Electrostatic properties of confluent Caco-2 cell layer correlates to their microvilli growth and determines underlying transcellular flow

Recently, Rajapaksa et al. (2010) showed that the rate of uptake of potential vaccine delivery nanoparticles in the mucosal layer is a function of the electrostatic properties of the corresponding solvent. This fundamentally implies that the dominant driving forces that may be capitalized on for mucosal vaccine strategies are electrostatic in nature. We hypothesize that the driving force normal to the cell (in the direction from apical to basolateral across the cell) is of particular importance. In addition, it has been theoretically shown that the electrostatic properties of mucosal cells are directly related to their development of brush border. Here we correlate the development of brush border on a human mucosal epithelial model (Caco-2) cultured in DMEM on 3.0 µm pore sized polycarbonate membranes to their corresponding electrostatic properties characterized by measuring their normal zeta potential. Properties of normal streaming potential, hydraulic permeability, and brush border development (as determined by size and number) were monitored for 2, 6, and 16 days (after cells were confluent). Human endothelial cells (HECs), which lack brush border, were used as the control. Our results demonstrate that normal zeta potential of Caco-2 cells significantly changed from -5.7 ± 0.11 mV to -3.4 ± 0.11 mV for a period between 2 and 16 days, respectively. The zeta potential of the control cell line, HECs, stayed constant (statistically not different, P > 0.05) for the duration of the experiments. Our results show that the calculated increase in surface area of the Caco-2 cells with microvilli from 6 to 16 days was directly proportional to the corresponding measured zeta potential difference. These results imply that microvilli alter the electrostatic local environment around Caco-2 cells and, hence, enhance the normal electrostatic selective transport of solute across the mucosal barrier.

Chemical modification and site-directed mutagenesis of human liver arginase: evidence that the imidazole group of histidine-141 is not involved in substrate binding

Native and wild-type recombinant human liver arginases (EC 3.5.3.1) were photoinactivated by Rose bengal, and protection was afforded by the competitive inhibitor l-lysine. The dissociation constant for the enzyme-protector complex was essentially equal to the corresponding K(i) value. Upon mutation of His141 by phenylalanine, the enzyme activity was reduced to 6-10% of wild-type activity, with no changes in K(m) for arginine or K(i) for l-lysine or l-ornithine. The subunit composition of active enzyme was not altered by mutation, but the mutant H141F was markedly more sensitive to trypsin inactivation and completely insensitive to inactivation by diethyl pyrocarbonate (DEPC) and photoinactivation. Species with histidine groups blocked with DEPC were also insensitive to photoinactivation. We conclude that His141, which is the target for both inactivating procedures, is not involved in substrate binding, but plays a critical, albeit not essential role in the hydrolysis of enzyme-bound substrate.

Manganese-dependent inhibition of human liver arginase by borate

Full activation of human liver arginase (EC 3.5.3.1), by incubation with 5 mM Mn2+ for 10 min at 60 degrees C, resulted in increased Vmax and a higher sensitivity of the enzyme to borate inhibition, with no change in the K(m) for arginine. Borate behaved as an S-hyperbolic I-hyperbolic non-competitive inhibitor and had no effect on the interaction of the enzyme with the competitive inhibitors L-ornithine (Ki = 2 +/- 0.5 mM), L-lysine (Ki = 2.5 +/- 0.4 mM), and guanidinium chloride (Ki = 100 +/- 10 mM). The pH dependence of the inhibition was consistent with tetrahedral B(OH)4- being the inhibitor, rather than trigonal B(OH)3. We suggest that arginase activity is associated with a tightly bound Mn2+ whose catalytic action may be stimulated by addition of a more loosely bound Mn2+, to generate a fully activated enzyme form. The Mn2+ dependence and partial character of borate inhibition are explained by assuming that borate binds in close proximity to the loosely bound Mn2+ and interferes with its stimulatory action. Although borate protects against inactivation of the enzyme by diethyl pyrocarbonate (DEPC), the DEPC-sensitive residue is not considered as a ligand for borate binding, since chemically modified species, which retain about 10% of enzymatic activity, were also sensitive to the inhibitor.

Evidence that histidine-163 is critical for catalytic activity, but not for substrate binding to Escherichia coli agmatinase

Agmatinase (agmatine ureohydrolase, EC 3.5.3.11) from Escherichia coli was inactivated by diethyl pyrocarbonate (DEPC) and illumination in the presence of Rose bengal. Protection against photoinactivation was afforded by the product putrescine, and the dissociation constant of the enzyme-protector complex (12 mM) was essentially equal to the K(i) value for this compound acting as a competitive inhibitor of agmatine hydrolysis. Upon mutation of His163 by phenylalanine, the agmatinase activity was reduced to 3-5% of wild-type activity, without any change in K(m) for agmatine or K(i) for putrescine inhibition. The mutant was insensitive to DEPC and dye-sensitized inactivations. We conclude that His163 plays an important role in the catalytic function of agmatinase, but it is not directly involved in substrate binding.

Manganese is essential for catalytic activity of Escherichia coli agmatinase

Purified Escherichia coli agmatinase (EC 3.5.3.11) expressed the same activity in the absence or presence of added Mn2+ (0-5mM). However, it was strongly inhibited by Co2+, Ni2+, and Zn2+ and almost half inactivated by EDTA. Partial inactivation by EDTA yielded enzyme species containing 0.85 +/- 0.1 Mn2+/subunit, and it was accompanied by a decrease in intensity of fluorescence emission and a red shift from the emission maximum of 340 nm to 346 nm, indicating the movement of tryptophane residues to a more polar environment. The activity and fluorescence properties of fully activated agmatinase were restored by incubation of dialysed species with Mn2+. Manganese-free species, obtained by treatment with EDTA and guanidinium chloride (3 M), were active only in the presence of added Mn2+. Results obtained, which represent the first demonstration of the essentiality of Mn2+ for agmatinase activity, are discussed in connection with a possible binuclear metal center in the enzyme.

Decreased heterogeneity of CS histone variants after hydrolysis of the ADP-ribose moiety

Sea urchin CS histone variants are electrophoretically heterogeneous when analyzed in two dimensional polyacrylamide gels (2D-PAGE). Previous results suggested that this heterogeneity is due to the poly (ADP-ribosylation) of these proteins. Consequently, native CS histone variants were subjected to different treatments to remove the ADP-ribose moiety. The incubation in 1 M hydroxylamine was not effective in eliminating the polymers of ADP-ribose from CS variants, and the treatment with sodium hydroxide was deleterious to the proteins. In contrast, the ADP-ribose moiety was successfully removed from the CS variants by incubation with phosphodiesterase (PDE). To eliminate contamination of CS histone variants with PDE extract, the enzyme was covalently bound to Sepharose 4B prior to its utilization. Treatment of native CS histone variants with this immobilized phosphodiesterase removed around 85% of the total ADP-ribose moiety from these proteins. After S-PDE treatment the complex electrophoretic pattern of CS histone variants in 2-D PAGE decreases to five major fractions. From these results we conclude that the electrophoretic heterogeneity of native CS histone variants is mainly due to the extent to which five main CS histone variants are poly(ADP)-ribosylated).

Interaction of arginase with metal ions: studies of the enzyme from human liver and comparison with other arginases

As determined by atomic absorption, fully activated human liver arginase contained 1.1 +/- 0.1 Mn2+/subunit. Upon dissociation to inactive subunits (< 0.01 Mn2+/subunit), there was decreased intensity and a red shift in the tryptophan fluorescence emission spectra of the enzyme, and the resulting species were markedly sensitive to thermal and proteolytic inactivation by trypsin. Arginine and lysine specifically protected the subunits from heat inactivation. Subunit activation by Mn2+ followed hyperbolic kinetics (Kd = 0.08 +/- 0.01 microM). In addition to Mn2+, Ni2+ and Co2+ converted inactive subunits into active monomers, and favoured their association to the oligomeric state of the enzyme (M(r) = 120,000 +/- 2000). The replacement of Mn2+ by Ni2+ or Co2+ resulted in significant changes in Vmax without any change in the Km values for the substrates (arginine or canavanine) or the Ki value for lysine inhibition. The results support our previous suggestion (Carvajal et al., 1994) that Mn2+ is not essential for substrate binding to arginase, and substantiates the conclusion that species differences may exist in the interaction of arginase with metal ions.

Subcellular localization and kinetic properties of arginase from the liver of Genypterus maculatus

1. From the liver of the teleost fish Genypterus maculatus, a partially purified preparation of arginase was obtained and characterized. 2. The Km value for arginine was found to be 9.1 mM at pH 7.5 and 11.5 mM at the optimum pH of 9.5. At both pH values, competitive inhibition was caused by ornithine and lysine, whereas proline, leucine, valine and isoleucine caused a non-competitive inhibitory effect. Branched chain amino acids were more inhibitory than proline. 3. The enzyme was found localized in the mitochondrial matrix of the liver of Genypterus maculatus. It is suggested that this localization would be of importance in the use of arginine as an energy source.

Kinetics of inhibition of rat liver and kidney arginases by proline and branched-chain amino acids

The effects of proline, leucine, isoleucine and valine on kidney and liver arginases were studied. At pH 7.5 and at nearly physiological concentrations, the branched-chain amino acids caused a significant inhibition of liver arginase A1 and only minor effects on kidney arginase A4. Kidney arginase was, however, much more sensitive to inhibition by proline than the liver enzyme. The inhibition of liver and kidney arginases by branched-chain amino acids was partial, indicating the existence of allosteric sites on both enzymes. The function of kidney arginase in proline biosynthesis and a possible role of branched-chain amino acids in the hydrolysis of arginine in liver is discussed.

Comparative kinetic studies of Mn2+-activated and fructose-1,6-P-modified Mg2+-activated pyruvate kinase from Concholepas concholepas

Initial velocity and product inhibition studies of Mn2+-activated and FDP-modified Mg2+-activated pyruvate kinase from Concholepas concholepas, were performed. Evidence is presented to show that the Mn2+-enzyme catalyzes an ordered sequential mechanism, with ADP being the first substrate and pyruvate the last product. The results presented are consistent with a random combination of reactants with the FDP-modified Mg2+-activated enzyme and the formation of the dead-end complexes enzyme ADP-ATP and enzyme-PEP-ATP.

Differences between magnesium-activated and manganese-activated pyruvate kinase from the muscle of Concholepas concholepas

In contrast to the Mg2+-activated enzyme, in the presence of Mn2+ pyruvate kinase exhibits hyperbolic kinetics with respect to the substrate phosphoenolpyruvate and is insensitive to fructose 1,6-biphosphate, phenylalanine and alanine. However, with both metal activated species inhibition by excess ADP is observed. In contrast with Mg2+, which affords significant protection against inactivation caused by 5,5'-dithiobis (2-nitrobenzoic acid), the rate of inactivation by this reagent is increased in the presence of Mn2+. Differences in conformational changes induced by combination of pyruvate kinase with Mg2+ or Mn2+ were indicated by u.v. difference spectra.

Negative cooperativity in human liver argininosuccinase

The kinetic properties of argininosuccinase (L-argininosuccinate arginine-lyase, EC 4.3.2.1.) were investigated. Negative cooperativity was observed in the response of the enzyme to the substrate argininosuccinate and GTP behaved as a positive allosteric effector. These effects were observed in 60 mM potassium phosphate but not in 50 mM Tris-HCl. Structural changes in the protein molecule are suggested to explain previous observations of Michaelis-Menten kinetics for this enzyme.

Hybrid, immobilised dimers of human liver arginase

The activity of matrix-bound monomers of arginase (L-arginine amidinohydrolase, EC 3.5.3.1) was not changed by incubation with p-hydroxymercuribenzoate. When the chemically modified, matrix-bound monomers were incubated with soluble subunits in the presence of Mn2+, dimers were obtained. These dimers were hybrids between modified and native monomers. The results obtained are in accord with a D2-symmetry where two dimers meet to form the tetrameric enzyme. From kinetic studies it is concluded that the structure of the active sites of arginase is not affected by the chemical modification with p-hydroxymercuribenzoate.

Evidence for cooperative effects in human liver arginase

The reaction kinetics of human liver arginase (L-arginine amidinohydrolase, EC 3.5.3.1) in terms of arginine concentration is strikingly altered by varying the pH. Lowering the pH from the optimum (9.5) toward a more physiological value (7.5) there is a transition from hyperbolic to sigmoidal kinetics. The cooperative effects are observed in the presence and absence of the product ornithine. Dimers of arginase exhibit typical Michaelis-Menten kinetics even in the presence of ornithine. Dimer-dimer interactions are suggested to explain the kinetic properties of arginase at pH 7.5.

Subunit interactions and immobilised dimers of human liver arginase

Incubation of soluble human liver arginase (L-arginine amidinohydrolase, EC 3.5.3.1) with p-hydroxymercuribenzoate resulted in the dissociation of the enzyme into active dimers. Addition of 2-mercaptoethanol resulted in the regeneration of the tetrameric enzyme. When arginase, bound covalently to nylon, was incubated with p-hydroxymercuribenzoate, matrix-bound dimers were obtained. Incubation of these species with 2-mercaptoethanol resulted in stable, unmodified dimers. Based on this dissociation of arginase, a model with D2-symmetry is suggested for this enzyme. The specific activity, the Km value for arginine, pH optimum and the inhibition constants for ornithine and lysine were determined for monomeric, dimeric and tetrameric forms. It is concluded that the behaviour of the active sites of the monomers is not substantially altered by the interaction of these species in the oligomeric molecule.

Immobilised monomers of human liver arginase

Human liver arginase (L-arginine amidinohydrolase, EC 3.5.3.1) was immobilised by attachment to nylon with glutaraldehyde as a crosslinking agent. Incubation of the immobilised tetrameric enzyme with EDTA followed by dialysis resulted in the dissociation of the enzyme into inactive matrix-bound and solubilised subunits. Both species recovered enzymatic activity after incubation with Mn2+, and the activity of the reactivated matrix-bound subunits was nearly 25% of that shown by the enzyme initially attached to the support in the tetrameric form. When the reactivated bound subunits were incubated with soluble subunits in the presence of Mn2+, they 'picked-up' from the solution an amount of protein and enzymatic activity almost identical to that initially lost by the immobilised tetramer after the dissociating treatment with EDTA. This occurred only in the presence of Mn2+. It is suggested that the reactivation of the subunits of arginase involves the initial formation of an active monomer, which then acquires a conformation that favours a reassociation to the tetrameric state.