Tryptophan Fluorescence Studies of Subunit Interaction and Rotational Dynamics of Human Luteinizing Hormone

Human luteinizing hormone (hLH) has a single tryptophan residue occurring in the β-subunit (βhLH). This provides an intrinsic fluorescent probe, in native hLH and βhLH, that is unambiguously assigned. The fluorescence intensities of hLH and βhLH are, however, significantly different. This difference has been utilized in studying the interaction of fluorescent βhLH with the nonfluorescent α-subunit. The accessibility of the tryptophan residue in native hLH and βhLH has been assessed by measuring the rate of collisional fluorescence quenching and by solvent perturbation (D₂O/H₂O) of fluorescence. Fluorescence anisotropy measurements have been used in studying the intramolecular dynamics and segmental tryptophan mobility in hLH and βhLH. Lifetime-resolved anisotropy, measured by the technique of oxygen quenching of fluorescence, has revealed the presence of segmental tryptophan motion. These data can be satisfactorily explained in terms of fast segmental tryptophan motion and rotational diffusion of the whole protein and do not require that intersubunit motion be invoked for intact hLH as it was suggested earlier on the basis of fluorescence depolarization of fluorescein-labeled hLH [Bishop, W. H., & Ryan, R. J. (1975) Biochem. Biophys. Res. Commun. 65, 1184–1190].