Time- and dose-dependency of intestinal lactase activity in adult rat on starch intake

Although it is generally accepted that lactase (β-d-galactosidase, EC 3.2.1.23) activity is not influenced by intake of saccharides containing α-linkages, an effect of these carbohydrates on lactase activity was never thoroughly investigated. Activity of lactase and sucrase (sucrose α-d-glucohydrolase, EC 3.2.1.48) was determined in proximal, middle and distal thirds of the jejunoilem of female, 12-week-old rats, fed for 2 weeks a low-starch (5 cal%), high-fat (73%) diet, and in rats, that after this introduction period were fed for 1,2 and 3 days, an isocaloric middle-starch (40%), middle-fat (36%) diet or an isocaloric high-starch (70%), low-fat (7%) diet. During the entire experimental period, the body weight changes, food intake and the amount of protein per segment were practically the same in all three dietary groups. In all intestinal segments, increased intake of starch was followed by an increase of lactase and sucroase activity (both expressed as per tissue protein or per intestinal segment) within the first day. The increase continued during the second day and leveled off during the third day. A highly significant linear correlation was found between the search content of the diets and the lactase activity in all three segments. A highly significant correlation was also established in all three segments between sucrase and lactase activities. These studies thus demonstrated a dose- and time-dependency between the intake of starch (a carbohydrate containing only α-linkages) and the activity of lactase, a neutral β-galactosidase in adult rats.     

Further characterization of intestinal lactase/phlorizin hydrolase

Pig intestinal lactase/phlorizin hydrolase (EC 3.2.1.23/62) was purified in its amphiphilic form by immunoadsorbent chromatography. The purified enzyme was free of other known brush border enzymes and appeared homogeneous in immunoelectrophoresis and polyacrylamide gel electrophoresis in the presence of SDS. Pig lactase/phlorizin hydrolase was shown to have the same quaternary structure as the human enzyme, i.e., built up of two polypeptides of the same molecular weight (160000). In addition to hydrolyzing lactose, phlorizin and a number of synthetic substrates, both the human and the pig enzyme were shown to have a considerable activity against cellotriose and cellotetraose, and a low but significant activity against cellulose. The lactase/phlorizin hydrolase isolated from pigs in which the pancreatic ducts had been disconnected 3 days before death and from Ca2+-precipitated enterocyte membranes (basolateral and intracellular membranes) exhibited in SDS-polyacrylamide gel electrophoresis the same size of constituent polypeptides and the same catalytic and immunological properties as a normal brush border lactase/phlorizin hydrolase.     

The nature of maturational decline of intestinal lactase activity

We have examined the nature of the decline of lactase (EC 3.2.1.23) activity in the maturing rat intestine. It was established in an initial study that the activity decline reflected a proportional reduction in the concentration of the enzyme protein. Accumulation patterns of label into lactase, total intestinal proteins and sucrase (EC 3.2.1.48)-isomaltase (EC 3.2.1.10) were compared, 4 h following administration of a tracer dose of [3H]leucine to weanling rats exhibiting a wide range of lactase decline. Accumulation of increasing amounts of label in total intestinal proteins and sucrase-isomaltase pools was found to accompany the lactase decline, in contrast to accumulation of a constant amount of label in the declining lactase pools. The pattern of increased label accumulation in total intestinal proteins was shown in a corollary study to reflect a corresponding acceleration of total protein synthesis. On this basis, the finding of a constant amount of label in the declining lactase pools suggested a constant synthesis of lactase. We proposed earlier that associated reductions in enterocyte life-span (leading to correspondingly less lactase accumulation) rather than suppressed synthesis may provide the primary causal basis of lactase decline in the postweaned mammal.     

Cortisone and thyroxine modulate intestinal lactase and sucrase mRNA levels and activities in the suckling rat.

Glucocorticoids and thyroxine modulate postnatal intestinal sucrase and lactase activities. Whether changes in enzyme activity are accompanied by changes in enzyme mRNA levels were determined in day 6 rats given thyroxine, cortisone, or thyroxine plus cortisone and killed 3 days later. Cortisone induced precocious expression of jejunal sucrase activity which was enhanced when cortisone plus thyroxine was administered; sucrase mRNA changed in parallel. Jejunal lactase activity was unaffected by thyroxine and was increased after cortisone, but not after thyroxine plus cortisone. Jejunal lactase mRNA levels increased equally after cortisone or after cortisone plus thyroxine. Thus, cortisone induces coordinated increases in sucrase and lactase activities and in corresponding mRNA levels. Thyroxine only enhances cortisone induced sucrase expression and antagonizes cortisone by depressing lactase activity post-translationally.     

Dietary-induced increase in lactase activity and in immunoreactive lactase in adult rat jejunum.

Adult rats that had been fed on a low-starch high-fat diet for 7 days were force-fed with either the same diet or isoenergetic diets containing 40% of energy as either sucrose or lactose. Within 12h, the increase in jejunal lactase activity in sucrose- and lactose-fed rats was accompanied by a corresponding increase in immunoreactive lactase protein.     

Gene-dosage effect on intestinal lactase activity demonstrated in vivo.

The activities of the disaccharidases lactase, maltase, and sucrase were determined in upper jejunal biopsies of 65 healthy adult German males. The study was an attempt to demonstrate the gene-dosage effect on lactase activity expected from the presence of a "hypolactasia" (l) and a "lactase-persistence" (L) allele in the German population. In contrast to lactase/sucrase ratios, lactose/maltose ratios showed a trimodal distribution in proportions of presumed genotypes LL, Ll, and ll compatible with Hardy-Weinberg equilibrium. The frequency of homozygotes ll (13.8%) was similar to the average frequency of lactose malabsorbers in Germany reported in the literature. The importance of considering the lactase gene-dosage effect in population studies of lactase activity is discussed.     

Recessive inheritance of the adult type of intestinal lactase deficiency.

In order to investigate the genetic control of the adult type of intestinal lactase deficiency, 61 families with 177 children over 6 years of age were investigated. The results strongly suggest that this deficiency is inherited as a simple Mendelian recessive trait.     

Improved procedure for the purification of hepatic lipase from rat liver homogenate

A procedure is described for the purification of hepatic lipase (HL)4 from rat liver homogenate which results in a high yield (41%) of electrophoretically homogeneous enzyme. The method is based on that of Twu et al. (Biochim. Biophys. Acta 1984: 792, 330), but it is more efficient with respect to yield (about 4-fold) and purity (1.6-fold). It includes the preparation of a high-speed supernatant, chromatography in series on octyl-, heparin- and concanavalin A-Sepharose, and finally gel filtration. On SDS-PAGE analysis, the purified enzyme exhibited an apparent molecular mass of 63.6 +/- 3.2 kDa. Heterogeneity was observed, when purified HL was subjected to isoelectric focussing. The enzyme displayed a specific catalytic activity of 23,000 U* (mumol fatty acid released per h at 37 degrees C) per mg protein, when assayed with trioleoyl glycerol suspensions in arabic gum. A highly specific antiserum against rat liver HL, capable of inhibiting 817 mU* HL per microliter antiserum, was raised in rabbits.     

High intestinal lactase concentrations in adult Arbs in Saudi Arabia.

The maximum rise in blood glucose after 50 g lactose by mouth was determined in 40 adult Arabs. Out of 30 Bedouin, urban Saudi, and Yemeni and 9 of mixed ancestry (usually partly African), 25 (83%) and 2 (22%) respectively showed an increase of over 1-1 mmol/1 (20 mg/100 ml). In common with most northern Europeans and Hamitic people of northern Africa, Arabs in Saudi Arabia usually have high intestinal lactase concentrations in adult life. This persistence of high levels probably originated in the Arabian peninsula. Its selective advantage may have been associated with the fluid and calorie content of camels'' milk, which is important for survival in desert nomads.     

Persistence of high intestinal lactase activity (lactose tolerance) in Afghanistan

Summary Two hundred and seventy apparently healthy adult subjects from Afghanistan, mainly from the central and eastern parts of the country, were subjected to a lactose tolerance test. The change of blood glucose from the fasting concentration at 20 min after the administration of lactose showed a bimodal distribution. Forty-seven subjects had a rise of blood glucose concentration of more than 1.1 mmol/l and were classified as persistence of high intestinal lactase activity (PHILA), a term which lays emphasis on the fact that high lactase activity in the adult is an unusual state whose prevalence in some populations requires explanation. In the Afghan sample there were no significant differences of the frequency of PHILA in different ethnic groups.Support by Stiftung Volkswagenwerk, Hannover.     

A proposed mechanism of normal intestinal lactase decline in the postweaned mammal

Intestinal lactase declines to low levels in the postweaned mammal as an adaptation to normal termination of lactose ingestion. Although a well known phenomenon and presumably the basis of milk intolerance among most human adults, the mechanism of this decline has remained obscure. Evidence is presented to support a proposal that temporally related cytokinetic changes, which effect a generally corresponding reduction in enterocyte life-span, serve as the causal basis of lactase decline in the postweaned mammal.