Dehydroquinate synthase: the role of divalent metal cations and of nicotinamide adenine dinucleotide in catalysis

The cofactor requirements of dehydroquinate synthase from Escherichia coli have been characterized. The homogeneous enzyme, purified from the overproducing strain RB791 (pJB14), is a monomeric metalloenzyme of Mr = 39,000 that contains 1 mol of tightly bound Co(II) according to atomic absorption analysis. The holoenzyme rapidly loses activity upon incubation with EDTA, giving rise to a stable but catalytically inactive apoenzyme. Activity is fully restored by reconstitution with Co(II) and partially restored with other divalent cations. Reconstitution of the apoenzyme with Zn(II) (which is probably the functioning metal in vivo) restores activity to 53% of the level observed with the Co(II)-holoenzyme. The presence of the substrate 3-deoxy-D-arabino-heptulosonate 7-phosphate (1) blocks the inactivation by EDTA. Dehydroquinate synthase also binds 1 mol of NAD+, the presence of which is essential for catalytic activity. The rate constant for the dissociation of NAD+ from the Co(II)-holoenzyme was found to be 0.024 min-1. Under turnover conditions with saturating levels of substrate, the dissociation rate of NAD+ increases by a factor of 40, to 1 min-1. Under these conditions (pH 7.5, 20 degrees C), the Km for NAD+ was determined to be 80 nM.     

Molecular cloning of covalently closed circular DNA of bovine leukemia virus

The two species of covalently closed circular DNA molecules of bovine leukemia virus were cloned in the lambda phage vector lambda gtWES X lambda B. Of the nine independent recombinant lambda-bovine leukemia virus clones that were analyzed, three were derived from the small and six were derived from the large circular molecules carrying, respectively, one and two copies of the long terminal repeat sequences. Comprehensive restriction endonuclease mapping of the unintegrated bovine leukemia virus and the cloned DNA molecules showed that eight of the nine clones carried viral information without any detectable deletions or insertions of more than ca. 50 base pairs. One of the nine clones, which carries a retroviral insert with one copy of the long terminal repeat, had a deletion of ca. 150 base pairs.     

The mechanism of inhibition of S-adenosyl-L-homocysteine hydrolase by fluorine-containing adenosine analogs

(Z)-4',5'-Didehydro-5'-deoxy-5'-fluoroadenosine (I), 5'-deoxy-5'-difluoroadenosine (II), and 4',5'-didehydro-5'-deoxy-5'-fluoroarabinosyl-adenosine (III) are inhibitors of rat liver S-adenosyl-L-homocysteine hydrolase. Compounds I and II are time-dependent and irreversible inhibitors of the enzyme. Both I and II are oxidized by E.NAD to produce E.NADH, and fluoride anion is formed in the inactivation reaction (0.7 to 1.0 mole fluoride/mole of enzyme subunit, and 1.7 moles fluoride/mole of enzyme subunit from I and II, respectively). The enzyme is stoichiometrically labeled with [8-3H]-I, but the label is lost upon denaturation of the protein either with or without treatment of the labeled complex with sodium borohydride. The compound III, the arabino derivative of I, is a competitive inhibitor of the enzyme. The mechanism of the inhibition of S-adenosyl-L-homocysteine hydrolase by these inhibitors is discussed.     

Inventory issues in incremental FMS implementation

In recent years, numerous studies have demonstrated convincingly that impressive benefits can be obtained by the adoption of flexible manufacturing systems (FMSs). To obtain the benefits of an FMS requires the development of a completely integrated system. However, FMS implementations are frequently done incrementally through the introduction of subsystems such as flexible machining centers into an existing conventional system. The purpose of this research is to investigate some of the operational issues associated with the introduction of a CNC (computer numerically controlled) machine tool into a conventional system. The primary objective of the present study is to explore the relative effects on inventory holding cost of installing a single CNC at different locations within three different system configurations. Additionally, the study examines the sensitivity of these impacts to changes in (1) System utilization; (2) the ratios of setup times to run times in the conventional work centers; and (3) the rates of increase in holding costs for parts as they move through the system. Results indicated that, in general, introduction of a CNC into an otherwise conventional system reduces inventory holding cost for the system as a whole. However, the degree of this reduction varies depending on the position of the CNC in the system. In some cases the reduction in inventory holding cost is substantial, while in other cases it is relatively small.     

Cellular and subcellular distribution of iron in the lamina propria of rat duodenum

Summary Cellular and subcellular distribution of iron in the lamina propria of rat duodenum was studied after a single i.p. injection of iron dextran, using electron microscopy and peroxidase cytochemistry. X-ray spectrum microanalysis was used for positive identification of iron. Ironcontaining particles (IP) were found in the cytoplasm of three cell types, viz. macrophages, pericytic reticular cells and sheathing fibrocytes. IP-containing organelles in lamina propria cells were more heterogeneous compared to absorptive cells and, in addition, some differences were noted in the subcellular distribution of IP in the 3 cell types. A common denominator in these 3 cell types was the presence of endogenous peroxidase, also shared by Kupffer cells which are known to be involved in iron storage. Peroxidase activity was absent in absorptive epithelial cells. It is hypothesized that the cells of the lamina propria, like Kupffer cells, may be the site of storage of excess iron absorbed, releasing iron upon demand and migrating into the lumen to prevent iron overload. In this fashion they may regulate the exchange of iron with the environment. The presence of peroxidase in these as well as Kupffer cells, and its absence in absorptive cells also raises the possibility that this enzyme may be related to certain aspects of iron storing process.     

Studies on bone marrow histogenesis: Morphometric and autoradiographic studies of regenerating marrow stroma in extramedullary autoimplants and after evacuation of marrow cavity

Summary The marrow cavity of the rat tibia was mechanically evacuated and autoimplanted to the subcutaneous tissue. The regenerative process which restored the integrity of marrow stroma and hemopoiesis, was morphometrically evaluated in whole mount of tibia. Following evacuation, the clot filled the cavity. The granulation tissue then appeared and expanded, penetrating and replacing the clot. The fibroblasts of the granulation tissue differentiated into osteoblasts forming osteoid bone. Within its interstices, the primordial marrow consisting of loose connective tissue and vascular sinuses appeared and hemopoiesis resumed. Expansion of hemopoiesis resulted in the resorption of bone and within three weeks the tibial cavity was restored to the pre-evacuation state.Autoradiography indicated that the labeling index was initially high in fibroblasts and osteoblasts but was subsequently reduced while it increased in osteocytes, cells of Haversian canals, stromal and hemopoietic cells of marrow. The finding is in disagreement with the view that the regenerative process originates from the Haversian canal. When the label was introduced on day 4 post-operatively, it subsequently appeared in osteocytes, cells of Haversian canal, stromal elements of the marrow, but not in the hemopoietic cells. This indicates complete dissociation of marrow stroma and hemopoietic stem cell.Supported by NASA Contract NSG 9061. Mehdi Tavassoli is the recipient of a CRD Award AM-70551     

Microbial transformation of sterols

Summary Arthrobacter simplex, Serratia marcescens, Fusarium and Mycobacterium were tested for their ability to transform phytosterol to Androsta 1, 4 diene 3, 17 dione (ADD). Arthrobacter simplex ATCC 6946 was found to be more efficient than the other species tested.     

Effect of high fluoride intake on haematological aspects of the mouse.

The effect of feeding adult Swiss albino mice of both sexes a diet supplemented with 0, 125, 250 and 500 parts/10(6) of fluoride for four and eight week periods on haemoglobin concentration (Hb), packed cell volume (PCV) and mean corpuscular haemoglobin concentration (MCHC) was investigated. Values of the three parameters were significantly lowered at both periods in the treated groups as compared with the controls. The extent of reduction in these values was, in general, dependent on the dose of supplemented dietary fluoride. Clinical symptoms were not observed before the end of the sixth week. However, appearance of the symptoms did not change the trend of variations in Hb, PCV and MCHC values. The reduced values could be the result of lowered haemoglobin synthesis and erythropoiesis. It was suggested that these haematological indices could serve to detect preclinical effects of high fluoride intake with an added dose of as low as 125 parts/10(6), or even less, for a period of four weeks or probably earlier.     

Biphasic uptake of iron-transferrin complex by L1210 murine leukemia cells and rat reticulocytes

The kinetics of the cellular uptake of iron-transferrin complex was studied in L1210 murine leukemia cells and rat reticulocytes using ¹²⁵I-transferrin. Saturation of transferrin with iron was necessary for optimal uptake. Following the incubation of cells with the radiolabeled complex a biphasic pattern of uptake was observed. The initial phase was rapid and relatively temperature-independent and was not altered by ethylamine, an inhibitor of transglutaminase activity which is necessary for receptor-mediated endocytosis. This phase was considered to result from receptor-ligand interaction which could be reversed to a great degree by replacement with unlabeled transferrin. A plateau was then reached, indicating a saturation of receptors. After 30 min a second phase of uptake was indicated by the second rise in the curve. This phase was slow, relatively temperature-dependent and could be abolished by ethylamine. It was interpreted as evidence of internalization of the ligand. Analysis of the data from competition studies with unlabeled transferrin indicated that the first phase might itself comprise a reversible and an irreversible step with a ratio of 5 to 1.4 for bound transferrin. Thus, the cellular uptake of iron-transferrin complex may consist of a reversible ligand-receptor interaction. Conformational changes may render this interaction irreversible and the internalization of the ligand may then follow.