Spectral studies on the calcium binding properties of bovine brain S-100b protein

The effect of Ca2+ binding on the circular dichroism (CD) and 270-MHz proton nuclear magnetic resonance (NMR) spectra of brain-specific S-100b calcium binding protein has been examined at two pH values, 8.5 and 7.5. At pH 8.5, S-100b protein binds two Ca2+ per monomer with Kd values of 6 x 10(-5) and 2 x 10(-4) M, whereas at pH 7.5, the protein binds only one Ca2+ per monomer with a Kd of 2 x 10(-4) M. The presence of K+ inhibits the binding of Ca2+ to the higher affinity site at pH 8.5, and the affinity for calcium is lowered to Kd = 8.5 x 10(-4) M. Mg2+ has no effect on protein conformation. In the absence of Ca2+, S-100b undergoes a conformational change when the protein is titrated from pH 8.6 to 6.0. Addition of Ca2+ perturbed the environment of tyrosine and phenylalanine residues as measured by ultraviolet difference spectroscopy and 1H NMR. CD melt experiments and far-ultraviolet CD studies at alkaline pH and NMR experiments suggest that the protein is more stable in the presence of Ca2+. The single tyrosine residue in the protein ionizes only after the protein is denatured by exposure to high pH.

Solid-phase radioimmunoassay for human placental lactogen in serum

We report a solid-phase radioimmunoassay for human placental lactogen, in which antibody-coated polystyrene tubes and polyurethane sponges are used. Incubation time and temperature, ionic strength of buffer, and nonspecific adsorption have been evaluated for optimum conditions required for routine clinical assays. The validity of the assay technique has been ascertained by its specificity, reproducibility, and recovery. The sensitivity of this method is 0.4 ng of placental lactogen per assay tube.

Solid-phase radioimmunoassay for human placental lactogen in serum

We report a solid-phase radioimmunoassay for human placental lactogen, in which antibody-coated polystyrene tubes and polyurethane sponges are used. Incubation time and temperature, ionic strength of buffer, and nonspecific adsorption have been evaluated for optimum conditions required for routine clinical assays. The validity of the assay technique has been ascertained by its specificity, reproducibility, and recovery. The sensitivity of this method is 0.4 ng of placental lactogen per assay tube.

Physicochemical and optical studies on calcium- and potassium-induced conformational changes in bovine brain S-100b protein

The brain-specific S-100 protein is a mixture of two components, S-100a and S-100b, with a subunit composition of alpha beta or beta 2, respectively. S-100b, isolated by using hydroxylapatite chromatography in its final purification, is homogeneous by the criteria of gel electrophoresis in the absence and presence of sodium dodecyl sulfate (NaDodSO4) and ultracentrifuge studies. Molecular weight studies by both sedimentation equilibrium in 6 M guanidine hydrochloride and 15% NaDodSO4 gels indicated the subunit molecular weight to be 10 500, and since a molecular weight of 21 000 was obtained in native solvents, the protein exists as a dimer in benign medium. The two subunits are held together by noncovalent forces. The S-100b protein undergoes a conformational change upon binding calcium, as revealed by ultraviolet difference spectroscopy and circular dichroism (CD) studies in the aromatic and far-ultraviolet (UV) range. Far-UV CD studies indicated the apparent helical content drops from approximately 58 to 52% in the presence of Ca2+. The effect of K+ on the protein was antagonistic to Ca2+, and the proteins affinity for calcium was lowered by the presence of K+. The conformational state of the protein is very much dependent upon the metal ions (Ca2+, K+) present, suggesting that changing conformation may be the way S-100 responds to local changes in ionic parameters. Fluorescence studies indicate the presence of an abnormal tyrosine in the protein with the emission maximum centered between 327 and 330 nm when the protein is excited at 280 nm.

Interaction of studies of a 100,000 dalton chymotryptic fragment of rabbit skeletal M-line protein with the S2 subfragment of myosin

Controlled chymotryptic digestion of the 165,000 dalton component of the M-line of rabbit skeletal muscle, followed by Biogel P-150 chromatography of the digest, has led to the isolation of a homogeneous 100,000 dalton species. This fragment was found, by both sedimentation equilibrium and gel filtration chromatography, to interact with heavy meromyosin subfragment 2 (HMM-S2). The persistence of this fragment, after chymotryptic digestion, to bind HMM-S2, along with the known insensitivity of the M-line to proteolysis, suggests a structural role for the parent 165,000 dalton component along the lines of the M-filament, as suggested by the Knappeis and Carlsen model for M-line structure.

Interaction studies of the 165 000 dalton protein component of the M-line with the S2 subfragment of myosin

The M-line protein component of molecular weight 165 000 was isolated and purified from rabbit skeletal muscle using ion exchange chromatography. Sodium dodecyl sulphate electrophoresis revealed that the protein was homogeneous. Circular dichroism measurements indicated that the protein interacts with myosin and heavy meromyosin subfragment 2 (S2). There was an increase in negative ellipticity at 221 nm upon interaction, relative to the calculated values assuming no interprotein interaction. The net increaes in negative ellipticity at 221 nm as a result of interaction of M-protein with myosin and subfragment 2 were 600 degrees and 800 degrees respectively. When the protein was mixed with subfragment 2 in a 1 : 1 mol ratio in 0.5 M KCl/25 mM Tris buffer at pH 8.0, low speed sedimentation equilibrium studies gave a molecular weight of 235 000 +/- 10 000 for the complex, indicative of an interaction of the two components. On a Bio gel A 0.5 m column, M-protein and S2 when applied in 1 : 1 mol ratio, were eluted as a single symmetrical peak and a molecular weight of 230 000 was obtained for the complex from the observed elution volume. Both circular dichroism and sedimentation equilibrium studies indicated no interaction of M-line protein with light meromyosin and subfragment 1. Interaction of the 165 000 component with the flexible hinge region of myosin may have special significance in terms of the mechanism accounting for the reversible expansion of the interfilament distance which occurs during contraction.

Isolation and characterization of the 165 000 dalton protein component of the M-line of rabbit skeletal muscle and its interaction with creatine kinase

The M-line protein component of molecular weight 165 000 was isolated and purified from rabbit skeletal muscle using ion exchange chromatography. Gel electrophoresis, in the presence and absence of sodium dodecyl sulfate, revealed the protein to be homogeneous. Sodium dodecyl sulfate gel electrophoresis and low speed sedimentation equilibrium studies in 0.5 M KCl, 50 mM potassium phosphate gave a molecular weight of 165 000 suggesting the protein to be made up of a single polypeptide chain. Circular dichroism spectra revealed the presence of two negative dichroic bands located at 216 and 208 nm, indicative of the presence of some beta-structure. Ellipticity values at these two wavelengths were --6500 +/- 400 and --7500 +/- 400 deg . cm2 . dmol-1, respectively. Addition of 165 000 component lowered the enzymatic activity of creatine kinase M-line protein and the nature of the inhibition was found to be a competitive one. When the protein was mixed with creatine kinase in a 1 : 1 mole ratio in a medium consisting of 0.2 M KCl, 25 mM Tris, 1 mM dithiothreitol (pH 8.0), low speed sedimentation equilibrium studies gave a molecular weight of 260 000 +/- 10 000 for the complex, indicative of an interaction of the two components of the M-line.

Physicochemical studies on the creatine kinase M-line protein and its interaction with myosin and myosin fragments

The M-line protein which is identical to the muscle form of creatine kinase was purified from rabbit skeletal muscle using ion exchange chromatography. Gel electrophoresis in the presence and absence of sodium dodecyl sulfate revealed the protein to be homogeneous. Sodium dodecyl sulfate gel electrophoresis gave 44 000 +/- 2000 as the minimum molecular weight while low speed sedimentation equilibrium experiments yielded a molecular weight of 84 000 +/- 4000, suggesting that the parent molecule is a dimer. Circular dichroism spectra revealed the presence of two negative dichroic bands located at 218 and 208 nm suggesting the presence of some beta-structure. Ellipticity values at these two wavelengths were -8000 +/- 400 and -9000 +/- 400 deg-cm2-dmol-1. Circular dichroism measurements indicated the protein to interact with myosin, heavy meromyosin and heavy meromyosin subfragment 1 (S1). The Ca2+-activated ATPase activities of myosin, heavy meromyosin and subfragment 1 were inhibited by the addition of M-line protein. When the protein was mixed with subfragment 1 in a 1:1 mole ratio in 0.15 M KC1, 50 mM Tris pH 8, low speed sedimentation equilibrium studies gave a molecular weight of 205 000 +/- 10 000 for the complex, indicative of an interaction of the two components. Both circular dichroism and sedimentation equilibrium studies indicated no interaction of M-line protein with light meromyosin.

Some physicochemical and chemical properties of light chains isolated from rabbit cardiac myosin

Cardiac light chains released from myosin at alkaline pH were isolated by ammonium sulfate fractionation. The isolated light chains were separated on a diethylaminoethylcellulose column using a phosphate gradient. Sodium dodecyl sulfate acrylamide gel electrophoresis indicated the presence of two light chains, LC1 and LC2 of molecular weight 27 000 and 18 000, respectively. Densitometer traces of the gels gave a molar ratio of 2 to 1 for the two respective protein components. Molecular weights of 24 000 ± 1000 and 16 000 ± 500 were obtained for these two components by high-speed sedimentation equilibrium studies. Amino acid analyses of the two fractions revealed a high ratio of phenylalanine to tyrosine (4.0 and 6.7 for LC1 and LC2, respectively), which also dominates the aromatic circular dichroic (CD.) spectrum. The C.D. spectra of LC1 and LC2 exhibited ellipticity bands at 220 and 206.5 nm, positions typical of the α-helical conformation, but the low ellipticity values at these positions indicated that only a small portion of their polypeptide chains are in the α-helical conformation consistent with their large content of proline.