The role of caprylate ligand ion on the stabilization of human serum albumin

Sodium caprylate was added to a pharmaceutical-grade human serum albumin (HSA) to stabilize the product. In this study we have aimed to establish how caprylate ligand protects HSA from thermal degradation. The fatty acid stabilizer was first removed from commercial HSA by charcoal treatment. Cleaned HSA was made to 10% w/v in pH 7.4 buffered solutions and doped with sodium caprylate in serial concentrations up to 0.16 mmol/g-protein. These solutions as well as a commercial HSA, human serum, and enriched-albumin fraction were subjected to differential scanning calorimetry (DSC) within the temperature range of 37-90°C at a 5.0°C/min scanning rate. The globular size of the cleaned HSA solutions was measured by dynamic light scattering. The denaturing temperatures for albumin with sodium caprylate and a commercial one were significantly higher than for albumin only. It was found that the protein globules of cleaned HSA were not as stable as that of the native one due to aggregation, and the caprylate ion may reduce the aggregation by enlarging the globules’ electrical double layer. A rational approximation of the Lumry-Eyring protein denaturation model was used to treat DSC denaturing endotherms. The system turned from irreversible dominant Scheme: N (k3K)→ P to reversible dominant Scheme:N (k1)→ P with the increase in caprylate concentration from null to ~0.08 mmol/g-protein. It was postulated that the caprylate ligand may decrease the rate of reversible unfolding as it binds to the IIIA domain which is prone to reversible unfolding/refolding and causes further difficulty for irreversible denaturation which, in turn, HSA can be stabilized.