The substance exchanging between two chemically different sites A and B gives two separate NMR signals at νA and νB, if the exchange rate is slower enough than the frequency separation ΔνAB. When the sample is irradiated at the frequency νB, the signal intensity of A occasionally becomes weaker or disappears. This phenomenon is called saturation transfer in NMR and is used to determine the sites and rate of the exchange process. This technique is only applicable when the life time of exchange is between the inverse of ΔνAB and the spin lattice relaxation time T1. By using this technique we found that there is direct proton exchange between the complementary nucleic acid bases in organic solvents. When the signal of the amino protons of an adenine derivative is irradiated, the imino proton signal of an uracil derivative disappears. By quantitative analysis following the Forsen and Hoffman's treatment, it was found that the exchange occurs ten times in a second at room temperature. Other applications related to biological systems are reviewed: the electron exchange between cytochromes and the proton exchange between peptides and solvent water. The rates of following reactions were examined: the synthesis of ATP from AMP and inorganic phosphate by ATPase, the disproportionation of two ADP to ATP and AMP by adenylate kinase and the hydroxylation of acetylaldehyde by carbonic anhydrase.
Muscle contraction occurs as a result of the interaction between thin and thick filaments in the presence of free Ca2+ and MgATP. It is thus very important to investigate the dynamic properties of thin filaments in the elementary process of muscle contraction, especially to correlate the tension development with the flexibility of the thin filament due to fluctuations of the imtermolecular bond-strength of actin, tropomyosin and/or troponin. When a striated muslce fiber is overstretched or ATP is removed from the myofibrillar space, the fiber can not contract on electric stimulation. The intensities of optical diffraction lines of such a fiber increase when an electric field is applied along the fiber axis. From this electro-optical effect, we have studied the flexural rigidity of thin filaments in vivo and its dependence on concentrations of free Ca2+. The flexural rigidity steeply changed from 3·10-17 dyn cmcm2 at free [Ca2+] above 3μM to 2·10-16 dyn cmcm2 at free [Ca2+] below 0.5μM. In this article we review the story of our study on this effect from its finding (Umazume & Fujime, 1975) to the recent progress (Fujime & Yoshino, 1978; Yoshino, Umazume, atori, Fujime & Chiba, 1978).
Sugar transport across erythrocyte membrane has been studied by many investigators. Analysis of the Michaelis-Menten kinetics has been applied to the sugar transport since Widdas formulated the carrier mediated transport model. However, many different values of Km and Vmax Were reported due to the different conditions of experiments. We found striking change in osmotic fragility of human red blood cells in the mixture of NaCl and saccharide solution. The sequence of hemolytic efficiency of saccharides is as follows; D-Xylose_??_L-Arabinose_??_D-Mannose_??_D-Galactose>D-Glucose>2-Deoxy-D-Glucose>[NaCl] _?? D-Ribose>D-Arabinose_??_L-Sorbose_??_L-Fucose>L-Xylose_??_ Xylitol>L-Glucose_??_D-Furctose_??_D-Sorbitol_??_D-Mannitol_??_D-Maltose_??_D-Sucrose. The effect is due to the sugar transport across the membrane, of which kinetics was shown to be a new type. The carrier is apparently a divalent carrier, or the glucose passes through the erythrocyte membrane as a dimer.
X-ray crystallographic studies show that the four domains of the H chain and two domains of the L chain in immunoglobulin G have very similar tertiary structures referred to as immunoglobulin fold and consist of two β-sheets. The states of tryptophyl, tyrosyl, hhistidyl residues, and disulfide bonds of Bence Jones proteins determined by physical and chemical methods are described. The VL domain is more resistant against denaturants than the CL domain. Studies on refolding of Bence Jones proteins from 4 M GuHCl shows that the VL domain refolds much faster than the CL domain. The noncovalent interaction of L chain to one of the two sites on H chain dimer decreases the affinity of the other site for L chain. The mechanism of formation of interchain disulfide bonds in Bence Jones proteins, Fab fragments, and immunoglobulin G is described.
Recent studies on chromatin structure have elucidated a subunit structure called nucleosome, which consists of 200 base pairs of DNA, two molecules each of the histones H2A, H2B, H3 and H4, and one molecule of histone H1. The nucleosome core contains 140 base pairs of DNA coiled on the surface of a histone octamer. Nucleosome cores have been crystallized and shown to be flat particles with dimensions of 110×110×57Å. Nucleosomes seem to persist in DNA replication and transcription. Judging from the fact that the DNA coding for active genes is preferentially digested by DNase I, the templateactive chromatin has an altered conformation of nucleosomes. Evidence is accumulating to suggest that the increase in DNase I susceptibility of active genes is attributable to histone acetylation or to interactions with nonhistone proteins. Poly ADP-ribosylation of nuclear proteins is also suggested to play a role in chromatin function.