Recent technological development has yielded a variety means of medical informations. Among these informations, imaging of in vivo phenomena within the body is the most appealing and comprehensive to a clinician, and, hence, impose rather difficult task on computer processing. However, the nuclear imaging, in essence, consists of digital data, so it is quite adaptable to be processed by the computer. Since nuclear data is a series of rapid time sequence imaging, a bulk of memory capacity with high speed processing time should be requird in order to construct such type of presentation as a functional imaging, electrocardiogram-gated radionuclide angiocardiogram and etc. In addition, recent trend to reconstruct 3 D image, further capacity should be necessary in the near future. Although computer application has not yet fully affected the present diagnostic process, it might potentiate much information efficacy of the nuclear imaging as has been evidenced in the field of the cardiovascular nuclear medicine.
The structure of the purple membrane is reviewed briefly. The purple membrane is the differentiated patch in the cell membrane of halobacteria which contains only one polypeptide, bacteriorhodopsin with a molecular weight of 26, 000. X-ray diffraction patterns demonstrated the presence of a two dimensional hexagonal lattice of the space group P3 in the purple membrane. The X-ray diffraction also indicates a high content of α-helix in the protein and the bilayer arrangement for the lipids. The three-dimensional structure of bacteriorhodopsin has been obtained at a resolution of 7Å from electron diffraction patterns and electron micrographs. One molecule conatins seven α-helical segments which pack neatly side by side and span the hydrophobic interior of the membrane. It is also described that reconstituted bacteriorhodopsin form the hexagonal lattice spontaneously in the isolated membrane patch whereas bacterioopsin do not. Recently, the amino acid sequence of bacteriorhodopsin has been determined. The retinal is bound as a protonated Schiff base to the ∈-amino group of a lysine residue.
Light-induced reaction of bacteriorhodopsin (bR) from Halobacterium halobium was sutdied by low temperature spectroscopy and by the analysis on retinal isomer composition. The choromophor of light-adapted form of bR (bRL) consists of all-trans retinal alone, while dark-adapted form of bR (bRD) contains about equal molar amounts of both 13-cis and all-trans retinals. Upon receiving light at -190°C, bRL forms a single species of red-shifted intermediate, batho-trans-bR. 13-Cis-bR in bRD behaves differently, forming another kind of the product, batho-13-cis-bR. Low temperature spectroscopic analyses show that batho-trans-bR first changes to lumi-intermediate in the dark. A part of the lumi-intermediate then changes directly to bRL and the remainder changes to bRL through meta-intermediate, the formation of which is an obligatory step for bR expressing its proton pump activity. Batho-13-cis-bR changes to bRL without forming a meta-type intermediate. Spectral analysis at 9K does not reveal the presence of a hypso-type intermediate or a direct precursor of batho-intermediate. Quantum efficiency for the formation of batho-trans-bR is practically independent upon a wide range of temperatures starting from 9K, while some temperature-dependence is noticed in the case of an artificial pigment synthesized from retinal2, indicating that the analog retinal can not be accommodated well within its binding site in bR. Monomerization of bR by Triton X-100 induces some decreases in its light-sensitivity and lead to the production of light-insensitive pigment at -190°C. A mechanism for proton transport process was discussed in view of an idea which emphasizes the participation of charged groups in bR. In this context, our recent finding on the light-induced formation of 9-cis and 11-cis retinal pigments at acidic pH's may be useful for the analysis on the mechanism of light-driven proton pump in bR.
The effects of ionizing radiation on biological membranes have been examined by various investigators from different points of view: changes in membrane permeability, cell surface charge, receptor functions and enzyme release, and inactivation. These studies were briefly reviewed. It was also described that radiation can induce significant changes in membrane structure. The nature of radiation damage to membranes might be radiation-induced oxidation of membrane-bound SH groups. In summary, it is clear that biomembranes are highly radiosensitive and that membrane damage may play an important role in cellular radiosensitivity. It is also discussed that the structural integrity of cell membranes may be essential for the repair of DNA in E. coli.
Macrolide antibiotics are widely known as the ionophoric substances which can form 1:1 complexes selectively with K+ ion and carry it through artificial and biological membranes. Molecular structures in crystals and molecular conformations in solutions are described for valinomycin and nactins, both in free and complexed states. Valinomycin molecule (oval shaped) possesses an approximate center of symmetry in the crystal, and the structure is stabilized by six intramolecular H-bonds; the four of those are 4→1 type H-bond and the rests are 5→1 type. In non-polar solvents, valinomycin possesses C3 symmetry, in which six 4→1 type intramolecular H-bonds are observed (A2 form). The structure A2 is also observed in the K+-valinomycin complex, in which six ester carbonyl oxygen atoms are directed to K+ ion forming an octahedron of coordination. In the crystals of nactins, on the other hand, the molecular shape of dinactin (cradle; C1 symmetry) is markedly different from those of nonactin (torus; C2 and an approximate S4 symmetry) and tetranactin (propeller; C2 symmetry), but the molecular conformation of dinactin can be regarded as a hybrid of those of the subunits in K+-nonactin complex and tetranactin. Intramolecular van der Waals contacts are observed in dinactin and tetranactin. In solutions, nactins are very flexible, and they have the molecular symmetry of S4. In the K+-nactin complexes, four carbonyl oxygen atoms and four ether oxygen atoms are directed to K+ ion to establish a cubic coordination.