Pixel-based reconstruction (PBR) promising simultaneous techniques for CT reconstructions.

Algorithms belonging to the class of pixel-based reconstruction (PBR) algorithms, which are similar to simultaneous iterative reconstruction techniques (SIRTs) for reconstruction of objects from their fan beam projections in X-ray transmission tomography, are discussed. The general logic of these algorithms is discussed. Simulation studies indicate that, contrary to previous results with parallel beam projections, the iterative algebraic algorithms do not diverge when a more logical technique of obtaining the pseudoprojections is used.

Replitase: a complex integrating dNTP synthesis and DNA replication.

Replitase is a multienzyme complex of mammalian cells that produces deoxynucleoside triphosphates and delivers them to the DNA polymerase activity, which also resides in the complex. Structural interactions within this complex form the basis of internal controls to keep these key biosynthetic processes efficient and in balance. The active complex is formed in the nuclear region in only the S phase of the cell cycle, when the cell's DNA is being replicated.

Light-induced proton permeability changes in retinal rod photoreceptor disk membranes.

We have used the membrane-permeant charged fluorescent dye, 3,3'-dipropylthiadicarbocyanine iodide (diS-C3[5]), to monitor electrical potentials across the membranes of isolated bovine disks. Calibration curves obtained from experiments where a potential was created across the disk membrane by a potassium concentration gradient and valinomycin showed an approximately linear relation between dye fluorescence and calculated membrane potential from 0 to -120 mV. Light exposure in the presence of the permeant buffer, imidazole, caused a rapid decay of the membrane potential to a new stable level.

Allosteric interaction of components of the replitase complex is responsible for enzyme cross-inhibition.

The enzymes of DNA polymerization and DNA precursor synthesis are assembled in the replitase complex during the S phase of the cell cycle. Cross-inhibition is a phenomenon shown by enzymes of the replitase complex, in which inhibition of one enzyme of the complex leads to inhibition of a second, unrelated enzyme. This inhibition occurs only in vivo and only during S phase.

Extended sensitivity for the calcium selective electrode.

Sensitivity of calcium-selective electrodes heretofore has been limited to calcium concentrations above 10(-8) M in the absence of competing ions. We describe the use of calcium buffers to stabilize the free calcium in the reference electrode. Electrode calibration is linear to 10(-8) M and is curvilinear to 10(-11) M in the presence of 0.

Simplified concanavalin-A sepharose adsorption method for separation of cone visual pigments from rhodopsin.

Batch adsorption of chicken photoreceptor extract using Concanavalin-A Sepharose enables separation of rod and cone pigments and separation of cone pigments of different color sensitivity. In earlier work, column separations using the same adsorption medium, although effective, with high resolution, were slow, demanding and required many differential bleachings of column fractions for analysis. It is shown here that affinity separations can be performed in the batch adsorption mode to purify cattle, frog and chicken rhodopsin.

Chromatographic resolution of the rod pigment from the four cone pigments of the chicken retina.

Chicken rod and cone pigments are chromatographically resolved using a combination of stepwise elution from a concanavalin A affinity column and a secondary ion exchange purification. The rod pigment rhodopsin and the cone pigment iodopsin are purified to homogeneity. There is a partial separation of three other cone pigments: chicken green, chicken blue, and chicken violet, making clear that this is at least a tetrachromatic cone system.

Localization and properties of bovine photoreceptor 5'-nucleotidase.

The main light-activated enzyme of the vertebrate photoreceptor is cGMP phosphodiesterase, whose product is GMP. GMP would be broken down to guanosine by the enzyme 5'-nucleotidase on the cytoplasmic (extradiscal) surface of the disks. The presence of 5'-nucleotidase on the cytoplasmic surface was verified by using sucrose continuous gradients to show its association with the photoreceptors and by using disk preparation and concanavalin A binding to demonstrate its presence on the extradiscal surface.

A large-scale purification of phosphatidylethanolamine, lysophosphatidylethanolamine, and phosphatidylethanolamine, and phosphatidylcholine by high performance liquid chromatography: a partial resolution of molecular species.

Egg yolk phospholipids, on a 10 g scale, were resolved by high-performance liquid chromatography on an 8-m silica column with elution by a stepwise chloroformmethanol gradient into homogenous phosphatidylcholine, phosphatidylinositol, phosphatidylethanolamine, and lysophosphatidylethanolamine fractions. Within these fractions, partial resolution on the basis of fatty acyl side chain composition was achieved.