Study of protein conformational stability and integrity using calorimetry and FT-Raman spectroscopy correlated with enzymatic activity

Maintaining protein conformational stability and integrity during formulation is critical for developing protein pharmaceuticals. Accordingly, high sensitivity differential scanning calorimetry (HSDSC) and Fourier transform (FT)-Raman spectroscopy were employed to assess conformational stabilities (thermal stability and folding reversibility) and structural integrities, respectively, for three model proteins: lysozyme, deoxyribonuclease I (DNase I) and lactate dehydrogenase (LDH) in lyophilised (as received) and spray-dried forms. Enzymatic assay after cooling of thermally denatured protein solutions from HSDSC determined if thermal transition reversibility was related to biological activity. HSDSC data showed that molecules from lyophilised lysozyme were able to refold better than the spray-dried form. This was confirmed by enzymatic assay. Moreover, enzymatic assay results revealed that lysozyme folding reversibility was related to the native structure of the protein that is essential for the biological activity. Thermal denaturation of DNase I and LDH samples in HSDSC was not reversible upon cooling of thermally denatured proteins (in contrast to lysozyme). Hence, it was decided to identify the effect of protein initial structures on its propensity to thermal denaturation via FT-Raman spectroscopy. In other words, proteins may denature with structural alterations due to stresses such as heat and the protein loses its enzymatic activity. Consequently, FT-Raman investigated the effects of spray drying and heating of solid DNase I and LDH samples, from differential scanning calorimetry, on protein conformational integrities. Lyophilised and spray-dried DNase I and LDH solid samples were heated to two temperatures, one before the apparent denaturation temperatures (Tm) and the other after the Tm. Samples heated below their Tm showed some alterations of the secondary structure and some enzymatic activity. HSDSC and FT-Raman spectroscopy are useful techniques to study protein conformations and their results correlate with those of enzymatic activity.