Vibrational spectroscopic studies of solid recombinant bovine growth hormone and related growth hormone analogs

Spectroscopic Analysis of Highly Concentrated Suspensions of Bovine Somatotropin in Sesame Oil

Spectroscopy was employed to analyze the structural and thermal stability of highly concentrated oil suspensions of bovine somatotropin (bST). These methods were then compared with more dilute aqueous solutions (1 and 10 mg/mL). All oil suspensions were opaque, viscous, and highly concentrated in bST (>300 mg/mL) and thus provided unique analytical challenges. Using front surface fluorescence and ATR-FTIR spectroscopy, protein structure and stability could be directly monitored in this environment. Differences were detected in structure between concentrated oil and dilute aqueous formulations. Fluorescence spectroscopy found that bST was highly thermally stabile within oil suspensions, since minimal changes in emission peak maxima and emission intensity were observed with increasing temperature when compared to dilute solutions. It was also observed that the amount of aggregate in a sample had some effect on the fluorescence spectra. As the amount of aggregated protein increased, the emission peak maximum and emission intensity changed. Employing ATR-FTIR, the secondary structure was examined with increasing temperature. The secondary structure of bST was also found to be very thermally stabile since no change in relative amount of helix/random structure is observed up to 70 degrees C while significant losses are observed in aqueous solution. This study demonstrates that conformational stability can be directly analyzed within highly concentrated, opaque environments using slight modifications of conventional methods.

Solid-state photodegradation of bovine somatotropin (bovine growth hormone): evidence for tryptophan-mediated photooxidation of disulfide bonds

Lyophilized recombinant bovine somatotropin (rbST; bovine growth hormone) is sensitive to photoinduced degradation. The underlying mechanisms of these processes are identified and presented. Lyophilized rbST was photolyzed with near-ultraviolet (UV) light between 305 and 410 nm, and the protein content was analyzed by various bioanalytical techniques, including tryptic mapping, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), amino acid analysis, and fluorescence, UV, Raman and Fourier transform infrared (FTIR) spectroscopy. The solid-state photodegradation of rbST by near-UV light exclusively targets the protein disulfide bonds. The reaction is initiated by photoionization of tryptophan (Trp) and one-electron reduction of the disulfide. However, in contrast to the behavior of other proteins in solution, rbST appears to undergo back electron transfer to restore Trp and yield a pair of cysteine (Cys) thiyL radicals, which add molecular oxygen and ultimately recombine to yield alpha-disulfoxide, thiosulfinate, and/or thiosulfonate. Photodegradation is strictly dependent on the presence of molecular oxygen, but does not involve singlet oxygen. Between 0.4 and 10%, residual moisture levels do not affect the rate of photodegradation. Our results show a novel mechanism for Trp-mediated photodegradation of protein disulfide bonds via formation of a pair of thiyL radicals followed by addition of molecular oxygen.