The activation of intestinal brush border sucrase by alkali metal ions: an allosteric mechanism similar to that for the Na+-activation of nonelectrolyte transport systems in intestine

pH-dependent inhibitory effects of tris and lithium ion on intestinal brush-border sucrase

Tris and two of its hydroxylated amine analogs were examined in a metal-free, universal n-butylamine buffer, for their interaction with intestinal brush border sucrase. Our recent three-proton-families model (Vasseur, van Melle, Frangne and Alvarado (1988) Biochem. J., 251, 667-675) has provided the sucrase pK values necessary to interpret the present work. At pH 5.2, 2-amino-2-methyl-l-propanol (PM) causes activation whereas Tris has a concentration-dependent biphasic effect, first causing activation, then fully competitive inhibition. The amine species causing activation is the protonated, cationic form. The difference between the two amines is related to the fact that Tris has a much lower pKa value than PM (respectively, 8.2 and 9.8). Even at pH 5.2, Tris (but not PM) exists as a significant proportion of the free base which, by inhibiting the enzyme fully competitively, overshadows the activating effect of the cationic, protonated amine. Above pH 6.8, both Tris and PM act as fully competitive inhibitors. These inhibitions increase monotonically between pH 6.5 and 8.0 but, above pH 8, inhibition by 2.5 mM Tris tends to diminish whereas inhibition by 40 mM PM increases abruptly to be essentially complete at pH 9.3 and above. As pH increases from 7.6 to 9.0, the apparent affinity of the free amine bases decreases whereas that of the cationic, protonated amines, increases. In this way, the protonated amines replace their corresponding free bases as the most potent inhibitors at high pH. The pH-dependent inhibition by 300 mM Li+ is essentially complete at pH 8, independent of the presence or absence of either 2.5 mM Tris or 40 mM PM. Even at pH 7.6, an excess (300 mM) of Li+ causes significant increases in the apparent Ki value of each Tris, PD (2-amino-2-methyl-1-3-propanediol) and PM, suggesting the possibility of a relation between the effects of Li+ and those of the hydroxylated amines which in fact are mutually exclusive inhibitors. The inhibitory results are interpreted in terms of a mechanistic model in which the free bases bind at two distinct sites in the enzyme's active center. Binding at the glucosyl sub-site occurs through the amine's free hydroxyl groups. This positioning facilitates the interaction between the lone electron pair of the deprotonated amino group with a proton donor in the enzyme's active center, characterized by a pK0 around 8.1. When this same group deprotonates, then the protonated amines acting as proton donors replace the free bases as the species giving fully competitive inhibition of sucrase.

Purification of the rabbit small intestinal sucrase-isomaltase complex: separation from other maltases

The rabbit intestinal sucrase-isomaltase complex has been purified to homogeneity after solubilization with Triton X 100 followed by chromatography on DEAE Sepharose CL 6B and a second solubilization with papain. After hydrophobic chromatography on Octyl Sepharose CL 6B, separation from other contaminating maltases was achieved by gel filtration on Ultrogel ACA 22. The final enzyme was purified 390 fold, with a specific activity of about 10 units per mg protein.

Alkali-metal-ion- and H+-dependent activation and/or inhibition of intestinal brush-border sucrase. A model involving three functionally distinct key prototropic groups

For rabbit intestinal brush-border sucrase, a model based on classical Michaelis-Dixon theory cannot fully explain the peculiar antagonistic relationship existing between the substrate and one key proton, Hx, which at acid pH values behaves as a fully competitive inhibitor. In the same pH range, a second proton, Hy, is responsible for changes in catalytic activity and behaves as a mixed-type partially non-competitive inhibitor [Vasseur, Tellier & Alvarado (1982) Arch. Biochem. Biophys. 218, 263-274]. Although involved in the same ionization reaction, these two protons have different kinetic functions, since they are responsible for affinity-type and capacity-type effects respectively. Depending on whether Hx is bound or not, we postulate the enzyme to alternate between two distinct forms differing in their binding properties. The alkali-metal ions Na+ and Li+ have a concentration-dependent biphasic effect on this equilibrium. At low concentrations they facilitate the release of Hx, resulting in K-type activation. At higher concentrations they favour enzyme reprotonation, causing K-type inhibition. On the basic side of the pH spectrum, our results confirm the existence of separate non-competitive effects of the alkali-metal ions, particularly Li+ [Alvarado & Mahmood (1979) J. Biol. Chem. 254, 9534-9541]. To explain the molecular mechanisms underlying the alkali-metal-ion- and H+-dependent effects, we formulate a sucrase model, the three-protons model, in which the acid and basic ionization constants involve respectively two and one key prototropic groups that are functionally distinguishable. A global iterative fit of the relevant general equation to our whole set of data has permitted us to estimate the numerical value of each of the constants constituting the model.

A rabbit jejunal isolated enterocyte preparation suitable for transport studies

A method is described for isolating viable enterocytes from rabbit jejunum. Estimates of sucrase and gamma-glutamyl transferase activities in cells isolated by this method suggest that they originate from the upper villus only. Isolated cells accumulate both alpha-methyl-D-glucoside and alanine, maintaining high intracellular concentrations for at least 60 and 40 min respectively. Accumulation of alpha-methyl-D-glucoside is inhibited by the presence of phloridzin. The cells accumulate 42K and 86Rb in an identical manner. This uptake, which is maintained for at least 60 min, is inhibited in the presence of ouabain. Passive efflux of 42K and 86Rb occurs with rate constants which are virtually identical. The efflux follows a single exponential suggesting that it originates from only one intracellular compartment. It is suggested that the preparation can be used to study the effect of sugars and amino acids on K efflux. The advantages of using such a preparation are discussed.

The distribution of (Na+ + K+)-ATPase along the villus crypt-axis in the rabbit small intestine

The migration of intestinal epithelial cells from the crypts to the tips of villi is associated with progressive cell differentiation. The changes in Na+-pump levels during migration have been measured in epithelial cells isolated from rabbit small intestine. A significant proportion of ouabain-sensitive (Na+ + K+)-ATPase in the cell homogenates was latent but could be unmasked by detergent treatment. Highest detergent activation was observed in villus cells. The distribution of pumping sites was also assessed by measuring ouabain binding to intact cells. The kinetics of specific binding was consistent with the interaction of the cardiac glycoside with a single population of binding sites with an apparent Kd of around 10(-7) M. Both enzyme assay and ouabain-binding measurements suggest that a 2-3-fold increase in the number of Na+-pumping sites accompanies cell differentiation in rabbit jejunal epithelium. This increase in pumping capacity might be an adaptation of the cells to their absorptive function.

Effect of a single oral dose of endosulfan on intestinal uptake of nutrients and on brush-border enzymes in rats

The effect of a single oral dose of endosulfan (5 mg/kg body weight) on the uptake of certain nutrients and brush-border enzymes has been studied in rat intestine. The uptake of glucose and alanine was elevated but that of leucine was decreased in endosulfan-fed rats. There was no change in the uptake of phenylalanine and lysine in insecticide-fed rats. The activities of brush-border sucrase and alkaline phosphatase were considerably increased while the activity of Na+ K+ ATPase was reduced in endosulfan-exposed animals. The leucine aminopeptidase activity was unaffected in pesticide-treated rats. There was a significant decrease in cellular LDH and GOT activities with no change in GPT activity. Neither was there a considerable increase in the cellular glucose-6-phosphatase activity (P less than 0.01) in the pesticide-fed rats. These results suggest that endosulfan toxicity induces certain functional changes in the intestine.

Transport studies and enzyme assays in mice infected with human Giardia lamblia

It is well established that Giardia infection causes malabsorption. However, the precise mechanism of such a malabsorption is not known. To investigate this, transport studies, using the tissue accumulation technique, were carried out in mice infected with G. lamblia obtained from human stools. There was a significant fall in the transport of D-glucose, L-alanine and glycine in the infected animals compared with the controls. Kinetics of the D-glucose and glycine transport system were examined by measuring the tissue uptake in the presence of different concentrations of the substrate. For glucose, the affinity constant (Km) for the transport site was the same (4 . 37mM) in normal and infected animals but the maximal transport rate (V max) was considerably reduced in infected animals (158 . 7 mu moles/hr/g tissue) compared with (357 . 1 microgram moles/hr/g tissue) in controls. Results with glycine were similar; the Km was similar in control and infected animals (5 . 7 mM) whereas the V max was reduced in infected animals (27 . 02 microgram moles/hr/g tissue) compared with controls (45 . 5 micrograms moles/hr/g tissue). Analysis of the intestinal enzymes showed a significant decrease in the levels of brush border sucrase, lactase and alkaline phosphatase in infected animals; the cellular enzymes, LDH, GOT and GPT remained unaffected. The observed aberrations in the transport functions and brush border enzymes suggest that G. lamblia causes malabsorption by damaging the epithelial membrane of the enterocyte.

Acute dieldrin toxicity: effect on the uptake of glucose and leucine and on brush border enzymes in monkey intestine

Administration of a single oral dose of dieldrin (20 mg/kg body wt.) to rhesus monkeys considerably elevated the uptake of glucose and the activities of brush border sucrase, lactase, maltase and alkaline phosphatase in intestine compared to control animals. Leucine uptake and leucine amino peptidase activity was significantly depressed in pesticide-treated animals. Kinetic studies with brush border sucrase revealed that augmentation of enzyme activity in pesticide-fed animals was due to an increase in the disaccharidase content.

Quantitative analysis of the mixed activating effects of the alkali metal ions on intestinal brush-border sucrase at pH 5.2

The activation of rabbit brush-border sucrase by the alkali metal ions, Li+, Na+ and K+, was analyzed using the equations of the random-order allosteric model previously proposed for sucrase (Mahmood, A. and Alvarado, F. (1975) Arch. Biochem. Biophys. 168, 585). The alkali metals have mixed activating effects in tert-butylamine buffers at pH 5.2, including: 1. Affinity-type activation, where the apparent Km decreases as a hyperbolic function of the metal concentration. 2. Capacity-type activation, where the apparent V increases with the metal concentration. These two effects were analyzed quantitatively: firstly, by using linear transformations that allowed us to solve each partial equation separately and secondly, by iteration of the general equation, which permits treating the mixed effects as a whole. Results are consistent with the interpretation that a single metal-binding (activator) site suffices to explain the simultaneous occurrence of the two types of kinetic effect. Nevertheless, complicating factors exist that may require the postulation of additional sites for monovalent cations. In particular, the tert-butylammonium ion appears to interface with the effects of the alkali metals, especially Li+.

A kinetic study of the interactions between amino acids and monosaccharides at the intestinal brush-border membrane

1. The influx of amino acids into guinea-pig intestinal rings in vitro is inhibited by monosaccharides, and that of monosaccharides by amino acids. Two hypotheses have been proposed to account for these heterologous interactions. According to the first, the cis hypothesis, there is an allosteric interaction between substrates binding to separate but related sites at the outer face of the brush-border membrane matrix. In contrast, the trans hypothesis envisages the interaction to result from a partial dissipation of the electrochemical sodium gradient due to the cotransport of each substrate with sodium ions. 2. In an attempt to distinguish between the merits of the two hypotheses, we examined the kinetics of the inhibition of phenylalanine influx by two sugars of widely different affinities, galactose and beta-methylglucoside. Since beta-methylglucoside carries more sodium into the cell than galactose, the trans hypothesis would predict it to be the stronger inhibitor, but in fact the opposite result is found. 3. Equations were developed to describe the inhibitions in accordance with the cis hypothesis. The satisfactory agreement between experimental observations and theoretical predictions provides support for the applicability of the model. Further implications of the polyfunctional carrier model are discussed.

A quantitative histochemical technique for the characterisation of alpha-glucosidases in the brush-border membrane of rat jejunum

A quantitative histochemical method to determine the Km and Vmax of alpha-glucosidases in the intestinal epithelium without disruption of the cellular structure is described. 2-Naphthyl-alpha-D-glucoside was used as substrate and hexazonium-p-rosaniline as coupling agent. Using a Leitz MPV2 microdensitometer and a field measuring 4 X 4 micrometers, and reading the test samples against a blank focused on the lamina propria, we observed that the intensity of the colour was a linear function of both the incubation time up to 20 min, and the thickness of the slice up to 20 micrometers. The ratio between the extinction at the absorption maximum and at a second wave-length was constant, whatever the intensity of the colour. By determining the relationship between the extinction and the substrate concentration under standard conditions (slice thickness of of 10 micrometers and incubation time of 10 min), we obtained a saturation curve described by a Km of 0.68 +/- 0.038 mM and a Vmax of 1.41 +/- 0.039 A lambda 480 . 10(-2) . micrometers-1 . min-1. When the hydrolysis of the same substrate by a homogenate of jejunal mucosa was examined biochemically under comparable conditions, a Km of 0.64 +/- 0.012 mM and a Vmax of 57.3 +/- 0.70 mU/mg protein were obtained. When the natural substrate, sucrose, was used in the biochemical study, a Km of 15 +/- 3.5 mM and a Vmax of 149 +/- 24.7 mU/mg protein were obtained. These experiments demonstrate that the kinetic constants of enzyme reactions can be assessed with equal accuracy on histochemical sections as in tissue homogenates.

Effects of DDT (chlorophenotane) administration on glucose uptake and brush border enzymes in monkey intestine

A considerable increase occurs in D-glucose uptake and brush border sucrase and lactase activities in the intestine of monkeys treated with a single oral dose of DDT. Brush border alkaline phosphatase activity remains unaffected in the pesticide treated animals. In vitro addiction of DDT has no effect on the sugar absorption and disaccharidase activities.

Effect of chronic alloxan diabetes and insulin administration on intestinal brush border enzymes

Brush border sucrase and lactase activities are significantly elevated in alloxan-induced chronic diabetes and are restored to control levels after insulin treatment. Alkaline phosphatase and Mg-ATPase levels remain unchanged in diabetes, compared to a control group. Insulin treatment alone to control animals also led to enhanced activities of these enzymes.

Harmaline interaction with sodium-binding sites in intestinal brush border sucrase

The effect of harmaline on rabbit brush border sucrase has been studied at pH 6.8. An initial analysis in classical kinetic terms revealed harmaline to be a fully competitive inhibitor of the substrate, sucrose. In spite of this result however, the following hypothesis has been tested. Harmaline, which is positively charged in the physiological range of pH, might in fact compete, not directly with the substrate site, but rather with an allosterically-related sodium-binding site which has been postulated to be involved in the activation of sucrase by the alkali-metal ions (Mahmood and Alvarado, Arch. Biochem. Biophys. 168, 585, 1975). Because of its size, harmaline, when bound to the metal site, could at least partially overlap with the substrate site, thereby behaving as if it were an authentic fully competitive inhibitor of the substrate. This hypothesis appears to be confirmed by the fact that the alkali metals can completely reverse the inhibition caused by harmaline.

Dose any enzyme follow the Michaelis-Menten equation?

A literature search has been conducted to see to what extent steady-state kinetics studies in the period 1965-1976 have revealed deviations from Michaelis-Menten kinetics. It was found that over 800 enzymes have been reported as giving complex curves for a variety of reasons and a group by group classification of all these enzymes has been carried out listing all the types of variations reported and the authors' explanations. In addition, for highly complex curves, we have determined the minimum degree of the rate equation. There were very few determined attempts to demonstrate adherence to the Michaelis-Menten equation over a wide variety of experimental conditions and substrate concentration and almost invariably detailed experimental work revealed unsuspected complexities. For these reasons, it is concluded that the assumption that most enzymes follow the Michaelis-Menten equation can not be supported by an appeal to the literature.