How gastric lipase, an interfacial enzyme with a Ser-His-Asp catalytic triad, acts optimally at acidic pH

Gastric lipase is active under acidic conditions and shows optimum activity on insoluble triglycerides at pH 4. The present results show that gastric lipase also acts in solution on vinyl butyrate, with an optimum activity above pH 7, which suggests that gastric lipase is able to hydrolyze ester bonds via the classical mechanism of serine hydrolases. These results support previous structural studies in which the catalytic triad of gastric lipase was reported to show no specific features. The optimum activity of gastric lipase shifted toward lower pH values, however, when the vinyl butyrate concentration was greater than the solubility limit. Experiments performed with long-chain triglycerides showed that gastric lipase binds optimally to the oil-water interface at low pH values. To study the effects of the pH on the adsorption step independently from substrate hydrolysis, gastric lipase adsorption on solid hydrophobic surfaces was monitored by total internal reflection fluorescence (TIRF), as well as using a quartz crystal microbalance. Both techniques showed a pH-dependent reversible gastric lipase adsorption process, which was optimum at pH 5 (Kd = 6.5 nM). Lipase adsorption and desorption constants (ka = 147,860 M(-1) s(-1) and kd = 139 x 10(-4) s(-1) at pH 6) were estimated from TIRF experiments. These results indicate that the optimum activity of gastric lipase at acidic pH is only "apparent" and results from the fact that lipase adsorption at lipid-water interfaces is the pH-dependent limiting step in the overall process of insoluble substrate hydrolysis. This specific kinetic feature of interfacial enzymology should be taken into account when studying any soluble enzyme acting on an insoluble substrate.     

Human gastric lipase. A kinetic study with dicaprin monolayers

The effects of several proteins on the hydrolysis at pH 3.0 of didecanoylglycerol monolayers by human gastric lipase were investigated. Among the six proteins tested (bovine serum albumin, myoglobin, a protein inhibiting lipase isolated from soya bean, melittin, beta-lactoglobulin and ovalbumin), only the first three proteins were found to inhibit lipase activity. The inhibition capacity of the proteins was not related to the decrease in interfacial tension or to their isoelectric points. However, inhibition of human gastric lipase by proteins may be correlated with the penetration power of the protein into the lipid interface. It is hypothesized that this lipase has a higher penetration power than that of pancreatic lipase, even though the former enzyme is more susceptible to interfacial denaturation.     

Rapid exchange of pancreatic lipase between triacylglycerol droplets

Two types of experiments were performed to study the reversibility of interfacial adsorption of pancreatic lipase (PL) to fat droplets during lipolysis. Lipolysis was measured in olive oil/gum arabic emulsions containing radiolabeled triolein in the presence of bile salts and lecithin at rate-limiting concentrations of porcine PL (PPL) or human PL (HPL). The lipolysis rate in a labeled emulsion, i.e. release of [(14)C]oleic acid, was immediately reduced by around 50% upon dilution with an equal amount of an unlabeled emulsion. Further, lipolysis was rapidly and completely suppressed when a non-exchanging lipase inhibitor was present in the second emulsion. These results indicate hopping of lipase between emulsion droplets. Alternative explanations were excluded. Hopping of PL between triolein droplets stabilized with gum arabic at supramicellar bile salt concentrations was observed only in the presence, not in the absence, of lecithin. Displacement from a trioctanoin-water interface of active HPL by an inactive mutant (S152G) was studied in the presence of bile salts by measuring HPL distribution between the water phase and the oil-water interface. Colipase was limiting for HPL binding to the oil-water interface (colipase to lipase molar ratio: 0.5) and, thus, for lipolysis. Upon adding S152G, which has the same affinity for colipase, inactive and active HPL were found to compete for binding at the oil-water interface. When equal amounts of HPL and HPL S152G were used, the lipolysis rate dropped to half the maximum rate recorded with HPL alone, suggesting that half the active HPL was rapidly desorbed from the oil-water interface. Therefore, under various conditions, PL does not remain irreversibly adsorbed to the oil-water interface, but can exchange rapidly between oil droplets, via an equilibrium between soluble and lipid-bound PL.     

不依赖油水界面激活的黑曲霉脂肪酶突变体的构建

为获得不依赖油水界面激活的黑曲霉脂肪酶 (ANL) 突变体,在生物信息学分析基础上,对黑曲霉脂肪酶盖子结构域两侧铰链区的氨基酸残基进行了置换突变,获得两个黑曲霉脂肪酶突变体 (ANL-Ser84Gly和ANL-Asp99Pro)。对不同浓度对硝基苯丁酸酯的水解活性检测结果表明：ANL-Ser84Gly的催化活性仍依赖油水界面,而ANL-Asp99Pro的催化活性不再依赖油水界面。底物特异性检测结果表明：较ANL而言,ANL-Ser84Gly的比活力显著降低,其水解对硝基苯棕榈酸酯、对硝基苯豆蔻酸酯、对硝基     

Purification and biochemical characterization of dog gastric lipase

A lipase was found to be present in dog stomach which appeared to be more abundant in the fundic than in the pyloric mucosa. Dog gastric lipase was extracted by soaking the gastric tissue and further purified after cation exchange, anion exchange and gel-filtration using fast protein liquid chromatography. The amino-acid composition, N-terminal amino-acid sequence, substrate specificity, interfacial and kinetic behavior and inactivation by sulfhydryl reagents were determined and compared with those of human and rabbit gastric lipases. We report for the first time that a gastric lipase is 13 times more active on long-chain than on short-chain triacylglycerols at pH 4.0, reaching a maximal specific activity of 950 U/mg on Intralipide emulsion.     

Stereoselectivity of lipases. II. Stereoselective hydrolysis of triglycerides by gastric and pancreatic lipases

In the present study, porcine pancreatic lipase, rabbit gastric lipase, and human gastric lipase stereospecificity toward chemically alike, but sterically nonequivalent ester groups within one single triglyceride molecule was investigated. Lipolysis reactions were carried out on synthetic trioctanoin or triolein, which are homogenous, prochiral triglycerides, chosen as models for physiological lipase substrates. Diglyceride mixtures resulting from lipolysis were derivatized with optically active R-(+)-1-phenylethylisocyanate, to give diastereomeric carbamate mixtures, which were further separated by high performance liquid chromatography. Resolution of diastereomeric carbamates gave enantiomeric excess values, which reflect the lipases stereobias and clearly demonstrate the existence of a stereopreference by both gastric lipases for the sn-3 position. The stereoselectivity of human and rabbit gastric lipases, expressed as the enantiomeric excess percentage, was 54% and 70% for trioctanoin and 74% and 47% for triolein, respectively. The corresponding values with porcine pancreatic lipase were 3% in the case of trioctanoin and 8% in that of triolein. It is worth noting that rabbit gastric lipase, unlike human gastric lipase, became more stereoselective for the triglyceride with shorter acyl chains (trioctanoin). This is one of the most striking catalytic differences observed between these two gastric lipases.     

Construction of Aspergillus niger lipase mutants with oil-water interface independence

Based on previous bioinformational analytical results [Shu ZY, et al. Biotechnol Prog 2009;25:409-16], four A. niger lipase (ANL) mutants, ANL-Ser84Gly, ANL-Asp99Pro, ANL-Lys108Glu and ANL-EαH (obtained by replacing the lid domain of ANL with the corresponding domain from A. niger feruloyl esterase), were constructed to screen out ANL mutants with oil-water interface independence. ANL-S84G displayed a pronounced interfacial activation, while ANL-D99P and ANL-K108E displayed no interfacial activation. The specific activity of ANL-S84G towards p-nitrophenyl esters decreased from 29.8% to 76.5% compared with that of ANL, while the specific activity of ANL-D99P towards p-nitrophenyl palmitate increased 2.2-fold. The thermostability of ANL-K108E was almost unchanged, while the thermostability of ANL-S84G and ANL-D99P significantly decreased compared with that of ANL. The construction of oil-water interface-independent ANL mutants would help to further understand the mechanism of lipase interfacial activation.     

Lingual and gastric lipases: species differences in the origin of prepancreatic digestive lipases and in the localization of gastric lipase

The source of the lipase(s) acting in the stomach was investigated in five animal species: rat, mouse (rodents), rabbit (lagomorphs), guinea pig (caviidae), baboon and human (primates). The activity of lingual and gastric lipases was quantitated in homogenates of lingual serous glands and of gastric mucosa, respectively, by the hydrolysis of tri[3H]oleylglycerol and is expressed in units/g (1 U = 1 mumol [3H]oleic acid released/min) per g tissue wet weight, mean +/- S.E. There were marked differences in the activity level of lingual and gastric lipases among species: mouse and rat had high levels of lingual lipase activity (250 +/- 20 and 824 +/- 224 U/g) and only traces of gastric lipase activity (4.5 +/- 0.9 and 0.04 U/g, respectively), whereas rabbit and guinea pig had no lingual lipase activity and only gastric lipase activity (78 +/- 48 and 27 +/- 7.4 U/g, respectively). In the baboon and human, gastric lipase was the predominant enzyme (109 +/- 20 U/g and 118 +/- 8.8 U/g, respectively), whereas lingual lipase activity was present in trace amounts only (0.04 U/g and 0.3 U/g, respectively). In addition to species differences in the origin of the preduodenal lipases, there were also species differences in the distribution of gastric lipase in the stomach. Thus, while in the rabbit, gastric lipase was localized exclusively in the cardia and body of the stomach, it was diffusely distributed in the entire stomach of the guinea pig and baboon. A comparison between the level of activity of lipase and pepsin (the two chief digestive enzymes secreted by the stomach), showed differences in their localization in the species studied. The difference in source (tongue vs. stomach) and site (cardia-body vs. entire stomach) of lipase secretion must be taken into account in future studies of these digestive enzymes. Although the exact contribution of lingual and gastric lipases individually to fat digestion in species which contain both enzymes cannot yet be evaluated, the markedly higher levels of gastric lipase activity in the baboon and human suggests that, in primates, gastric lipase is probably the major non-pancreatic digestive lipase.     

A comparative study on two fungal lipases from Thermomyces lanuginosus and Yarrowia lipolytica shows the combined effects of detergents and pH on lipase adsorption and activity

The effects of various detergents and pH on the interfacial binding and activity of two fungal lipases from Yarrowia lipolytica (YLLIP2) and Thermomyces lanuginosus (TLL) were investigated using trioctanoin emulsions as well as monomolecular films spread at the air-water interface. Contrary to TLL, YLLIP2 was found to be more sensitive than TLL to interfacial denaturation but it was protected by detergent monomers and lowering the temperature. At pH 7.0, both the interfacial binding and the activities on trioctanoin of YLLIP2 and TLL were inhibited by sodium taurodeoxycholate (NaTDC). At pH 6.0, however, YLLIP2 remained active on trioctanoin in the presence of NaTDC, whereas TLL did not. YLLIP2 activity on trioctanoin was associated with strong interfacial binding of the enzyme to trioctanoin emulsion, whereas TLL was mostly detected in the water phase. The combined effects of bile salts and pH on lipase activity were therefore enzyme-dependent. YLLIP2 binds more strongly than TLL at oil-water interfaces at low pH when detergents are present. These findings are particularly important for lipase applications, in particular for enzyme replacement therapy in patients with pancreatic enzyme insufficiency since high detergent concentrations and highly variable pH values can be encountered in the GI tract.     

Human gastric lipase. The effect of amphiphiles

Human gastric lipase (HGL) activity on tributyrin emulsion was detected only in the presence of amphiphiles such as bile salts, proteins (serum albumin, beta-lactoglobulin or ovalbumin) or phosphatidylcholine. These findings are contrary to the strong inhibitory effect of amphiphiles observed on pure pancreatic lipase. To reveal HGL activity, amphiphiles should be added prior to HGL. This may prevent irreversible interfacial denaturation. HGL activity was found to be restricted to a triacylglycerol/water surface tension ranging from 8 dyn/cm to 13 dyn/cm. All amphiphiles, which decrease the interfacial tension below 8 dyn/cm, act as irreversible inhibitors of HGL in the absence and in the presence of bile salts. Our results confirm that HGL is capable of hydrolysing triacylglycerol in the presence of the physiological concentration of bile salts prevailing in the upper small intestine and in the presence of alimentary proteins. These observations could explain the high dietary lipid absorption observed under pancreatic lipase deficiency.     

Instability development in heated human erthrocytes.

Heated human erythrocytes gradually lose their form-maintaining structure as the temperature is increased to 50 degrees C and can behave in some respects as a viscous fluid. We have developed a technique for heating and stressing these cells that is novel, simple and quantitatively precise. We have applied this technique to heated human erythrocytes and have measured instability development in cells. We have employed instability growth theory to calculate a value for an effective surface tension which, in contrast to other methods of membrane surface tension measurement sought to minimize the effects of membrane supporting structural elements. The value obtained for the surface tension of the heated erythrocyte membrane was 0.9 . 10(-6) N/m with a range of variation from 0.4 . 10(-6)N/m to 1.4 . 10(-6) N/m. The methods described may be useful for determining fundamental physical parameters such as internal viscosity and interfacial tension in other systems.     

A physicochemical investigation of two phosphatidylcholine/bile salt interfaces: implications for lipase activation

Within the gastrointestinal tract ingested lipids are broken down into their constituent mono-acylglycerides and fatty acids by the enzyme family of lipases. In this study we have investigated the interfacial composition and structure of two phospholipid/bile salt (BS) systems that display significant differences in the duration of the lag phase of porcine pancreatic lipase kinetics. The interfacial tension of the single BSs, and their binary mixtures with phospholipid is reported at an n-tetradecane/water interface as a function of phospholipid mole fraction and total surfactant concentration. The structuring of the interface was probed by characterisation of the thin liquid film formation, thickness and stability. Lateral interactions were quantified by measurement of the diffusion coefficient of a probe fluorophore. We conclude that interfacial tension was not a factor in lag time duration as there was no significant difference in the minimum interfacial tension for the phosphatidylcholine (PC)/sodium taurocholate and the PC/sodium taurodeoxycholate system. No correlation was found between lag phase duration and the physiochemical properties of the interface, i.e. lateral diffusion, thin liquid film formation or interfacial tension. This is in agreement with our previous study that the lag time duration was directly related to the phospholipid content of the interface.     

Inhibition of human gastric and pancreatic lipases by chiral alkylphosphonates. A kinetic study with 1,2-didecanoyl-sn-glycerol monolayer

Enantiomerically pure alkylphosphonate compounds RR′P(O)PNP (R=C_nH_2n+1, R′=OY with Y=C_n′H_2n′+1 with n=n′ or n≠n′; PNP=p-nitrophenoxy) noted (RY), mimicking the transition state occurring during the carboxyester hydrolysis were synthesized and investigated as potential inhibitors of human gastric lipase (HGL) and human pancreatic lipase (HPL). The inhibitory properties of each enantiomer have been tested with the monomolecular films technique in addition to an enyzme linked immunosorbent assay (ELISA) in order to estimate simultaneously the residual enzymatic activity as well as the interfacial lipase binding. With both lipases, no obvious correlation between the inhibitor molar fraction (₅₀) leading to half inhibition, and the chain length, R or Y was observed. (R₁₁Y₁₆)s were the best inhibitor of HPL and (R₁₀Y₁₁)s were the best inhibitors of HGL. We observed a highly enantioselective discrimination, both with the pure enantiomeric alkylphosphonate inhibitors as well as a scalemic mixture. We also showed, for the first time, that this enantioselective recognition can occur either during the catalytic step or during the initial interfacial adsorption step of the lipases. These experimental results were analyzed with two kinetic models of covalent as well as pseudo-competitive inhibition of lipolytic enzymes by two enantiomeric inhibitors.     

Stereoselectivity of lipases. I. Hydrolysis of enantiomeric glyceride analogues by gastric and pancreatic lipases, a kinetic study using the monomolecular film technique

In the present study, porcine pancreatic lipase, rabbit gastric lipase, and human gastric lipase stereospecificity toward enantiomeric glyceride derivatives was kinetically investigated using the monomolecular film technique. Pseudoglycerides such as enantiomeric 1(3)-alkyl-2,3(1,2)-diacyl-sn-glycerol, enantiomeric 1(3)-alkyl-2-acyl-sn-glycerol, or enantiomeric 1(3)-acyl-2-acylamino-2-deoxy-sn-glycerol were synthesized in order to assess the lipase stereoselectivity during the hydrolysis of either the primary or the secondary ester position of these glycerides analogues. The cleaved acyl moiety was the same in both enantiomers, thereby excluding the possibility of effects occurring due to fatty acid specificity. We observed a porcine pancreatic lipase sn-3 stereoselectivity when using the enantiomeric 1(3)-alkyl-2-acylamino-2-deoxy-sn-glycerol (diglyceride analogue) which contrasted with the lack of stereoselectivity observed when using the enantiomeric 1(3)-alkyl-2,3(1,2)-diacyl-sn-glycerol (triglyceride analogue). The gastric lipases, in contrast to the pancreatic lipase, preferentially catalyze the hydrolysis of the primary sn-3 ester bond of the enantiomeric monoakyl-diacyl pair tested. From these kinetic data, high hydrolysis rates and no chiral discrimination were observed in the case of rabbit gastric lipase, whereas low rates and a clear chiral discrimination was noticed in the case of human gastric lipase during hydrolysis of the acyl chain from the secondary ester bond of 1(3)-alkyl-2-acyl enantiomers. It is particularly obvious that in the case of human gastric lipase decreasing the lipid packing increases the lipase sn-3 stereopreference during hydrolysis of the primary ester bond of the enantiomeric 2-acylamino derivatives (diglyceride analogue).     

Effects of gum arabic on lipase interfacial binding and activity.

We investigated the surface behavior of gum Arabic (GA) as well as its effects on the lipolytic activity of human pancreatic lipase (HPL) and Humicola lanuginosa lipase (HLL), using emulsions of triacylglycerols (TAG) with various chain lengths. The effects of GA on the interfacial binding of HPL were also investigated. In the presence of 4 mM sodium taurodeoxycholate (NaTDC), GA (3% w/v, final concentration) had no effect on the HPL activity measured in the presence of colipase, whatever the type of TAG used. However, in the absence of bile salts or at low bile salt concentrations, GA inhibited the HPL activity when trioctanoin (TC8) and purified soybean oil (PSO) were used as substrates. At 3% (w/v, final concentration), GA strongly desorbed pure HPL from the TC8 interface and the classical anchoring effect of colipase was clearly observed. Both crude and dialyzed GA solutions were found to be highly tensioactive at the air-water as well as the oil-water interface using the drop technique. In conclusion, GA, or a putative contaminant present in GA, was found to be surface active and to have similar effects to those of bile salts on the interfacial binding and activity of HPL.     

The influence of short-chain alcohols on interfacial tension, mechanical properties, area/molecule, and permeability of fluid lipid bilayers

We used micropipette aspiration to directly measure the area compressibility modulus, bending modulus, lysis tension, lysis strain, and area expansion of fluid phase 1-stearoyl, 2-oleoyl phosphatidylcholine (SOPC) lipid bilayers exposed to aqueous solutions of short-chain alcohols at alcohol concentrations ranging from 0.1 to 9.8 M. The order of effectiveness in decreasing mechanical properties and increasing area per molecule was butanol>propanol>ethanol>methanol, although the lysis strain was invariant to alcohol chain-length. Quantitatively, the trend in area compressibility modulus follows Traube's rule of interfacial tension reduction, i.e., for each additional alcohol CH(2) group, the concentration required to reach the same area compressibility modulus was reduced roughly by a factor of 3. We convert our area compressibility data into interfacial tension values to: confirm that Traube's rule is followed for bilayers; show that alcohols decrease the interfacial tension of bilayer-water interfaces less effectively than oil-water interfaces; determine the partition coefficients and standard Gibbs adsorption energy per CH(2) group for adsorption of alcohol into the lipid headgroup region; and predict the increase in area per headgroup as well as the critical radius and line tension of a membrane pore for each concentration and chain-length of alcohol. The area expansion predictions were confirmed by direct measurements of the area expansion of vesicles exposed to flowing alcohol solutions. These measurements were fitted to a membrane kinetic model to find membrane permeability coefficients of short-chain alcohols. Taken together, the evidence presented here supports a view that alcohol partitioning into the bilayer headgroup region, with enhanced partitioning as the chain-length of the alcohol increases, results in chain-length-dependent interfacial tension reduction with concomitant chain-length-dependent reduction in mechanical moduli and membrane thickness.     

Exploring the specific features of interfacial enzymology based on lipase studies

Many enzymes are active at interfaces in the living world (such as in the signaling processes at the surface of cell membranes, digestion of dietary lipids, starch and cellulose degradation, etc.), but fundamental enzymology remains largely focused on the interactions between enzymes and soluble substrates. The biochemical and kinetic characterization of lipolytic enzymes has opened up new paths of research in the field of interfacial enzymology. Lipases are water-soluble enzymes hydrolyzing insoluble triglyceride substrates, and studies on these enzymes have led to the development of specific interfacial kinetic models. Structure-function studies on lipases have thrown light on the interfacial recognition sites present in the molecular structure of these enzymes, the conformational changes occurring in the presence of lipids and amphiphiles, and the stability of the enzymes present at interfaces. The pH-dependent activity, substrate specificity and inhibition of these enzymes can all result from both "classical" interactions between a substrate or inhibitor and the active site, as well as from the adsorption of the enzymes at the surface of aggregated substrate particles such as oil drops, lipid bilayers or monomolecular lipid films. The adsorption step can provide an alternative target for improving substrate specificity and developing specific enzyme inhibitors. Several data obtained with gastric lipase, classical pancreatic lipase, pancreatic lipase-related protein 2 and phosphatidylserine-specific phospholipase A1 were chosen here to illustrate these specific features of interfacial enzymology.     

Isoform purification of gastric lipases. Towards crystallization.

Several isoforms of rabbit and human gastric lipases have been purified. These isoforms have the same apparent molecular weight (Mr approximately 50,000), but very different isoelectric points. Some of these isoforms were purified: pI 7.2 and 6.5 in the case of rabbit gastric lipase; and pI 7.4 and 7.2 in that of human gastric lipase. All the purified isoforms were found to have the same specific lipase activity (around 1200 units per mg of protein, measured on tributyrin as substrate). The isoforms of dog gastric lipase are more closely related, and could not be separated. Partial enzymatic deglycosylation of human gastric lipase reduced the apparent molecular weight from Mr approximately 50,000 to Mr approximately 43,000 and induced a change in the isoelectrofocusing pattern and the emergence of a new isoform (pI 7.3). It is concluded that the charge heterogeneity of gastric lipases is at least partly due to the glycan moiety of the molecule, which amounts to approximately 14% of the total molecular weight. Several crystallization trials on purified native preparations of rabbit and human gastric lipases were unsuccessful, whereas crystals were obtained from native dog gastric lipase and all the purified isoforms of rabbit and human gastric lipases, some of which were crystallographically characterized.     

Kinetic studies of Chromobacterium viscosum lipase in AOT water in oil microemulsions and gelatin microemulsion-based organogels

Kinetic studies have shown that octyl decanoate synthesis by Chromobacterium viscosum (CV) lipase in sodium bis-2-(ethylhexyl) sulfosuccinate (AOT) water in oil (w/o) microemulsions occurs via the nonsequential (ping-pong) bi bi mechanism. There was evidence of single substrate inhibition by decanoic acid at high concentrations. Initial rate data yielded estimates for acid and alcohol Michaelis constants of ca. 10(-1) mol dm(-3) and a maximum rate under saturation conditions of ca. 10(-3) mol dm(-3) s(-1) for a lipase concentration of 0.36 mg cm(-3). CV lipase immobilized in AOT microemulsion-based organogels (MBGs) was also found to catalyze the synthesis of octyl decanoate according to the ping-pong bi bi mechanism. Reaction rates were similar in the free and immobilized systems under comparable conditions. Initial rates at saturating (but noninhibiting) substrate concentrations were first order with respect to CV lipase concentration in both w/o microemulsions and the MBG/oil systems. Gradients yielded an apparent k(cat) = 4.4 x 10(-4) mol g(-1) s(-1) in the case of w/o microemulsions, and 6.1 x 10(-4) mol g(-1) s(-1) for CV lipase immobilized in the MBGs. A third system comprising w/o microemulsions containing substrates and gelatin at concentrations comparable to those employed in the MBG formulations, provided a useful link between the conventional liquid microemulsion medium and the solid organogels. The nongelation of these intermediate systems stems from the early inclusion of substrate during a modified preparative protocol. The presence of substrate appears to prevent the development of a percolated microstructure that is thought to be a prerequisite for MBG formation. FT-NMR was employed as a semicontinuous in situ assay procedure. The apparent activity expressed by CV lipase in compositionally equivalent liquid and solid phase gelatin-containing systems was similar. An apparent activation energy of 24 +/- 2 kJ mol(-1) was determined by (1)H-NMR for esterification in gelatin-containing w/o microemulsions. This value agrees with previous determinations for CV lipase-catalyzed synthesis of octyl decanoate in "conventional" w/o microemulsions and MBG/oil systems. The similarities in lipase behavior are consistent with the claim, based largely on structural measurements, that the physico-chemical properties of the lipase-containing w/o microemulsion are to a large extent preserved on transformation to the daughter organogel. The close agreement of apparrent activation energies suggests that substrate mass transfer is not rate determining in the three studied systems. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 54:416-427, 1997.     

Interfacial tension of aqueous biopolymer mixtures close to the critical point

Proteins and polysaccharides, being the main constructional materials in many biological structures, have a limited compatibility in aqueous media. At sufficiently high concentrations, they form water-in-water emulsions. Interfacial tension is an important parameter in such systems since it is a controlling factor in the morphology development during processing. In this work a rheo-optical methodology, based on the analysis of small angle light scattering (SALS) patterns during fibril break-up, is used to study the interfacial tension of water-sodium caseinate-sodium alginate systems located close to and relatively far from the binodal. The interfacial tension close to the critical point was approximately 10(-8) N/m, and it increased considerably, to a value of up to 5.2 x 10(-6) N/m farther from the critical point. For the scaling of the interfacial tension with the density difference between the phases, a scaling exponent of 3.1 +/- 0.3 was found, in agreement with the critical mean-field scaling exponent of 3.