Irreversible aggregation of recombinant bovine granulocyte-colony stimulating factor (bG-CSF) and implications for...

The kinetics of irreversible aggregation of bovine Granulocyte-Colony Stimulating Factor (bG-CSF) in solution were investigated as a function of temperature (T), concentration, and pH, and analyzed in terms of an Extended Lumry-Eyring model of protein aggregation proceeding via a non-native conformational state. In the spirit of classic Lumry-Eyring models, the observed kinetics are separated into contributions from thermodynamic or conformational stability of unaggregated native and non-native states, and the intrinsic aggregation kinetics of non-native molecules. It is found that a detailed treatment of the intrinsic kinetics coupled with a two-state approximation of the reversible unfolding transition is sufficient to allow quantitative prediction of low-T stability from high-T data despite highly non-Arrhenius kinetics. Accounting for shifts in conformational equilibrium quantitatively captures the non-Arrhenius T dependence, without requiring the assumption of a change in the rate-determining step with T. From a more general perspective, the observed aggregation behavior of bG-CSF is consistent with the rate-determining step being aggregation at T below a crossover temperature T(x) that is inversely related to initial protein concentration. Above T(x), irreversible unfolding is presumably the rate-determining step. The results illustrate that protein aggregation kinetics can, in principle, be predicted quantitatively from so-called accelerated data provided the thermodynamic and kinetic components can be separately extrapolated to longer term storage conditions.