Neuroglobin is a recently discovered member of the globin superfamily that is suggested to enhance the O 2 supply of the vertebrate brain. Spectral measurements with human and mouse recombinant neuroglobin provide evidence for a hexacoordinated deoxy ferrous (Fe 2؉ ) form, indicating a His-Fe 2؉ -His binding scheme. O 2 or CO can displace the endogenous protein ligand, which is identified as the distal histidine by mutagenesis. The ferric (Fe 3؉ ) form of neuroglobin is also hexacoordinated with the protein ligand E7-His and does not exhibit pH dependence. Flash photolysis studies show a high recombination rate (k on ) and a slow dissociation rate (k off ) for both O 2 and CO, indicating a high intrinsic affinity for these ligands. However, because the ratelimiting step in ligand combination with the deoxy hexacoordinated form involves the dissociation of the protein ligand, O 2 and CO binding is suggested to be slow in vivo. Because of this competition, the observed O 2 affinity of recombinant human neuroglobin is average (1 torr at 37°C). Neuroglobin has a high autoxidation rate, resulting in an oxidation at 37°C by air within a few minutes. The oxidation/reduction potential of mouse neuroglobin (E o ‫؍‬ ؊129 mV) lies within the physiological range. Under natural conditions, recombinant mouse neuroglobin occurs as a monomer with disulfidedependent formation of dimers. The biochemical and kinetic characteristics are discussed in view of the possible functions of neuroglobin in the vertebrate brain.In addition to the well known hemoglobins (Hbs) 1 and myoglobins (Mbs), a third type of globin has recently been described in vertebrates that is predominantly expressed in the brain and other nerve tissues (1). These neuroglobins (NGBs) consist of single chains with 151 amino acids (M r ϭ ϳ17,000) that share only little sequence similarity with the vertebrate globins (Mb Ͻ 21%; Hb Ͻ 25%). Nevertheless, all key determinants of genuine globins are conserved (2). Although NGB was initially discovered in mouse and man, recent data show its presence in many different mammalian species as well as in fish, suggesting the universal occurrence of NGB in vertebrate brains. Nerve-specific globins have been sporadically observed in mollusc, annelid, arthropod, and nemertean species (3-5). These invertebrate nerve globins reach high local concentrations up to the millimolar range, which may be sufficient to facilitate O 2 diffusion or store O 2 that supports cell function during temporary hypoxia (5). The latter assumption is supported by the observation that the nervous function in the mollusc Tellina alternata under anoxic conditions depends on the oxygenation of a nerve globin (6, 7). However, the estimated amount of NGB in the vertebrate brain under nonpathological conditions is only in the micromolar range and thus is much lower than that of a typical invertebrate nerve globin (1). The physiological role of such lowly expressed globins is not well understood. Wittenberg (8) proposed that cytoplasmic globins at low concentrat...

Plant stress responses are a key factor in steering the development of cells, tissues, and organs. However, the stress-induced signal transduction cascades that control localized growth and cell size/differentiation are not well understood. It is reported here that oxidative stress, exerted by paraquat or alloxan, induced localized cell proliferation in intact seedlings, in isolated root segments, and at the single cell level. Analysis of the stress-induced mitotic activity revealed that oxidative stress enhances auxin-dependent growth cycle reactivation. Based on the similarities between responses at plant, tissue, or single cell level, it is hypothesized that a common mechanism of reactive oxygen species enhanced auxin-responsiveness underlies the stress-induced re-orientation of growth, and that stress-induced effects on the protoplast growth cycle are directly relevant in terms of understanding whole plant behaviour.

Glutathione is generally accepted as the principal electron donor for dehydroascorbate (DHA) reduction. Moreover, both glutathione and DHA affect cell cycle progression in plant cells. But other mechanisms for DHA reduction have been proposed. To investigate the connection between DHA and glutathione, we have evaluated cellular ascorbate and glutathione concentrations and their redox status after addition of dehydroascorbate to medium of tobacco (Nicotiana tabacum) L. cv Bright Yellow-2 (BY-2) cells. Addition of 1 mM DHA did not change the endogenous glutathione concentration. Total glutathione depletion of BY-2 cells was achieved after 24-h incubation with 1 mM of the glutathione biosynthesis inhibitor L-buthionine sulfoximine. Even in these cells devoid of glutathione, complete uptake and internal reduction of 1 mM DHA was observed within 6 h, although the initial reduction rate was slower. Addition of DHA to a synchronized BY-2 culture, or depleting its glutathione content, had a synergistic effect on cell cycle progression. Moreover, increased intracellular glutathione concentrations did not prevent exogenous DHA from inducing a cell cycle shift. It is therefore concluded that, together with a glutathione-driven DHA reduction, a glutathione-independent pathway for DHA reduction exists in vivo, and that both compounds act independently in growth control.