Dietary lactose and the aetiology of human small-intestinal hypolactasia

Impact of molecularly defined hypolactasia, self-perceived milk intolerance and milk consumption on bone mineral density in a population sample in Northern Europe

OBJECTIVE

Milk intake as a source of calcium is considered an important factor for bone mineral metabolism. Low lactase activity in adult-type hypolactasia (HL) and self-perceived lactose intolerance (LI) are the main limiting factors of milk intake. The aim of this study was to examine the relationship of HL, LI, individual milk consumption and bone mineral density in a population with high milk consumption and a prevalent vitamin D deficiency.

MATERIAL AND METHODS

A population-based study of 367 men and women aged 25-70 years was conducted in Estonia. HL was diagnosed by direct sequencing of the LCT gene, bone mineral density and body composition measured by dual energy X-ray absorptiometry (DXA). An original health questionnaire was used to collect data on milk and dairy consumption, self-perceived milk intolerance, supplement usage and fracture history.

RESULTS

Lactase genotype and phenotype had no effect on bone mineral density in this high milk consumption population with a prevalent vitamin D insufficiency. Milk consumption was a significant determinant of bone mineral density in Estonia. Self-perceived milk intolerance leads to self-imposed reductions in milk consumption, increases in bone turnover and an increased risk of fracture.

CONCLUSIONS

Self-perceived milk-intolerance rather than HL influences milk consumption and has deleterious effects on bone metabolism.

Review article: lactose--a potential prebiotic

Lactose maldigestion, which affects a large majority of the world's population, has been mostly linked with uncomfortable symptoms. In addition, dairy consumption is variably blamed or recommended for a number of ill effects. There is, however, emerging evidence that certain lactic acid-producing bacteria, which selectively consume prebiotics, may be beneficial against some lower intestinal diseases. Lactose maldigestion and lactose should perhaps be re-evaluated as a potential provider of such a prebiotic. This historical and observational review discusses lactose and argues the opinion that it has prebiotic potential. Moreover, in maldigesters, natural ingestion or lack thereof may be relevant in the pathogenesis of diseases such as colorectal cancer and inflammatory bowel diseases.

The genetically programmed down-regulation of lactase in children

BACKGROUND & AIMS

Intestinal lactase activity is high in all healthy human babies, but in adults a genetic polymorphism, which acts in cis to the lactase gene, determines high or low messenger RNA (mRNA) expression and activity (lactase persistence and nonpersistence, respectively). Our aim was to investigate the onset of expression of this polymorphism in children.

METHODS

Activities were analyzed in relation to age in normal biopsy specimens from a 20-year collection of diagnostic specimens. In a smaller set of 32 samples, aged 2-132 months, RNA was extracted for semiquantitative reverse-transcription polymerase chain reaction. Marker polymorphisms were used to determine the allelic origin of lactase mRNA transcripts.

RESULTS

Analysis of 866 children showed evidence that the lactase persistence/nonpersistence polymorphism began before 5 years of age. The 32 children tested had high lactase mRNA and activity. Six children aged 2-16 months showed equal expression of two alleles, 2 children aged 7 and 14 months showed slightly asymmetric expression, and 7 children aged 22-132 months showed very asymmetric expression, the second allele being undetectable in the 11-year-old, as previously seen in lactase-persistent heterozygote adults.

CONCLUSIONS

Genetically programmed down-regulation of the lactase gene is detectable in children from the second year of life, although the onset and extent are somewhat variable.

Hypolactasia and lactase persistence. Historical review and the terminology

Lactase (more precisely lactase-phlorizin hydrolase) is located in the brush border of the small intestinal enterocytes and is responsible for the hydrolysis of dietary lactose. The earliest studies on lactase activity in mammals were published around the turn of the century. In 1903, it was found that the dog had a very low lactase activity and therefore lactose remained unhydrolysed, causing diarrhoea. Human hypolactasia was demonstrated in 1963, and it was soon found that it is very common, commoner than lactase persistence in most parts of the world. In 1973, adult-type hypolactasia was shown to be inherited by an autosomal recessive single gene. This article reviews the early expansion of the knowledge on lactase and hypolactasia as well as the correct terminology.

Genetics and epidemiology of adult-type hypolactasia

The prevalence of adult-type hypolactasia varies from less than 5% to almost 100% between different populations of the world. The lowest prevalence has been found in northwestern Europe, around the North Sea, and the highest prevalence in the Far East. The reason for the variation is that selective (primary) hypolactasia is genetically determined by an autosomal recessive single gene. It is assumed that thousands of years ago all people had hypolactasia in the same way as most mammals do today. At that time in cultures where milk consumption was started after childhood, lactase persistence had a selective advantage. Those people with lactase persistence were healthier and had more children than people with hypolactasia, and the frequency of the lactase persistence gene started to increase. The present prevalence of hypolactasia can be explained fairly well by this culture historical hypothesis. This article reviews the evidence for the inheritance of adult-type hypolactasia and the occurrence of hypolactasia in different countries and populations of the world.

Lactase deficiency in patients with intestinal schistosomiasis

Sixty patients having schistosomiasis mansoni infection were studied for lactase deficiency by different methods including lactase assay in intestinal biopsies. Thirty patients suffering from simple intestinal (Group I) and 30 patients suffering from hepatointestinal (Group II) schistosomiasis were compared to 60 controls (Group III). Lactase deficiency was evaluated by symptoms of lactose intolerance, stool pH and osmolarity, oral lactose tolerance test (OLLT), oral lactose hydrogen breath test (OLHBT) and lactase activity (LA) in small intestinal biopsies. OLTT showed intolerance in 36.7% of Group I and 40% of Group II compared to 23.3% of the controls. OLHBT showed intolerance in 26.7%, 30% and 18% for the three groups respectively, indicating a significance in Groups I and II. Abdominal symptoms of hypolactasia had higher scores in the patient groups compared to the controls, and was even higher in Group I than Group II. The mean lactase activity measured by micromol/gm wet wt/min was not statistically higher in Group I but higher in Group II than the controls. No correlation was found between lactase activity in the mucosa and the symptoms of hypolactasia in Group I. Hence, other factors in intestinal schistosomiasis may affect lactose hydrolysis. The study showed also that OLHBT is the most sensitive, specific and efficient among the non-invasive tests.

Mechanism of maturational decline of rat intestinal lactase-phlorizin hydrolase

The maturational decline in lactase-phlorizin hydrolase (LPH) activity was studied in groups of young rats ranging from suckling to early post-weaned states. Associated maturational increases in sucrase-isomaltase (SI) and maltase-glucoamylase (MG) activities were also examined as a comparison. Over this time period changes in cellular concentrations of the three enzymes were observed, reflecting corresponding changes in enzyme activities. Synthesis patterns accompanying these maturational changes in concentration were examined using labelled leucine as a marker. Synthesis of LPH was found to be maintained at constant rates independent of the maturation-associated decline in its concentration, whereas the increases in cellular concentrations of SI and MG were due to accelerated synthesis of the enzyme. Turnover of LPH, based on both the fractional synthesis rate and the disappearance rate of labelled leucine from prelabelled LPH pools, was increased in a quantitatively similar way to the decline in LPH concentration. These findings are consistent with our earlier proposal that the maturational decline of LPH occurs because of accelerated turnover, without a decrease in its rate of synthesis.

Biosynthesis and maturation of lactase-phlorizin hydrolase in the human small intestinal epithelial cells

The biosynthesis and maturation of the human intestinal lactase-phlorizin hydrolase (LPH; EC 3.2.1.23-3.2.1.62) has been studied in cultured intestinal biopsies and mucosal explants. Short time pulse labelling revealed on high mannose intermediate of Mr 215,000 which was converted upon endo-beta-N-acetylglucosaminidase H (endo-H) digestion to a polypeptide of Mr 200,000. The brush border form of LPH was revealed after longer pulse periods and has Mr 160,000. It possesses mainly complex oligosaccharide chains and, owing to its partial endo-H sensitivity, at least one chain of the high mannose type. Leupeptin partially inhibited the appearance of the Mr-160,000 polypeptide. Monensin treatment of biopsies resulted in the modification of the Mr-160,000 species to the Mr-140,000 molecule, which was endo-H sensitive. Pulse-chase analysis indicated a slow post-translational processing of the high mannose precursor (Mr 215,000) to yield the mature brush-border form (Mr 160,000) of LPH. Our results further indicate that LPH is synthesized as a single polypeptide precursor which is intracellularly cleaved to yield the mature brush border of LPH. The data presented suggest that this cleavage occurs during the translocation of the molecule across the Golgi complex.

Effects of prolonged alcohol administration and a high carbohydrate-low protein diet on the activities of the jejunal brush border enzymes in the rat

The effects of prolonged alcohol administration were studied on the brush border enzyme activities of the jejunum in rats receiving either a normal laboratory diet or a high carbohydrate-low protein for several weeks. Alcohol (15%) given in association with the normal diet provoked a stimulation of sucrase, maltase, and lactase activities after four weeks, but no significant modification in aminopeptidase activity. These results obtained for the disaccharidases were very similar to those observed with the high carbohydrate-low protein diet given without alcohol, although major differences were obvious in the timing of enzyme stimulation. In contrast, this dietary condition initiated a drop in aminopeptidase activity. When alcohol was given in association with the high carbohydrate-low protein diet, no modification in aminopeptidase activity was detected and the stimulation for the disaccharidase activities was similar to that observed with the high carbohydrate-low protein diet given alone. The present results suggest that the mechanisms involved in the stimulation of brush border disaccharidase activities were different for alcohol and for the high carbohydrate-low protein diet.

Purification and characterisation of amphiphilic lactase/phlorizin hydrolase from human small intestine

Human intestinal lactase/phlorizin hydrolase (EC 3.2.1.23/62) was purified in its amphiphilic form by immunoadsorbent chromatography. The purification factor was approximately 600 and the recovery 14%. The enzyme was essentially free from other known brush-border peptidases and disaccharidases and appeared homogeneous in crossed immunoelectrophoresis and polyacrylamide gel electrophoresis in sodium dodecylsulphate. The purified enzyme hydrolyzed lactose (pH optimum 5.8--6.0, Km 21 mM), phlorizin (Km 0.44mM) and other beta-galactosides and beta-glucosides. Tris inhibited the hydrolysis of lactose whereas phlorizin hydrolysis was almost unaffected. The activity against these two substrates also showed different thermal stability. It is suggested that the human enzyme has two different sites: one for lactose hydrolysis, inhibited by phlorizin and one for phlorizin hydrolysis. By gel filtration on Ultrogel AcA 34 the amphiphilic form of the enzyme had a molecular weight of 320000 while the hydrophilic form (papain-treated) had a molecular weight of 280000. This indicates that the anchoring segment(s) plus the bound detergent has a molecular weight of approximately 40000. In polyacrylamide gel electrophoresis in sodium dodecylsulphate the fully denatured enzyme had an apparent molecular weight of 160000. It is therefore suggested that the human lactase/phlorizin hydrolase is composed of two monomers each with a molecular weight of 160000. The electromicroscopic picture gives further evidence for this suggestion. In addition the possibility of a high molecular weight, one polypeptide chain is discussed.

Isolation and characterization of the proximal and distal forms of lactase-phlorizin hydrolase from the small intestine of the suckling rat

The complex between lactase (beta-D-galactoside galactohydrolase, EC 3.2.1.23) and phlorizin hydrolase (glycosyl-N-acylsphingosine glycohydrolase, EC 3.2.1.62) has been purified from the proximal and distal regions of the small intestine of suckling rats. The two enzymes behaved differently on DEAE-cellulose ion-exchange chromatography and during electrophoresis in the presence and absence of sodium dodecyl sulphate (SDS), but they have very similar cyanoge bromide cleavage patterns. Kinetic studies on the proximal and distal enzymes showed the same pH optimum of 6.0 and the same heat stability at 45 degrees C, but a small difference in Km. Treatment of both enzymes with fucosidase, mannosidase or N-acetylhexosaminidase did not affect enzymic activity or electrophoretic mobility. Neuraminidase digestion abolished the electrophoretic differences and gave two active enzymes with similar isoelectric points.

Immunoelectrophoretic studies on human small intestinal brush border proteins--amount of lactase protein in adult-type hypolactasia

Jejunal biopsies from 15 Greenlandic and three Danish patients with adult type hypolactasia and nine Greenlandic and 15 Danish patients with normal lactase activity were analysed quantitatively for lactase protein by crossed immunoelectrophoresis. A constant correlation between the amount of lactase activity and immunologically reactive lactase protein was demonstrated irrespective of the lactase activity level. As immunoelectrophoresis expresses the amount of enzyme protein independent of the enzymatic activity, it is concluded that the low enzymatic activity in adults with hypolactasia is caused by a low amount of lactase and not by a modified inactive enzyme.