Silicone oil, which is used as a lubricant or coating in devices such as syringes, needles and pharmaceutical containers, has been implicated in aggregation and particulation of proteins and antibodies. Aggregation of therapeutic protein products induced by silicone oil can pose a challenge to their development and commercialization. To systematically characterize the role of silicone oil on protein aggregation, the effects of agitation, temperature, pH and ionic strength on silicone oil-induced loss of monomeric anti-streptavidin IgG 1 antibody were examined. Additionally, the influences of excipients polysorbate20 and sucrose on protein aggregation were investigated. In the absence of agitation, protein absorbed to silicone oil with approximately monolayer coverage, however silicone oil did not stimulate aggregation during isothermal incubation unless samples were also agitated. A synergistic stimulation of aggregation by a combination of agitation and silicone oil was observed. Solution conditions which reduced colloidal stability of the antibody, as assessed by determination of osmotic second virial coefficients, accelerated aggregation during agitation with silicone oil. Polysorbate20 completely inhibited silicone oil-induced monomer loss during agitation. A formulation strategy optimizing colloidal stability of the antibody as well as incorporation of surfactants such as polysorbate20 is proposed to reduce silicone oil-induced aggregation of therapeutic protein products.

The residues responsible for the pHdependent stability of the helix formed by the isdlated Cpeptide (residues 1-13 of ribonuclease A) have been identified by chemical synthesis of analogues and measurement of their helix-forming properties. Each of the residues ionizing between pH 2 and pH 8 has been replaced separately by an uncharged residue. Protonation of Glu-2-is responsible for the sharp decrease in helix stability between pH 5 and pH 2, and deprotonation of His-12+ causes a similar decrease between pH 5 and pH 8. Glu-9-is not needed for helix stability. The results cannot be explained by the Zimm-Bragg model and host-guest data for a-helix formation, which predict that the stability of the C-peptide helix should increase when Glu-2-is protonated or when His-12+ is deprotonated. Moreover, histidine' is a strong helix-breaker in host-guest studies. In proteins, acidic and basic residues tend to occur at opposite ends of a-helices: acidic residues occur preferentially near the NH2-terminal end and basic residues near the COOH-terminal end. A possible explanation, based on a helix dipole model, has been given [Blagdon, D. E. & Goodman; Biopolymers 14,[241][242][243][244][245]. Our results are consistent with the helix dipole model and they support the suggestion that the distribution of charged residues in protein helices reflects the helix-stabilizing propensity of those residues. Because Glu-9 is not needed for helix stability, a possible Glu-9-His-12+ salt bridge does not contribute significantly to helix stability. The role of a possible Glu-2-Arg-10 salt bridge has not yet been evaluated. A charged-group effect on a-helix stability in water has also been observed in a different peptide system [Ihara, S., Ooi, T. & Takahashi, S. (1982) Biopolymers 21, 131-145]: block copolymers containing (Ala)20 and (Glu)20 show partial helix formation at low temperatures, pH 7.5, where the glutamic acid residues are ionized. (Glu)20(Ala)20Phe forms a helix that is markedly more stable than (Ala)20(Glu)20Phe. The results are consistent with a helix dipole model. Isolated C-peptide (residues 1-13 of RNase A, terminating in homoserine lactone at residue 13) shows partial a-helix formation in water near 0C, conditions where C-peptide remains monomeric (1-3). This behavior is surprising because, as noted earlier (2), all short peptides (s20 residues) are predicted not to show measurable a-helix formation in water, according to the Zimm-Bragg equation (4) and host-guest data (5) for helix formation. This point is illustrated in Fig. 1. To obtain 30% helix, which is the value shown by C-peptide at 00C, pH 5.0, 0.1 M NaCl (2), Fig.

The Fc region has two highly conserved methionine residues, Met 33 (C(H)3 domain) and Met 209 (C(H)3 domain), which are important for the Fc's structure and biological function. To understand the effect of methionine oxidation on the structure and stability of the human IgG1 Fc expressed in Escherichia coli, we have characterized the fully oxidized Fc using biophysical (DSC, CD, and NMR) and bioanalytical (SEC and RP-HPLC-MS) methods. Methionine oxidation resulted in a detectable secondary and tertiary structural alteration measured by circular dichroism. This is further supported by the NMR data. The HSQC spectral changes indicate the structures of both C(H)2 and C(H)3 domains are affected by methionine oxidation. The melting temperature (Tm) of the C(H)2 domain of the human IgG1 Fc was significantly reduced upon methionine oxidation, while the melting temperature of the C(H)3 domain was only affected slightly. The change in the C(H)2 domain T m depended on the extent of oxidation of both Met 33 and Met 209. This was confirmed by DSC analysis of methionine-oxidized samples of two site specific methionine mutants. When incubated at 45 degrees C, the oxidized Fc exhibited an increased aggregation rate. In addition, the oxidized Fc displayed an increased deamidation (at pH 7.4) rate at the Asn 67 and Asn 96 sites, both located on the C(H)2 domain, while the deamidation rates of the other residues were not affected. The methionine oxidation resulted in changes in the structure and stability of the Fc, which are primarily localized to the C(H)2 domain. These changes can impact the Fc's physical and covalent stability and potentially its biological functions; therefore, it is critical to monitor and control methionine oxidation during manufacturing and storage of protein therapeutics.

The importance of ProB28 and LysB29 on the self-association of insulin was established by systematically truncating the C terminus of the B chain. The relationship between structure and association was further explored by making numerous amino acid replacements at B28 and B29. Association was studied by circular dichroism, size-exclusion chromatography and ultracentrifugation. Our results show that the location of a prolyl residue at B28 is critical for high-affinity self-association. Removal of ProB28 in a series of C-terminal truncated insulins, or amino acid replacement of ProB28, greatly reduced association. The largest disruption to association was achieved by replacing LysB29 with Pro and varying the amino acid at B28. Several of the analogs were predominantly monomers in solutions up to 3 mg/ml. These amino acid substitutions decreased association by primarily disrupting the formation of dimers. Such amino acid substitutions also substantially reduced the Zn-induced insulin hexamer formation. The formation of monomeric insulins through amino acid replacements was accompanied by conformational changes that may be the cause for decreased association. It is demonstrated that self-association of insulin can be drastically altered by substitution of one or two key amino acids.

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The concentration-dependence of the diffusion and sedimentation coefficients (k(D) and k(s), respectively) of a protein can be used to determine the second virial coefficient (B₂), a parameter valuable in predicting protein-protein interactions. Accurate measurement of B₂ under physiologically and pharmaceutically relevant conditions, however, requires independent measurement of k(D) and k(s) via orthogonal techniques. We demonstrate this by utilizing sedimentation velocity (SV) and dynamic light scattering (DLS) to analyze solutions of hen-egg white lysozyme (HEWL) and a monoclonal antibody (mAb1) in different salt solutions. The accuracy of the SV-DLS method was established by comparing measured and literature B₂ values for HEWL. In contrast to the assumptions necessary for determining k(D) and k(s) via SV alone, k(D) and ks were of comparable magnitudes, and solution conditions were noted for both HEWL and mAb1 under which 1), k(D) and k(s) assumed opposite signs; and 2), k(D) ≥k(s). Further, we demonstrate the utility of k(D) and k(s) as qualitative predictors of protein aggregation through agitation and accelerated stability studies. Aggregation of mAb1 correlated well with B₂, k(D), and k(s), thus establishing the potential for k(D) to serve as a high-throughput predictor of protein aggregation.

Specific-ion effects are ubiquitous in nature; however, their underlying mechanisms remain elusive. Although Hofmeister-ion effects on proteins are observed at higher (>0.3M) salt concentrations, in dilute (<0.1M) salt solutions nonspecific electrostatic screening is considered to be dominant. Here, using effective charge (Q*) measurements of hen-egg white lysozyme (HEWL) as a direct and differential measure of ion-association, we experimentally show that anions selectively and preferentially accumulate at the protein surface even at low (<100 mM) salt concentrations. At a given ion normality (50 mN), the HEWL Q* was dependent on anion, but not cation (Li 1 , Na 1 , K 1 , Rb 1 , Cs 1 , GdnH 1 , and Ca 21 ), identity. The Q* decreased in the order, demonstrating progressively greater binding of the monovalent anions to HEWL and also show that the SO 2À 4 anion, despite being strongly hydrated, interacts directly with the HEWL surface. Under our experimental conditions, we observe a remarkable asymmetry between anions and cations in their interactions with the HEWL surface.

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