AOCS Division Newsletter




	printer-friendly version . send feedback on this newsletter . unsubscribe AOCS Phospholipid Division Newsletter April 2007 A number of drivers affect the protein bioadhesives market Edgar J. Acosta *from inform* 18(1) p. 56 Proteins and polysaccharides were the adhesive material of choice before World War II. At that time, Henry Ford, inventor and automobile manufacturer, and George Washington Carver, agricultural researcher and peanut advocate, were among the proponents of “chemurgy,” a concept by which fuels and chemical products (adhesives included) derived from plants and other renewable sources would sustain the growing manufacturing and transport industry. During World War II, inexpensive, moisture-resistant, and high bond strength formaldehyde-based resins were developed to replace “protein glues.” Sustained by the Haber-Bosch ammonia process (which can introduce nitrogen from air into fertilizers) and developments in biotechnology, the worldwide production of protein-rich grain meal is now close to 100 million tons per year. It is estimated that more than one third of this protein source is underutilized and could be used as raw material for new protein-based industrial products. The current and future developments on protein-based adhesives for industrial and biomedical applications are two topics of major interest. Proteins as biologically derived adhesive material In formaldehyde-based resins, the interpenetration and covalent bonding (cross-linking) of polymer chains produce adhesives with high bond strength and moisture resistance. In proteins, however, chain interpenetration is hindered by protein molecular folding, which is the reason why denatured proteins are better adhesives. Hy- drogen bonding, electrostatic, and van der Waals interactions are responsible for cross-linking protein chains. The interaction between proteins is weakened by water molecules, owing to the hydration of ionic groups and hydrogen bonding moieties on the protein chains. Nature provides evidence that it is possible to produce protein-based high- strength adhesives. The best example of these adhesives is mussel adhesive proteins (MAP) that are capable of adhering marine mussels to various surfaces (including glass and Teflon®). It is believed that MAP owe their strength to a combination of various factors: the shape of the byssus (filamentous secretions) of the mussels, the tight packing of protein bundles in the byssus, and the presence of a water-resistant protein “glue” at the end of the byssus. The cross-linkage of hydrophobic amino acid residues (especially l-3,4 dihydroxy-phenylalanine, l-DOPA) is responsible for the water resistance of MAP. Although various research groups have tried to synthesize these proteins in genetically modified bacteria, their commercial use is limited owing to high production costs. The research into protein-based adhesives has been driven by economics and environmental concerns (replacement of formaldehyde-based adhesives). In the United States alone, an estimated 3 million tons of adhesives are sold every year. Wood processing and pulp and paper industries are the largest consumers of adhesives. Among the various protein-based adhesive alternatives, a few representative examples are worthy of discussion. The Thames-Rawlins group at the University of Southern Mississippi, Hattiesburg, Mississippi, USA, has patented the use of maleinized methyl ester of tung oil (MMETO) as an additive to improve the bond strength and moisture resistance of soy protein-based adhesives. These researchers use glycerol as a plasticizer to unfold the proteins. Protein unfolding improves MMETO penetration and may induce an “entanglement” effect once the MMETO monomer is polymerized, increasing the bond strength of these mixtures. Similarly, a mixture of monomer (phenol formaldehyde), resorcinol, and soy protein is used in a commercial product, the PRF/Soy 2000 wood finger joint adhesive developed at the Battelle Memorial Institute at Columbus, Ohio, USA (Fig. 1). Xiuzhi Sun’s research group at Kansas State University, Manhattan, Kansas, USA, has mixed various anionic and cationic surfactants that bind to charged groups in the protein molecule, promoting protein unfolding and increasing protein hydrophobicity. Another approach used by Sun’s group is protein esterification with ethanol. This group has noted that there is an optimal level of protein hydrophobicity for maximal bond strength and moisture resistance. Kaichang Li’s research group at Oregon State University, Corvallis, Oregon, USA, has grafted DOPA-like catechol (pyrocatechol; 1,2-dihydroxybenzene) groups from lignin residues into soy protein isolates in the presence of polyethylenimine. The resulting polymers have superior bond strength and moisture resistance. At optimal formulation conditions the bond strength (measured as shear strength) of these soy protein-modified adhesives is close to 5 MPa (compared with 6 MPa for phenol-formaldehyde). There are other protein modification approaches not described in this article that produce similar effects to those mentioned before. While all these approaches seem different, they share a number of common principles: (i) need to unfold the proteins to improve protein-protein interactions; (ii) reduction of protein-water interaction through hydrophobic additives, while maintaining a certain balance; (iii) promotion of protein-protein interaction through aromatic and amino groups introduced either as additives or chemically attached to the protein chain. Proteins as adhesives for biological substrates** Proteins are gifted with different moieties that allow them to adhere to a wide range of substrates through van der Waals, electrostatic, and hydrogen-bonding interactions. The adhesive properties of proteins to biological substrates are evidenced by numerous phenomena such as the adhesion of bacterial films to dental surfaces. One example of the biomedical use of protein bioadhesives as a dental adhesive in wound healing applications is MAP obtained from genetically modified bacteria using recombinant protein adhesive technology developed by Dong Soo Hwang and colleagues at Pohang University of Science and Technology at Pohang, Korea (Kollodis BioScience, Inc.). Another commercial example is Bioglue® (Protein Polymer Technologies, Inc., based at San Diego, California, USA), which, similar to PRF/ Soy 2000, consists of a mixture of proteins (collagen, albumin) with aldehydes, and is used for tissue adhesion. One area of growing interest is the use of plant-extracted proteins, lectins in particular, as bioadhesives for drug delivery. Lectins (glycoproteins that bind to polysaccharides) adhere to mucous (mucin polysaccharides) membranes such as those found in airways, oral cavities, and in the small intestine. Research in the area of lectin-enhanced drug delivery (including oral and transnasal) began in the mid-1980s and was actively promoted by Barbara Naisbett and John Woodley at the University of Keele in Staffordshire, United Kingdom. This work was driven by the need to improve the bioavailability of hydrophobic drugs and proteins. Currently, oral and transnasal delivery formulations account for 66% of the drug delivery market, which is estimated at $100 billion/year worldwide. Lectins can be found in numerous legumes, and most of them can go through the digestive system without experiencing significant modification. Lectins have been considered an undesirable family of proteins for nutritional purposes but are associated with numerous immunological processes, including triggering allergic reactions and preventing absorption of nutrients. At low concentrations, however, lectins improve the adsorption of lectin-coated nano- and micro-drug carriers on the small intestine. The increased retention of drug carriers (e.g., liposomes) enhances the uptake of drugs by passive diffusion through the intestinal membrane. In addition, lectins can be covalently bonded to prodrugs and therapeutic proteins to achieve the same effect. The adhesion of lectins to polysaccharides is mediated through a carbohydrate recognition domain (CRD) located in the beta sheet-loops regions of the protein. Figure 2 presents a schematic of the adhesion and drug release mechanism in lectin-mediated drug delivery systems. The conjugation of the prodrug or the carrier to lectin can be achieved through the carbon diimide method that produces an amide bond (shown in Fig. 2). Lectin can also adhere to the carrier through hydrogen bonding and electrostatic interactions. The latter approach is, however, less efficient and requires higher lectin concentration. Among the different lectins tested, wheat germ agglutinin (WGA) and tomato lectin are the most efficient owing to their high binding capacity and relatively low toxicity. The concept illustrated in Figure 2 has been confirmed in in vitro and in animal studies, but no human tests have been conducted thus far. We recently posed the question at the 2006 AOCS Annual Meeting & Expo held in St. Louis, Missouri, USA, of whether the bioadhesive properties of lectins could be used to improve the retention of oil-swollen micelles formulated with lecithin (a generic delivery vehicle for hydrophobic drugs). To answer this question, we mixed lectin and lecithin micelles loaded with isopropyl myristate (IPM, solvent oil), in hopes that lectin would adhere to the surface of lecithin micelles and the surface of the small intestine. The protein-micelle mixture was contacted with a section of pig jejunum using a flow-through dialyzer operated in recirculation (batch) mode. Figure 3 shows carrier adsorption (retention) as a function of contact time for lecithin micelles (black curve), lecithin micelles with soy-extracted lectin (red curve, 0.007 mg lectin/mg carrier), and lecithin micelles with soluble mustard protein isolate (green curve, 0.7 mg protein/mg carrier). In the case of lecithin micelles alone (black curve), the carrier absorbed relatively fast within the first two minutes, and then continued to adsorb at a slower rate. Soy lectin increased the instantaneous adsorption of lecithin micelles, but due to the weak bonding between micelles and lectin, this structure dissociated. An interesting finding was that the same adsorption-desorption trend was obtained using high concentrations of water-soluble mustard protein isolate (donated by Levente Diosady at the University of Toronto, Canada). The fact that an inexpensive protein isolate can produce the same effect as purified (and expensive) lectins opens a door to explore new protein-enhanced oral delivery formulations. It is necessary to investigate the use of covalent bonding between the lectin-enriched protein isolate and the carrier to prevent carrier desorption. The examples just discussed barely give a taste of recent developments in the area of protein bioadhesives. The conjunction of high oil prices, environmental concerns, new protein modification techniques, and an abundant source of protein-rich grain meals is stimulating the resurgence of the “chemurgy” concept of a bio-based economy. Edgar J. Acosta, Assistant Professor, Department of Chemical Engineering and Applied Chemistry, University of Toronto, Canada, can be reached via e-mail at acosta@chem-eng.utoronto.ca. information • Mittal, K.L., and A. Pizzi (Editors), Handbook of Adhesive Technology, Marcel Dekker, New York, USA, 2003. • Mathiowitz, E., D. E. Chickering, and C.-M. Lehr, Bioadhesive Drug Delivery Systems: Fundamentals, Novel Approaches, and Development, Marcel Dekker, New York, 1999. Other stories in this newsletter: Greetings From the Chair, by Bruce Sebree Short Course on Lecithin Properties and Technological Functions GRAS Approval for Enzyme Technology USB Recognizes List Announcement of Research Funds Three successful lipid conferences in Pécs AOCS, 2710 S. Boulder, Urbana, IL 61802-6996 USA	Division Officers Chairperson Bruce Sebree phone: +1-217-451-4067 fax: +1-217-451-2693 e-mail: sebree@admworld.com Vice Chairperson Jonathan Maynes phone: +1-314-982-3162 fax: +1-314-982-2899 e-mail: Jmaynes@solae.com Secretary Gary List phone: +1-309-681-6388 fax: +1-309-681-6340 e-mail: listgr@ncaur.usda.gov Treasurer Moghis Ahmad phone: +1-847-573-0707 fax: +1-847-573-0770 Email: Moghis@jinapharma.com Member-at-Large Bernie Szuhaj phone: +1-260-489-6849 Member-at-Large Lance Colbert phone: +1-217-451-5802 fax: +1-217-451-2457 e-mail: colbert@admworld.com Member-at-Large Randy Zigmont phone: +1-203-262-7100 fax: +1-203-262-7101 e-mail: rzigmont@aol.com Member-at-Large Walt Shaw phone: +1-205-663-2494 fax: +1-205-663-0756 e-mail: waltshaw@avantilipids.com Member-at-Large Susan Gurkin phone: +1-770-986-6268 fax: +1-770-986-6282 e-mail: susan.gurkin@degussa.com Member-at-Large Willem van Nieuwenhuyzen phone: +31 72 505 5617 fax. +31 72 505 5618 e-mail: willem@lecipro.nl Immediate Past Chair Susan Carlson phone: +1-913-588-5359 fax: +1-913-588-8946 e-mail: scarlson@kumc.edu Past Chairperson Joseph Casey e-mail: JCasey@solae.com