption of glucose and galactose, xylose, and fat was normal in the 31 patients. Daily intake of calcium was below the normal range in 3 subjects with and in 6 subjects without lactose intolerance. The association between lactase deficiency and osteoporosis is stressed.
Adult↗
Lactose malabsorption. Optimalization of investigational methods.
Although the prevalence of lactose malabsorption is high and the importance of this diagnosis is established in gastroenterological practice, still a variety of poorly standardized tests is used to investigate the ability of the small bowel to digest lactose. In this report the various most commonly used lactose malabsorption tests are described and compared with each other knowing that a 'gold standard' does not exist. The hydrogen breath test is the test most used in clinical practice. On the basis of recent investigations the recommendation is made to improve the accuracy of this test by prolonging it to 4 hours and by simultaneous measuring of the oro-coecal transit time.
Blood Glucose↗
[Assessment of the lactose test and its value in detecting intolerance to milk sugar].
In 109 healthy and sick subjects suffering from diverse affections of the digestive organs the lactase activity in the mucosa of the large intestine was determined and a lactose test set up (1 g of lactose per 1 kg of body weight). In the test the following factors were considered: clinical symptoms, the rate of passage in the intestines (till the appearance of carmine), blood glucose content, pH and sugar in the feces. To appraise the activity of lactase the most accurate indicator in the test appeared to be the estimated degree of the rising blood glucose concentration as against the area of its ascent (23% of erroneous results). Because of a great variability of the test indicators the assessment of the lactase activity by combination of the latter did not increase the total number of correct results. However, in cases when there appeared combinations of indicators most characteristic for different activity of the lactase, the significance of the test results proved much higher (11.5-7% of erroneous results). Since in the lactose tolerance tests it is the total lactase activity, covering the entire surface of the small intestine that is recorded and subject to assessment are also possible changes, adaptive inclusive, of the gastro-intestinal tract motoricity, this test characterizes in the final analysis more correctly the tolerance of food products containing lactic sugar, than does direct determination of the lactase activity.
Humans↗
Construction of a lactose-assimilating strain of baker's yeast.
A recombinant strain of baker's yeast has been constructed which can assimilate lactose efficiently. This strain has been designed to allow its propagation in whey, the byproduct resulting from cheese-making. The ability to metabolize lactose is conferred by the functional expression of two genes from Kluyveromyces lactis, LAC12 and LAC4, which encode a lactose permease and a beta-galactosidase, respectively. To make the recombinant strain more acceptable for its use in bread-making, the genetic transformation of the host baker's yeast was carried out with linear fragments of DNA of defined sequence, carrying as the only heterologous material the coding regions of the two K. lactis genes. Growth of the new strain on cheese whey affected neither the quality of bread nor the yeast gassing power. The significance of the newly developed strain is two-fold: it affords a cheap alternative to the procedure generally used for the propagation of baker's yeast, and it offers a profitable use for cheese whey.
Lactose↗
Closer look at lactose-mediated support of retinal morphogenesis.
We have previously shown in intact isolated eye rudiments from Xenopus laevis that lactose, but not mannose, permits the formation of organized photoreceptor outer segments in the absence of the retinal pigment epithelium (RPE). The purpose of this study was to determine, using electron microscopic analysis, the key ultrastructural differences between healthy retinas, lactose-protected retinas, and retinas that developed aberrantly to reveal which subcellular structures were exclusively present in healthy retinas. Filamentous actin was also localized in retinas to determine its distribution under the various conditions. In healthy retinas, calycal processes extending approximately three-fourths of the length of the outer segment surrounded highly organized photoreceptor outer segments. Adherens junctions were localized between adjacent photoreceptors and Müller cells at the outer limiting membrane. In addition, Müller cells possessed apical processes that extended for a short distance beyond the adherens junctions. These fine cytoarchitectural details were missing in retinas that completed differentiation in the absence of the RPE; both calycal and apical processes were no longer present and adherens junctions were sparsely intermittent. Müller cells appeared atrophic. Similarly, mannose promoted none of the fine cytoarchitectural details of the retina. Lactose, however, supported the formation of the proper subcellular cytoarchitecture of both photoreceptor and Müller cells. These results suggest that these subcellular structures may be fundamental for the proper assembly and stability of organized outer segments and are necessary to allow for normal cytogenesis of the outer retina.
Actins↗
Oxidation of lactose to lactobionic acid by a Microdochium nivale carbohydrate oxidase: kinetics and operational stability.
Oxidation of lactose to lactobionic acid by a Microdochium nivale carbohydrate oxidase was studied. The K(m)-value for lactose, obtained by a traditional enzymatic assay, was 0.066 mM at pH 6.4 and 38 degrees C. The effect of oxygen on the enzymatic rate of reaction as well as the operational stability of the enzyme was studied by performing reactions at constant pH and temperature in a stirred tank reactor. Catalase was included in all reactions to avoid inhibition and deactivation of the oxidase by hydrogen peroxide. At pH 6.4 and 38 degrees C, K(m) for oxygen was 0.97 mM, while the catalytical rate constant, k(cat), was 94 s(-1). Furthermore, we found that the operational stability of the oxidase was dependent on the type of base used for neutralization of the acid produced. Thus, when 2 M NaOH was used for neutralization of a reaction medium containing 50 mM phosphate buffer, significant deactivation of the oxidase was observed. Also, we found that the oxidase was protected against deactivation by base at high lactose concentrations. A simple model is proposed to explain the obtained results.
Ascomycota↗
[The degradation of lactose in the intestines of rats on human and cow's milk diets].
Female lactose adapted rats were kept for 10 days on human milk (HM), a milk diet adapted to human milk (MD1), and a milk diet rich in protein and phosphate (MD2), the lactose supply being always the same. In the caecum, colon and faeces, the pH value, the phosphorus content, the buffer capacity and the numbers of microorganisms with proteolytic activity (Bacteroides, Klebsiella, Proteus) were lower and the lactose concentration and the beta-galactosidase activity were higher on HM and MD1 than on MD2.
Animals↗
Solid-state glycation of beta-lactoglobulin by lactose and galactose: localization of the modified amino acids using mass spectrometric techniques.
The Maillard reaction is commonly encountered during food processing or storage, and also in human nutrition, hence there is a need for analytical methodologies to identify and characterize the modified proteins. This paper reports specific methods using mass spectrometric techniques to localize protein modifications induced by lactose and galactose on beta-lactoglobulin (beta-Lg) under solid-state glycation conditions. The extent of glycation was first determined by liquid chromatography/electrospray ionization mass spectrometry (LC/ESI-MS). The specific identification of lactose-modified amino acid residues was realized using both NanoESI-MS, NanoESI-MS/MS (neutral loss scanning modes) and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) (with and without guanidination of lysine residues) on unfractionated digests. The results indicated that, after 8.25 h of incubation, the lysine residues were the main targets of lactose-induced modification. In addition to the 15 lysine residues, Leu1 (NH2 terminal) and the Arg124 were also found to be modified, thus leading to a total of 17 different modified amino acid residues (versus 15 found by LC/ESI-MS measurement). In a second set of experiments, different strategies consisting of constant neutral loss and precursor ion scanning were compared to characterize galactose-induced modifications. Owing to the high level of beta-Lg glycation, the combined use of these different strategies appeared to be necessary for determining the galactose-modified sites after 8.25 h of incubation. Thus, among the 22 galactose adducts deduced from the LC/ESI-MS measurement, apart from the N-terminal and classical lysine residues, we also observed a few arginine residues (Arg40, Arg124 and Arg148) that were modified, and also dialkylations on specific lysine residues (Lys47, Lys75).
Amino Acids↗
Moisture-induced surface crystallization of spray-dried amorphous lactose particles studied by atomic force microscopy.
The aim of this study was to show that atomic force microscopy (AFM) can be used to obtain mechanistic and kinetic information about the process of moisture-induced surface crystallization of single particles of amorphous lactose. Completely amorphous lactose particles were prepared by spray-drying a solution of alpha-lactose monohydrate, and moisture-induced crystallization was monitored for a bed of particles by microcalorimetry and for single particles by AFM. From the AFM images it was found that crystallization of the surface of single particles can be described in terms of a sequence of three events: an initial smoothening of the surface, formation of crystalline nanostructures dispersed in amorphous material, and growth of these structures to a complete crystalline surface. The surface roughness parameter rugosity was used to estimate the fraction crystalline surface, and the growth kinetics were found to obey the JMAK equation. The fraction crystalline surface at different times could also be estimated by determining the growth rate of individual crystals. It was concluded that AFM offers a unique means of visualizing the process of moisture-induced surface crystallization of amorphous particles and enables mechanistic and kinetic information about the process to be extracted.
Calorimetry↗
Cysteine scanning mutagenesis of the N-terminal 32 amino acid residues in the lactose permease of Escherichia coli.
Using a functional lactose permease mutant devoid of Cys residues (C-less permease), each amino acid residue in the hydrophilic N-terminus and the first putative transmembrane helix was systematically replaced with Cys (from Tyr-2 to Trp-33). Twenty-three of 32 mutants exhibit high lactose accumulation (70-100% or more of C-less), and an additional 8 mutants accumulate to lower but highly significant levels. Surprisingly, Cys replacement for Gly-24 or Tyr-26 yields fully active permease molecules, and permease with Cys in place of Pro-28 also exhibits significant transport activity, although previous mutagenesis studies on these residues suggested that they may be required for lactose transport. As expected, Cys replacement for Pro-31 completely inactivates, in agreement with previous findings indicating that "helix-breaking" propensity at this position is necessary for full activity (Consler TG, Tsolas O, Kaback HR, 1991, Biochemistry 30:1291-1297). Twenty-nine mutants are present in the membrane in amounts comparable to C-less permease, whereas membrane levels of mutants Tyr-3-->Cys and Phe-12-->Cys are slightly reduced, as judged by immunological techniques. Dramatically, mutant Phe-9-->Cys is hardly detectable when expressed from the lac promoter/operator at a relatively low rate, but is present in the membrane in a stable form when expressed at a high rate from the T7 promoter. Finally, studies with N-ethylmalemide show that 6 Cys-replacement mutants that cluster at the C-terminal end of putative helix I are inactivated significantly.(ABSTRACT TRUNCATED AT 250 WORDS)
Amino Acid Sequence↗
Protein domain mapping by lambda phage display: the minimal lactose-binding domain of galectin-3.
Mapping of protein domains having a distinct function is essential to understanding the protein's structure-function relationship. We used a bacteriophage lambda surface expression vector, lambdafoo, in order to determine the minimal carbohydrate-binding domain of human galectin-3 (Gal-3). Gal-3 cDNA was randomly digested by DNase I and cloned into the phage vector. The library generated was screened by affinity selection using lactose immobilized on agarose beads. DNA sequence analysis of a set of isolated clones defined the minimal folding domain of Gal-3 required for lactose binding, which consisted of 136 amino-acid residues. Using the phage clones isolated, we also determined relative dissociation constants in solution between lactose and the minimal domain expressed on the phage surface. This technique does not require either purified or labeled proteins, and bacteriophage lambda surface display may, therefore, be useful for protein domain mapping and in vitro studies of various macromolecular interactions.
Antigens, Differentiation↗
Dependence of lactose metabolism upon mutarotase encoded in the gal operon in Escherichia coli.
A new gene (galM) has been identified as the fourth cistron of the gal operon, encoding enzymes for the metabolism of galactose and lactose in Escherichia coli. Induction of the gal operon either from the gal promoters or from a neighboring prophage lambda promoter expresses the galM gene as well. The new structure of the gal operon from the promoter end is galE-galT-galK-galM in counter-clockwise orientation on the chromosome. Genetic and biochemical analyses have revealed that the galM gene product has mutarotase activity, which converts alpha-aldose to the beta-anomer. Unlike mutarotase from other bacteria in which the enzyme is primarily processed for export and secretion, the mutarotase from E. coli does not appear to be processed and yet is still found in periplasm (and culture media when overexpressed) in significant amounts. Although the interconversion of the sugar anomers occurs spontaneously in pure water in vitro, the in vivo formation of alpha-D-galactopyranose (the substrate for phosphorylation) from beta-D-galactopyranose (generated by beta-galactosidase hydrolysis of lactose) is largely dependent upon the presence of the mutarotase. This shows that efficient lactose metabolism requires mutarotase. These results give credence to the idea that the activity of intracellular water is not high enough to permit a simple extrapolation of observed in vitro reactions to in vivo situations in every case.
Amino Acid Sequence↗