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Human high-density lipoproteins (HDL), but not other lipoprotein classes, bind bovine thyrotropin (TSH) with moderately high affinity. Binding of 125I-labeled HDL to TSH has been measured in a solid-phase assay; it is saturable and can be displaced by unlabeled HDL but not by other lipoproteins or bovine serum albumin. The interaction of HDL with TSH has been studied by fluorescence spectroscopy: HDL specifically modifies the fluorescence properties of the biologically active dansyl derivative (DNS, (5-dimethyl-aminonaphtalene-1-sulfonyl) chloride) of TSH (DNS-TSH) causing a 12 nm shift to lower wavelength of the emission maximum, a two-fold increase of the quantum yield and a significant increase of fluorescence polarization. The primary site of TSH binding on the HDL particle is likely to be located on its protein moieties, since other lipoprotein classes, which share similar lipids with HDL, do not bind TSH. 125I-labeled apolipoprotein A-I binds TSH in the solid-phase assay and titration of DNS-TSH with apolipoprotein A-I causes perturbations nearly identical to those observed with intact HDL. One HDL particle has at least 12 binding sites for TSH with an association constant, K = 10(7) M-1 whereas one apolipoprotein A-I molecule binds one or two TSH molecules with an association constant slightly lower than that for HDL (K = 10(6) M-1). The lipid moieties of HDL also appears to be perturbed by the interaction with TSH.

The perturbing effect of guanidium hydrochloride and pH on the molecular structure of water buffalo apomyoglobin has been investigated by circular dichroism in the far and near ultraviolet and by fluorescence. In the wavelength region between 320 and 260 nm the circular dichroic spectrum of the globin is highly structured and the contributions of the aromatic chromophores have been resolved. Buffalo apomyoglobin undergoes a structural transition at neutral pH which involves elements of the secondary and tertiary structure, as indicated by changes of dichroic activity of the peptide and aromatic chromophores and the fluorescence of the two tryptophanyl residues. The possibility of charge-transfer complex between indole and imidazole is discussed. A major structural transition with abrupt unfolding takes place in the pH region between 5.6 and 4.3. Below pH 4.3 the peptide helical residues, which survive the acid transition, appear to be resistent to further acidification to pH 2.0 while tryptophanyl emission is quenched and shifted to longer wavelengths. A structural transition occurs also in alkali above pH 10, which has been detected by the same techniques. The relationships between buffalo and sperm whale apomyoglobin are discussed.