Fluorescence studies of human growth hormone

Oxidation of human growth hormone by oxygen-centered radicals: formation of Leu-101 hydroperoxide and Tyr-103 oxidation products

Human growth hormone (hGH) was exposed to oxygen-centered radicals generated through the thermolysis of AAPH in the presence of dioxygen. Such conditions mimic oxidative processes which protein pharmaceuticals can encounter during formulation in the presence of polysorbates. We detected the oxidation of Met to Met sulfoxide, the formation of protein carbonyls, the oxidation of Tyr to dityrosine and several additional Tyr oxidation products, the conformation-dependent oxidation of Trp, and the site-specific formation of protein hydroperoxides. The sensitivity of Met oxidation correlates with their solvent accessible surface, i.e. the yields of MetSO decreased in the order Met-14 > Met-125 > Met-170. Trp oxidation in native hGH was negligible, but was enhanced through denaturation. Dityrosine formed predominantly intramolecularly but did not contribute significantly to protein cross-linking. Hydroperoxides formed selectively on Leu-101 and were generated specifically by alkoxyl radicals, generated through the decomposition of peroxyl radicals. Tyr-103 was converted into a series of oxidation products characterized by mass shifts of Tyr + 14 Da and Tyr + 16 Da.

Ovine placental lactogen and ovine prolactin: partial proteolysis and conformational stability

The high-resolution structure of ovine placental lactogen (oPL) and ovine prolactin (oPRL), not yet established in detail, was probed by limited proteolysis with the Glu-specific protease from Staphylococcus aureus V8. While in hGH there were no cleavage sites inside of any of the four alpha-helices, the analysis of the fragments obtained after partial proteolysis of oPL showed a site of cleavage at the putative third helix, suggesting that this helix is partially unwound at this point. The partial proteolysis of the rest of the molecule was compatible with a similar folding pattern for oPL, hGH and pGH, on the basis of the crystal structure of these last hormones. In the case of oPRL, proteolytic cleavage occurred at Glu residues which would be located at the end of the first helix and the beginning of the second in the hGH folding model, suggesting that these helices are shorter in oPRL than in hGH. In order to gain further insight on the folding of these molecules, circular dichroism and intrinsic fluorescence measurements were used to examine the effect of denaturing conditions on oPL and oPRL. After exposure to 6 M guanidine the unfolding of both proteins was completely reversed upon elimination of the denaturing agent. In contrast, exposure to pH 3.0 caused an irreversible decrease in the alpha-helical content in both hormones, most striking for oPL, indicating that this hormone is less stable than oPRL or hGH.

Properties of the tryptophan residue in rabbit liver microsomal cytochrome P-450 isozyme 2 as determined by fluorescence

Cytochrome P-450 isozyme 2 from rabbit liver microsomes fluoresces upon excitation at 295 nm due to the single tryptophyl residue (Trp121) in the protein. The fluorescence spectrum, which is not altered by the presence of phospholipid or substrates, has a maximum at 335 nm, which suggests that the environment of the residue is hydrophobic. The fluorescence intensity decreases linearly with increase of specific content of the cytochrome preparations, and the holoenzyme was estimated to exhibit, at most, 6% as much fluorescence as the apoenzyme. This indicates that the fluorescence of the tryptophan is quenched by energy transfer to the heme. The distance between the tryptophyl residue and the heme was estimated to be less than 40 A. From enhancement of the fluorescence by methanol and ethanol, 30 to 50% of the Trp residue was found to be accessible to these solvents. On the other hand, the accessibility to iodide and cesium ions, as estimated by quenching effects, is less than 14%. From such evidence, the tryptophyl residue is believed to be partly buried. Since Trp121 is conserved at or near the same position in all mammalian P-450's so far sequenced, the results obtained may be applicable to these related cytochromes as well.

Quenching of tryptophanyl fluorescence of human growth hormone by iodide

Quenching of tryptophanyl fluorescence of human growth hormone by I- followed saturation kinetics and was abolished by KSCN. In the presence of 6 M guanidine hydrochloride quenching was linear between 0 to 0.2 M KI. These results suggest that I- quenched the fluorescence of the native hormone by binding at or near the single tryptophanyl residue. Quenching by 0.1 M KI decreased exponentially with increasing concentrations of human and bovine growth hormones. Acidification did not have a significant effect on quenching of the human hormone, but it markedly decreased quenching of the bovine hormone. Conformational differences at the vicinity of the lone tryptophanyl residue that could be inferred by these and other experiments may be contributing to the biological specificity of native human and bovine growth hormones.