Selective oxidation of methionine residues in Kunitz-type protease inhibitors

Production of pharmaceutical-grade recombinant aprotinin and a monoclonal antibody product using plant-based transient expression systems

Plants have been proposed as an attractive alternative for pharmaceutical protein production to current mammalian or microbial cell-based systems. Eukaryotic protein processing coupled with reduced production costs and low risk for mammalian pathogen contamination and other impurities have led many to predict that agricultural systems may offer the next wave for pharmaceutical product production. However, for this to become a reality, the quality of products produced at a relevant scale must equal or exceed the predetermined release criteria of identity, purity, potency and safety as required by pharmaceutical regulatory agencies. In this article, the ability of transient plant virus expression systems to produce a wide range of products at high purity and activity is reviewed. The production of different recombinant proteins is described along with comparisons with established standards, including high purity, specific activity and promising preclinical outcomes. Adaptation of transient plant virus systems to large-scale manufacturing formats required development of virus particle and Agrobacterium inoculation methods. One transient plant system case study illustrates the properties of greenhouse and field-produced recombinant aprotinin compared with an US Food and Drug Administration-approved pharmaceutical product and found them to be highly comparable in all properties evaluated. A second transient plant system case study demonstrates a fully functional monoclonal antibody conforming to release specifications. In conclusion, the production capacity of large quantities of recombinant protein offered by transient plant expression systems, coupled with robust downstream purification approaches, offers a promising solution to recombinant protein production that compares favourably to cell-based systems in scale, cost and quality.

Efficient peroxide-mediated oxidative refolding of a protein at physiological pH and implications for oxidative folding in the endoplasmic reticulum

The majority of secreted and outer membrane eukaryotic proteins contain disulfide bonds, formed by complex interdependent pathways in the endoplasmic reticulum. The current model for the major route of disulfide formation is the regulated flow of oxidizing equivalents from molecular oxygen to the membrane-associated enzyme Ero1 to protein disulfide isomerase, and hence to substrate proteins. One molecule of hydrogen peroxide is produced by Ero1 per disulfide bond made. This peroxide is usually considered to be a dangerous by-product. Here we show that peroxide, added to a refolding buffer or generated enzymatically in situ, results in the efficient refolding of a model protein to the native state. At pH 7.0, the kinetics of obtaining the native folded state are more efficient using peroxide than by the use of a glutathione redox buffer. Disulfide bond formation by peroxide is kinetically favored over oxidation of cysteine to cysteine sulfinic acid and over the oxidation of other amino acids in the proteins such as methionine. Hence, unless peroxides are added in excess, oxidative damage to the folding protein is minimal. Our results offer insights into potential mechanisms for disulfide bond formation in vivo.

Translocation of aprotinin, a therapeutic protease inhibitor, into the thylakoid lumen of genetically engineered tobacco chloroplasts

Aprotinin, a bovine protease inhibitor of important therapeutic value, was expressed in tobacco plastid transformants. This disulphide bond-containing protein was targeted to the lumen of thylakoids using signal peptides derived from nuclear genes which encode lumenal proteins. Translocation was attempted via either the general secretion (Sec) or the twin-arginine translocation (Tat) pathway. In both cases, this strategy allowed the production of genuine aprotinin with its N-terminal arginine residue. The recombinant protease inhibitor was efficiently secreted within the lumen of thylakoids, accumulated in older leaves and was bound to trypsin, suggesting that the three disulphide bonds of aprotinin are correctly folded and paired in this chloroplast compartment. Mass spectrometric analysis indicated that translocation via the Sec pathway, unlike the Tat pathway, led predominantly to an oxidized protein. Translocation via the Tat pathway was linked to a slightly decreased growth rate, a pale-green leaf phenotype and supplementary expression products associated with the thylakoids.

Heterotropic modulation of the protease-inhibitor-recognition process. Cations effect the binding properties of alpha-chymotrypsin

The effect of calcium and lanthanide ions (e.g. terbium) on the binding properties of alpha-chymotrypsin has been studied focussing on the modulation exerted by cations on the interaction of the enzyme with the bovine pancreatic trypsin inhibitor (BPTI or Kunitz inhibitor). The results obtained indicate that the cation binding induces conformational transitions, on the enzyme molecule, which destabilize the enzyme-inhibitor complex formation affecting the interaction of the inhibitor with the secondary specificity site of the proteolytic enzyme. This negative heterotropic effect can be observed only with macromolecular inhibitors (or substrates), displaying an extended interacting surface with the enzyme, and it seems linked to the number of positive charges carried by the cations. Thus, owing to the large conformational changes induced by the binding of trivalent cations, the divalent ones (e.g. calcium) appear to be more suitable for a fine regulation of the enzyme activity. The mutual correlation between inhibitors binding to (and calcium release by) the proteolytic enzymes (and vice versa) could assume an important physiological significance linking parameters, such as calcium concentration and the activity levels of proteolytic enzymes, which are both of great importance for the cell life.

Proteinase inhibitors of the Kunitz family in fallow deer organs: a comparative study

Protein proteinase inhibitors belonging to the Kunitz family have been isolated and characterized in several fallow deer organs. In all the organs studied we found the basic pancreatic trypsin inhibitor (BPTI) while its isoforms, previously isolated and characterized in organs of other ruminant species (bovids and ovids), were absent. In the kidney, in addition to BPTI, active fragments of the inter-alpha-trypsin inhibitor were also isolated. The distribution of Kunitz-type inhibitors in different species of ruminants is compared and discussed on the basis of the expression of their encoding genes.