Stability of food allergens to digestion in vitro.

An integral part of the safety assessment of genetically modified plants is consideration of possible human health effects, especially food allergy. Prospective testing for allergenicity of proteins obtained from sources with no prior history of causing allergy has been difficult because of the absence of valid methods and models. Food allergens may share physicochemical properties that distinguish them from nonallergens, properties that may be used as a tool to predict the inherent allergenicity of proteins newly introduced into the food supply by genetic engineering. One candidate property is stability to digestion. We have systematically evaluated the stability of food allergens that are active via the gastrointestinal tract in a simple model of gastric digestion, emphasizing the major allergens of plant-derived foods such as legumes (peanuts and soybean). Important food allergens were stable to digestion in the gastric model (simulated gastric fluid). For example, soybean beta-conglycinin was stable for 60 min. In contrast, nonallergenic food proteins, such as spinach ribulose bis-phosphate carboxylase/oxygenase, were digested in simulated gastric fluid within 15 sec. The data are consistent with the hypothesis that food allergens must exhibit sufficient gastric stability to reach the intestinal mucosa where absorption and sensitization (development of atopy) can occur. Thus, the stability to digestion is a significant and valid parameter that distinguishes food allergens from nonallergens.

Induction of replicative competence ("priming") in normal liver.

We have used a system of nutritional manipulation to investigate whether hepatocytes of the normal liver can be primed for replication in vivo. In this system, rats that are denied protein for 3 days undergo a burst of hepatic DNA synthesis and mitosis when they are refed amino acids, while normally fed or starved rats do not respond. To determine if hepatocytes of protein deprived (PD) rats have been "primed" for replication, we examined changes in protooncogene expression in livers of PD rats to see if they would mimic the pattern of gene expression that is induced early after partial hepatectomy. c-jun, c-myc, and p53 mRNAs were elevated in livers of PD rats, while c-fos and c-ras genes were not expressed. The administration of amino acids to PD rats stimulated hepatic DNA synthesis in a shorter period than is required after partial hepatectomy and induced p53 and c-ras expression. In culture, hepatocytes from PD rats had higher levels of c-myc mRNA, underwent morphological changes more rapidly, and reached maximum rates of DNA synthesis earlier than normal hepatocytes. In both normal and primed hepatocyte cultures, transforming growth factor alpha stimulated DNA synthesis more effectively than epidermal growth factor. We conclude that hepatocytes pass through a priming stage before they proliferate and that replicative competence without DNA synthesis can be induced in hepatocytes in the normal liver.

Prediction of folding stability and degradability of the de novo designed protein MB-1 in cow rumen.

The authors have recently reported on the design of a protein (MB-1) enriched in methionine, threonine, lysine, and leucine. The protein is intended to be produced by rumen bacteria, in a way that would provide high producing lactating cows with limiting amino acids. In this report, MB-1 stability in the rumen is assessed, i.e., where the protein might be found after cell lysis or after being secreted by rumen bacteria. Current in vitro methods used to predict proteolytic degradability in the rumen were used for MB-1, as well as other natural proteins for comparison. MB-1 was found to be more susceptible to degradation than cytochrome c and ribonuclease A. Data indicate that MB-1 will be rapidly degraded if exposed to the rumen environment without protection. The contribution of folding stability to proteolytic stability was also examined. Rumen liquor components were selected to formulate a solution compatible with constraints of thermal denaturation studies. Denaturation curves show that the natural proteins were folded at rumen temperature. The MB-1 denaturation curves indicated that MB-1 does not unfold in a cooperative transition when heated from 20 to 70 degrees C. This suggests that MB-1 structure may be progressively modified as temperature increases, and that a continuum of conformations are available to MB-1. At 39 degrees C, a significant (50%) portion of MB-1 molecules had their tertiary structure unfolded, contributing to proteolytic degradability. Despite the unusual constraints used in MB-1 design (i.e., a maximized content in selected essential amino acids), results show that MB-1 has structural properties similar to previously reported de novo designed proteins.

Opportunities for manipulating the seed protein composition of wheat and barley in order to improve quality.

Wheat and barley are the major temperate cereals, being used for food, feed and industrial raw material. However, in all cases the quality may be limited by the amount, composition and properties of the grain storage proteins. We describe how a combination of biochemical and molecular studies has led to an understanding of the molecular basis for breadmaking quality in wheat and feed quality in barley, and also provided genes encoding key proteins that determine quality. The control of expression of these genes has been studied in transgenic tobacco plants and by transient expression in cereal protoplasts, providing the basis for the production of transgenic cereals with improved quality characteristics.

Addition of a poly-(6X) His tag to Milk Bundle-1 and purification using immobilized metal-affinity chromatography.

Milk Bundle 1 (MB-1) is a de novo designed protein enriched in M, T, K, and L. Its future application is as a high-quality dietary protein source for ruminants. The protein is currently expressed in Escherichia coli and is being characterized to solve its folded conformation. MB-1 has marginal stability at room temperature, which has hindered our attempts at characterization. To increase the stability of the protein at room temperature, the purification procedure was examined and changed to hopefully increase its effectiveness. We describe here the production and purification of a new MB-1 with six His residues at the C-terminal end. This allows the new mutant (MB-1-His) to bind metal ions and to be purified with immobilized metal-affinity chromatography (IMAC). MB-1-His obtained using IMAC was purer on SDS-PAGE than both MB-1 or MB-1-His isolated using the current protocol. The IMAC protocol is more economical and more efficient; preliminary results show that the protein purified by this method is also quite stable at room temperature.

Expression and analysis of recombinant salmon parvalbumin, the major allergen in Atlantic salmon (Salmo salar).

The parvalbumin from white muscle of Atlantic salmon was previously found to be a major allergen, and designated Sal s1. Two distinct cDNAs, 14.1 and 24.1, which comprise the entire parvalbumin-encoding regions, were cloned, revealing transcripts from two different parvalbumin genes. In the present study, the protein-coding regions of these cDNAs were subcloned into an Escherichia coli expression vector (pET-19b). Both proteins were expressed and the generated target proteins were localized in both soluble and insoluble fractions of the expression host. The recombinant products in the soluble fraction were purified using the His tag-purification system and analysed on Western blots with anti-salmon parvalbumin polyclonal rabbit sera and sera from patients allergic to fish. Both recombinant products (His10-14.1 and His10-24.1) reacted positively with salmon parvalbumin-specific immunoglobulin G (IgG) from rabbits, and with specific immunoglobulin E (IgE) from the sera of six fish-allergic patients. The allergenicity of His10-14.1 was confirmed using enzyme-linked immunosorbent assay (ELISA). The 14.1 cDNA of salmon parvalbumin was shown to be the dominant type represented in a muscle cDNA library.