The polar interactions between pancreatic lipase, colipase and the triglyceride substrate

Lipid-Based Nanoparticles as Oral Drug Delivery Systems: Overcoming Poor Gastrointestinal Absorption and Enhancing Bioavailability of Peptide and Protein Therapeutics

Delivery and formulation of oral peptide and protein therapeutics have always been a challenge for the pharmaceutical industry. The oral bioavailability of peptide and protein therapeutics mainly relies on their gastrointestinal solubility and permeability which are affected by their poor membrane penetration, high molecular weight and proteolytic (chemical and enzymatic) degradation resulting in limited delivery and therapeutic efficacy. The present review article highlights the challenges and limitations of oral delivery of peptide and protein therapeutics focusing on the application, potential and importance of solid lipid nanoparticles (SLNs) and nanostructured lipid carriers (NLCs) as lipid-based drug delivery systems (LBDDSs) and their advantages and drawbacks. LBDDSs, due to their lipid-based matrix can encapsulate both lipophilic and hydrophilic drugs, and by reducing the first-pass effect and avoiding proteolytic degradation offer improved drug stability, dissolution rate, absorption, bioavailability and controlled drug release. Furthermore, their small size, high surface area and surface modification increase their mucosal adhesion, tissue-targeted distribution, physiological function and half-life. Properties such as simple preparation, high-scale manufacturing, biodegradability, biocompatibility, prolonged half-life, lower toxicity, lower adverse effects, lipid-based structure, higher drug encapsulation rate and various drug release profile compared to other similar carrier systems makes LBDDSs a promising drug delivery system (DDS). Nevertheless, undesired physicochemical features of peptide and protein drug development and discovery such as plasma stability, membrane permeability and circulation half-life remain a serious challenge which should be addressed in future.

Insights into the protective role of solid lipid nanoparticles on rosmarinic acid bioactivity during exposure to simulated gastrointestinal conditions

The evaluation of the digestion effects on bioactive solid lipid nanoparticles (SLN) was performed. For this purpose, witepsol and carnauba SLN loaded with rosmarinic acid (RA) were exposed to the simulated gastrointestinal tract (GIT) conditions prevailing in stomach and small intestine. The simulation of intestinal epithelium was made with a dialysis bag and intestinal cell culture lines. Changes on SLN physical properties, RA release and absorption profiles were followed at each step. Combination of digestion pH and enzymes showed a significant effect upon SLN physical properties. Zeta potential values increased at stomach conditions and decreased at small intestine simulation. Also, at intestine, SLN increased their sizes and released 40-60% of RA, maintaining its initial antioxidant activity values. Sustained release of 40% of RA from SLN was also observed in dialysis tube. At CaCo-2 cell line, both types of SLN showed similar absorbed RA % (ca. 30%). Nevertheless, in CaCo-2/HT29x mix cell lines, for carnauba SLN a lower adsorption RA % was observed than for witepsol SLN. Solid lipid nanoparticles protected RA bioactivity (in terms of antioxidant activity) until reaching the intestine. A controlled release of RA from SLN was achieved and a significant absorption was observed at intestinal cells. Overall, SLN produced with witepsol showed a higher stability than carnauba SLN.

Computational study of colipase interaction with lipid droplets and bile salt micelles

Colipase is a key element in the lipase-catalyzed hydrolysis of dietary lipids. Although devoid of enzymatic activity, colipase promotes the pancreatic lipase activity in physiological intestinal conditions by anchoring the enzyme at the surface of lipid droplets. Analysis of structures of NMR colipase models and simulations of their interactions with various lipid aggregates, lipid droplet, and bile salt micelle, were carried out to determine and to map the lipid binding sites on colipase. We show that the micelle and the oil droplet bind to the same side of colipase 3D structure, mainly the hydrophobic fingers. Moreover, it appears that, although colipase has a single direction of interaction with a lipid interface, it does not bind in a specific way but rather oscillates between different positions. Indeed, different NMR models of colipase insert different fragments of sequence in the interface, either simultaneously or independently. This supports the idea that colipase finger plasticity may be crucial to adapt the lipase activity to different lipid aggregates.

Stereoselectivity of microbial lipases. The substitution at position sn-2 of triacylglycerol analogs influences the stereoselectivity of different microbial lipases

In the present study, the stereoselectivity of purified lipases from Candida rugosa, Chromobacterium viscosum, Pseudomonas species and Rhizopus arrhizus towards triacylglycerols in comparison to various structural analogs were investigated. Different triacylglycerol analogs with distinct polarities at position sn-2 of the glycerol backbone (1,3-diacyl-2-X-glycerol, where 2-X = 2-acyloxy, 2-alkyloxy, 2-deoxy-2-alkyl, or 2-deoxy-2-phenyl) were synthesized. Substrate hydrophobicity and steric requirement was modified by variation of the alkyl and acyl chain length. Hydrolysis of these substrates demonstrated that minor structural variations at C2 of triacylglycerol strongly affect the stereoselectivity of the lipases tested. It was noteworthy that the variation of substrate structure did not only affect the quantity of stereoselectivity expressed as percentage enantiomeric excess, but also resulted in a reversal of stereopreference in some cases. Replacement of the acylester in position 2 of glycerol by a non-ester-linked aliphatic moiety shifted the preference of Chromobacterium viscosum lipase from sn-3 to sn-1. Lipases from Chromobacterium viscosum. Pseudomonas species and Rhizopus arrhizus exhibited sn-3 preference with 2-deoxy-2-phenyl analogs, while towards substrates with a 2-deoxy-2-alkyl moiety sn-1 stereobias was recorded. Candida rugosa lipase was rather insensitive to substrate variations concerning the polarity at position 2 of the glycerol backbone. However, variation of the acyl chain length significantly influenced stereoselectivity of this lipase.

Carba analogues of triglycerides--isosteric mimics for natural lipids. Novel substrates for the determination of regio- and enantioselectivities displayed by lipases

The novel carba analogues 1a,b and 2 were synthesized and demonstrated to be isosteric identical mimics for natural triglycerides. They can be used for the system independent determination of regio- and enantioselectivities displayed by lipases. They are, moreover, attractive starting materials for novel carba analogues of phospholipids, PAF analogues and PAF antagonists.

Effect of sodium taurodeoxycholate, CaCl2 and albumin on the action of pancreatic lipase on droplets of trioleoylglycerol and the release of lipolytic products into aqueous media

1. Effects of various substances on the activity of pancreatic lipase and on the release of lipolytic products into aqueous media were studied with droplets of trioleoylglycerol suspended from a membrane filter at the top of a flow-through chamber. The droplets were perifused for 7 min with a commercial preparation of pancreatic lipase in 0.15 M NaCl solution at pH 6.5 and then perifused for 60 min with lipase-free media, either 0.15 M NaCl at pH 6.5 or basal medium at pH 7.4 (70 mM sodium barbital) containing different additives. 2. About 6% of the trioleoylglycerol in droplets was hydrolyzed during the perifusion with lipase. Another 15% was hydrolyzed in 30 min, but none thereafter, when the droplets were perifused with 0.15 M NaCl alone. The rate of hydrolysis was doubled and prolonged when droplets were perifused with basal medium at pH 7.4. Lipolytic products formed at pH 7.4 were 62% oleic acid, 20% monooleoylglycerol and 18% dioleoylglycerol, yet only 4% of the lipolytic products were released into the perifusate. 3. Sodium taurodeoxycholate (TDC) (17 mM ) added to basal medium increased 18 x the amount of lipolytic products released into the perifusate but increased lipolysis only 13%. The molar ratio of oleic acid to monooleoylglycerol in the perifusate was 5.7 during the first 30 min and 4.0 during the last 30 min. 4. Ca2+ (3.3 mM) added to basal medium increased lipolysis 87% but did not affect the amount (4%) of lipolytic products released into the perifusing medium. 5. TDC and Ca2+ added to basal medium produced the largest increase in lipolysis, with 59% of trioleoylglycerol hydrolyzed in 15 min and 91% in 60 min. The amount of lipolytic products released into the perifusing medium, however, was not increased above that released into medium containing TDC alone. 6. Serum albumin (0.6 mM) and Ca2+ added to basal medium increased 14 x the amount of lipolytic products released into the perifusate without affecting the basal lipolytic rate. Albumin, however, suppressed by 40% the stimulatory effect of Ca2+ on pancreatic lipase activity.

The effects of pH and salt on the lipid binding and enzyme activity of lipoprotein lipase

This paper demonstrates a striking difference between the effects of salt and pH on the activity of lipoprotein lipase against two different substrates: Intralipid and bovine milk fat droplets. With the former substrate 1 M NaCl caused only a slight reduction in enzyme activity and the stimulation by apolipoprotein C-II was the same from 0.1 to 1.1 M NaCl. In contrast, 0.5 M or more NaCl virtually abolished the enzyme activity in the milk system. In this system the salt also abolished binding of the enzyme to the lipid droplets, whereas in the Intralipid system most of the enzyme remained bound even at 1 M NaCl. A similar picture was obtained with respect to effects of pH. In the milk system the activity decreased sharply at pH values above 8.5, whereas in the Intralipid system it continues to rise to pH 10, and the stimulation by activator protein is the same at all pH values. Correlating with this, the binding of the enzyme to the lipid droplets was highly dependent on pH values in the milk systems, with optimum binding around pH 8, whereas in the Intralipid system most of the enzyme remained bound to the lipid droplets at all pH values. These studies demonstrate that apolipoprotein C-II can activate lipoprotein lipase at a wide range of salt concentrations and of pH. They suggest that the well-known effects of high salt concentrations and of high pH to decrease lipoprotein lipase activity are exerted primarily on the enzyme itself.

Importance of the N-terminal sequence in porcine pancreatic colipase

Colipase exists in pancreatic juice in a pro-form which is activated by limited trypsin hydrolysis. During this activation, the N-terminal pentapeptide 1Val-Pro-Asp-Pro-5Arg is cleaved. The new N-terminal sequence formed, 6Gly-Ile-Ile-Ile-10Asn, contains three isoleucine residues. The importance of these for stimulating lipase activity has been investigated by successive Edman degradation of epsilon-acetimidolysine residues followed by limited trypsin hydrolysis. The epsilon-amidinated colipase obtained was fully active both with a phospholipid-covered triacylglycerol (Intralipid) and tributyrin as substrate. After removal of the three isoleucine residues, the activity of colipase was lost with Intralipid but not with tributyrin as substrate. The shortened colipases regained their Intralipid activity upon addition of long-chain fatty acids. The binding of colipase to lipase was not affected by removal of the isoleucine residues.

Measurement of the binding of colipase to a triacylglycerol substrate

The binding between colipase and two triacylglycerol substrates, tributyrin and Intralipid, in the presence of bile salts have been determined quantitatively by a method based on equilibrium partition in an aqueous two-phase system. In the model proposed the triacylglycerol, in the form of spherical droplets covered with bile salt, is assumed to have a certain number of independent binding sites at the surface for colipase. The binding of colipase to tributyrin at pH 7.0 in the presence of 4 mM sodium taurodeoxycholate and 150 mM NaCl had a dissociation constant Kd = 3.3 . 10(-7) M; the concentration of binding sites was 1.2 . 10(-6) M in a 102 mM tributyrin emulsion. When tributyrin was dispersed in 1 mM and 12 mM sodium taurodeoxycholate the dissociation constant was somewhat higher, 6.3 . 10(-7) M and 6.0 . 10(-7) M, respectively. Thus the binding strength was optimal at 4 mM sodium taurodeoxycholate. At the same time the concentration of binding sites decreased from 4.1 . 10(-6) M for 1 mM sodium taurodeoxycholate to 1.4 . 10(-6) M for 12 mM sodium taurodeoxycholate. This indicated that at higher bile salt concentration the bile salt acted as non-competitive inhibitors on the binding of colipase to the substrate, thus binding to other sites than colipase to the substrate. The binding of colipase to Intralipid, an emulsion of a long-chain triacylglycerol stabilized with phosphatidylcholine and glycerol, was more complex with indications of several different binding sites with different affinity. The majority of these had a dissociation constant Kd = 1.2 . 10(-6) M in the presence of 4 mM sodium taurodeoxycholate and 150 mM. With each droplet having a diameter of 10(-4) cm, the number of binding sites on each droplet was determined to 1.96 . 10(5) and the average area available for each colipase molecule to 1600 A at saturation. Colipase on denaturation has a surface of 1320 A.

Lipase-colipase interactions during gel filtration. High and low affinity binding situations

The interaction of porcine pancreatic lipase and colipase was studied during gel filtration in columns eluted with a variety of buffers. High and low affinity binding situations were observed under different conditions. Low affinity binding could only be detected at the high lipase-colipase concentrations encountered during batch purification (10(-3)-10(-4) M). Even in this situation the rapid dissociation of the weak complex during filtration resulted in considerable separation of the two proteins. High affinity binding of lipase to colipase was observed at protein eluant concentrations as low as 10(-8) M on columns equilibrated with oleic acid-taurodeoxycholate mixed micelles. This binding did not take place on columns equilibrated with simple bile salt and mixed phosphatidylcholine-cholesterol-bile salt micelles. Colipase alone exhibited strong binding to phosphatidylcholine and fatty acid mixed bile salt micelles when applied together in a sample on columns eluted with pure bile salt micelles, lipase did not. The relevance of the high affinity complex to the lipase . colipase . substrate complex is discussed.