The kinetics of rabbit muscle pyruvate kinase. Initial-velocity, substrate- and product-inhibition and isotopic-exchange studies of the reverse reaction

1. An assay, based on the transfer of label from [gamma-32P]ATP to [32P]phosphoenolpyruvate, suitable for a steady-state kinetic analysis of pyruvate kinase in the reverse direction (i.e. phosphoenolpyruvate synthesis), is described. 2. This assay was used in a kinetic investigation of the rabbit muscle enzyme including initial-rate and product-inhibition experiments, at a pH of 7.4 and constant concentrations of total K+ and free Mg2+. 3. These studies indicate that there is a random release of ADP and phosphoenolpyruvate from the enzyme and that there is a competitive substrate inhibition by ATP. Some of the results were suggestive that the rapid-equilibrium assumption, generally used for this enzyme was not valid. 4. Techniques were developed to measure the rate of isotopic exchange between all the substrate-product pairs. 5. By using these techniques the rates of isotopic exchange at chemical equilibrium were measured. The results indicate that this enzyme does not catalyse a truly rapid-equilibrium random mechanism, although in the forward reaction all initial-rate data obtained to date are consistent with this assumption.     

Responses of complex cells in the visual cortex of the cat as a function of the length of moving slits

(1) As a step towards specifying the spatial selectivity characteristics of complex cells with spatially periodic substructures, we have studied single cell responses to narrow slits of variable length moved across the receptive field in the preferred direction. In general, the length-response curves were linear over a considerable and sometimes full range until an optimal slit length was reached. (2) In those cells in which the rate of rise of the slit length-response functions decreased before the optimal length was reached, at least 3 factors contribute to the shape of the curve. First, the receptive field shapes of some complex cells are more ovoid or rounded than rectangular, and the summation of responses from excitatory zones of varying optimal lengths itself results in a nonlinear slit length-response function at long slit lengths. Second, central regions may contribute more to cell response than do more lateral regions along the length dimension. Third, a nonlinearity in the slit length-response curve may occur in the upper range of slit lengths as a saturation effect because discharge rates may reach 600/sec, which appears to be close to a limiting firing rate. (3) Some cells believed to be complex during preliminary receptive field testing showed weak inhibitory regions beyond the region of the optimal slit length. Many of these cells also displayed periodic average response histograms to moving slits. The extent and magnitude of the inhibition were variable from cell to cell. In terms of receptive field properties, these cells and 'regular' complex cells seem part of a continuum.     

New species of Actinomadura coeruleoviolacea sp. nov. and its antagonistic properties

Eight strains of actinomycetes isolated from soil samples were described as representatives of Actinomadura coeruleoviolacea sp.nov. The species was characterized by straight and sometimes branching short chains of spores, smooth spores, blue aerial mycellium and violet soluble pigment. All the strains showed antibacterial activity when cultivated in liquid media. The pigment differed from all known pigments. It is probably a new natural compound.     

Histones associated with non-nucleosomal rat ribosomal genes are acetylated while those bound to nucleosome-organized gene copies are not

Acetylation of histones bound to rat rRNA genes has been studied relative to their organization in chromatin, either as canonical nucleosomes, containing the inactive copies, or as anucleosomal nonrepeating structures, corresponding to the transcribed genes (Conconi, A., Widmer, R. M., Koller, T., and Sogo, J. M. (1989) Cell 57, 753-761). Nuclei from butyrate-treated rat tumor cells were irradiated with a UV laser to cross-link proteins to DNA, and the purified covalent complexes were immunofractionated by an antibody that specifically recognized the acetylated histones. Upon probing with sequences coding for mature rat 28 S RNA, DNA of the antibody-bound complexes was 5-20-fold enriched relative to the total rat DNA. Since the laser cross-links histones to DNA in both active and inactive genes, one cannot distinguish which one of them, or both, are bound to acetylated histones. Alternatively, purified mononucleosomes were immunofractionated, but DNA from the antibody-bound monosomes was not enriched in coding rDNA. Taken together, these results suggest that nucleosome-organized rRNA genes are bound to nonmodified histones and that the acetylated histones are associated with the active, anucleosomal gene copies.     

Examples of recapitulation of the phylogenetic stages of development of the wakefulness-sleep cycle in mammalian ontogenesis

The purpose of this study was to determine the relationship between the ulnar nerve and the cubital tunnel during flexion of the elbow with use of magnetic resonance imaging and measurements of intraneural and extraneural interstitial pressure. Twenty specimens from human cadavera were studied with the elbow in positions of incremental flexion. With use of magnetic resonance imaging, cross-sectional images were made at each of three anatomical regions of the cubital tunnel: the medial epicondyle, deep to the cubital tunnel aponeurosis, and deep to the flexor carpi ulnaris muscle. The cross-sectional areas of the cubital tunnel and the ulnar nerve were calculated and compared for different positions of elbow flexion. Interstitial pressures were measured with use of ultrasonographic imaging to allow a minimally invasive method of placement of the pressure catheter, both within the cubital tunnel and four centimeters proximal to it, at 10-degree increments from 0 to 130 degrees of elbow flexion. As the elbow was moved from full extension to 135 degrees of flexion, the mean cross-sectional area of the three regions of the cubital tunnel decreased by 30, 39, and 41 per cent and the mean area of the ulnar nerve decreased by 33, 50, and 34 per cent. These changes were significant in all three regions of the cubital tunnel (p < 0.05). The greatest changes occurred in the region beneath the aponeurosis of the cubital tunnel with the elbow at 135 degrees of flexion. The mean intraneural pressure within the cubital tunnel was significantly higher than the mean extraneural pressure when the elbow was flexed 90, 100, 110, and 130 degrees (p < 0.05). With the elbow flexed 130 degrees, the mean intraneural pressure was 45 per cent higher than the mean extraneural pressure (p < 0.001). Similarly, with the elbow flexed 120 degrees or more, the mean intraneural pressure four centimeters proximal to the cubital tunnel was significantly higher than the mean extraneural pressure (p < 0.01). Relative to their lowest values, intraneural pressure increased at smaller angles of flexion than did extraneural pressure, both within the cubital tunnel and proximal to it. With the numbers available, we could not detect any significant difference in intraneural pressure measured, either at the level of the cubital tunnel or four centimeters proximal to it, after release of the aponeurotic roof of the cubital tunnel.