Activation of wheat chloroplast sedoheptulose bisphosphatase: a continuous spectrophotometric assay

A new continuous spectrophotometric assay for sedoheptulose 1,7-bisphosphatase, applied to studies of the activation and steady-state kinetics of the wheat enzyme, is described. The assay enzyme sequence couples the formation of sedoheptulose 7-phosphate to the oxidation of NADH. The recycling of the reaction substrate enables measurements to be made at essentially constant substrate concentrations. Activation of wheat chloroplast sedoheptulose 1,7-bisphosphatase required a reducing agent and could be described by a first-order rate constant. The rate of activation was greatly increased in the presence of Mg²⁺ and sedoheptulose 1,7-bisphosphate. The K_m of the activated enzyme for sedoheptulose 1,7-bisphosphate. and its S_0.5 for Mg²⁺ were found to be 13.3 μm and 1.6 mm respectively. A high recovery method for purifying wheat chloroplast sedoheptulose 1,7-bisphosphatase is also detailed.     

Transferrin binding to K562 cell line : Effect of heme and sodium butyrate induction

Experiments demonstrating the existence of receptors for iron-saturated transferrin on K562 cells are described. Binding of ¹²⁵I-labelled transferrin is rapid, saturable and reversible, and can be specifically inhibited by unlabelled transferrin, but not by other proteins. The number of receptors determined by Scatchard analysis significantly decreased when K562 cells moved from the exponential to the quiescent phase of growth. Induction by hemin or sodium butyrate resulted in a marked reduction of transferrin binding. This phenomenon was due entirely to reduction in the number of receptors and was without effect on the affinity of interaction. The effect of butyrate and hemin on the number of transferrin receptors in other hematopoietic cell lines was investigated. Butyrate on the various cell lines was variable in its effect, whereas hemin constantly elicited a significant reduction in the number of transferrin receptors.     

Influence de la teneur en oxygène sur la sensibilité de certains stades juvéniles deSitophilus oryzae etS. granarius au dioxyde de carbone

Résumé Certains stades juvéniles de charançons des céréales du genreSitophilus ont été soumis à un séjour prolongé dans des compositions gazeuses modifiées par le dioxyde de carbone (CO₂).Les auteurs se sont placés dans des conditions de teneur en CO₂ constante (50%), associée à des teneurs en oxygène comprises entre 4 et 20%, pour étudier les conditions d'une synergie entre l'effet spécifique du CO₂ et la présence d'une forte teneur résiduelle en oxygène. Les effets ont été observés sur les stades juvéniles les plus évolués (larves et nymphes) des espècesS. oryzae (L.) etS. granarius (L.), (Coleoptera: Curculionidae).L'accroissement de la vitesse de mortalité chezS. oryzae est significative avec l'augmentation de la pression partielle d'oxygène. Cet effet de synergie provoqué par l'oxygène n'est pas significatif avecS. granarius, bien qu'il existe aussi avec cette espèce une tendance à l'amélioration de l'efficacité insecticide à court terme avec les mélanges à forte teneur résiduelle en oxygène.Il en est déduit des hypothèses sur les effets des mélanges gazeux à composition modifiée par le CO₂ au niveau de la physiologie générale de ces insectes, qui est très difficile à appréhender directement à cause du mode de développement des stades juvéniles des charançons (formes cachées dans le grain).     

Histological study of embryogenesis and organogenesis from anthers ofVitis rupestris du Lot cultured in vitro

Summary Anthers ofVitis rupestris du Lot were cultured in vitro at the uninucleate stage of the microspore, in order to investigate the histology of embryogenic and organogenic processes in this genotype. Microspores divided in the anther loculi resulting in the formation of globular structures with a ruptured exine. Somatic embryogenesis and, occasionally, caulogenesis and rhizogenesis occurred in calli produced from all anther tissues except the endothecium. The initial cell of the embryoid was surrounded by a jacket layer when situated deep within the callus. When the embryoid's initial cell was situated in the peripheral callus, a cutinized wall was present and the three-celled proembryoid was almost always segmented, showing the same embryonal type as the zygotic proembryo. Root differentiation and elongation and cap differentiation occurred during the growth phase in liquid medium. The mature root was diarch and contained cells with calcium-oxalate raphides, as seen in vivo. No starch or tannin deposition was ever observed in the mature embryoids.Abbreviations 6-BAP 6-benzylaminopurine - CC3 Nitsch and Nitsch (1969) basal medium - CS cross section - 2,4-D 2,4-dichlorophenoxy-acetic acid - LS longitudinal section     

Long-term effects of dexamethasone and nerve growth factor on adrenal medullary cells cultured from young adult rats

Summary Normal postnatal rat chromaffin cells and rat pheochromocytoma cells are known to show extensive Nerve Growth Factor (NGF)-induced process outgrowth in culture, and this outgrowth from the postnatal chromaffin cells is abolished by the corticosteroid dexamethasone. To determine whether adult rat chromaffin cells respond to NGF and dexamethasone, dissociated adrenal medullary cells from 3-month-old rats were cultured for 30 days in the presence or absence of these agents. Such cultures contained typical chromaffin cells, chromaffin cells with processes, and neurons. Fewer than 2 % of normal adult chromaffin cells formed processes under any of the conditions studied, and statistically significant changes in this proportion were not detectable in the presence of NGF or dexamethasone. Adrenal medullary neurons, however, were observed only in the presence of NGF, in cultures with or without dexamethasone, and thus appear to be previously unreported NGF targets which require NGF for survival or process outgrowth. Dexamethasone markedly increased total catecholamine content, total content of epinephrine, and tyrosine hydroxylase activity in cultures with or without NGF. In contrast, postnatal rat chromaffin and rat pheochromocytoma cells which have been studied in culture do not produce epinephrine under any of these conditions. It is concluded that rat adrenal chromaffin cells undergo age-related changes in both structural and functional plasticity. The in vitro characteristics of rat pheochromocytoma cells more closely resemble those of postnatal than of adult rat chromaffin cells, but may not entirely reflect the properties of the majority of chromaffin cells in either age group.     

Stimulation of the anaerobic growth ofSalmonella typhimurium by reduction ofl-carnitine,carnitine derivatives and structure-related trimethylammonium compounds

In view of the development of al-carnitine deficiency, the metabolism ofl-carnitine and structure-related trimethylammonium compounds was studied inSalmonella typhimurium LT₂ by means of thin-layer chromatography (TLC).l-Carnitine, crotonobetaine and acetyl-l-carnitine stimulated the anaerobic growth in a complex medium significantly. The stimulation depended on the formation of -butyrobetaine. The reduction ofl-carnitine proceeded in two steps: (1) Dehydration of thel-carnitine to crotonobetaine, (2) hydrogenation of crotonobetaine to -butyrobetaine. The reduction of crotonobetaine was responsible for the growth stimulation. Terminal electron acceptors of the anaerobic respiration such as nitrate and trimethylamine N-oxide, but not fumarate, suppressed the catabolism ofl-carnitine completely. Glucose fermentation, too, inhibited the reduction ofl-carnitine but optimal growth with a high carnitine catabolism was achieved byd-ribose. The esters of carnitine with medium- and long-chain fatty acids inhibited the growth considerably because of their detergent properties.Abbreviations TLC thin-layer chromatography     

Origines du peuplement mammalien en Afrique du Nord durant le miocène terminal

The late Miocene North African mammalianassemblage is considered here from three viewpoints: survivals, extinctions, and immigrations. The Eurasiatic affinities of the large mammals slightly prevail over the Ethiopian affinities. Amongst the North African large mammals, 4 to 8 taxa are Eurasiatic immigrants, while 4 to 6 are of Subsaharian origin. Contrarily, the micromammalian fauna is highly endemic, with only one species, a murid (Paraethomys miocaenicus), considered here as being related to an Asiatic form (Karnimata darwini). Our study of Eurasian and African Miocene faunas reveals that during the late Astaracian-early Turolian interval, the Saharo-Arabic belt permitted very little latitudinal faunal exchanges. However, during the middle and late Turolian such faunal exchanges became frequent. The micromammal record unequivocally indicates that a brief period of faunal exchange occurred between Africa and western Europe at the end of the Miocene, corresponding with the Messinian Salinity Crisis. The increased intercontinental faunal exchange between Africa and Eurasia during the late Miocene coincides with, and counterbalances the extinction of more than 10 taxa at the Mio/Pliocene boundary.     

Secondary structure in ribonuclease. II. Relations between folding kinetics and secondary structure elements

The kinetics of regain of 2′-CMP binding are monitored during renaturation of RNAase S. Experiments were performed by mixing equimolar amounts of S-peptide with S-protein. The S-protein fragment was incubated initially (i.e. before mixing with S-peptide) at pH 6.2 or 1.7 and various guanidine hydrochloride (GuHCl) concentrations. Three well-resolved phases are observed. The fastest phase is second-order. The reciprocal half-time increases linearly with fragment concentration and is independent of initial conditions for the S-protein fragment. An apparent on rate of k_on = 2 × 10⁵m^?1s^?1 is measured in 0.5 m-GuHCl (pH 6.2) and 20 ° C. Identical association kinetics are observed by changes in tyrosine absorbance. The fraction of native RNAase S formed in this second-order reaction strictly equals the fraction of S-protein molecules with intact β-sheet in initial conditions. The relation holds for different pH values, GuHCl concentrations and temperatures. The fraction of apparent helical content of S-protein in initial conditions may also vary but this is not reflected by the association reaction. We interpret this to mean that the β-sheet but not the α-helices must be preformed in initial conditions in order to generate the high-affinity peptide binding site of S-protein. Furthermore, it is concluded that the S-protein moiety β-sheet forms or unfolds in a single one-step reaction. 2′-CMP binding reports, additionally, two slower phases of renaturation. These are produced by S-protein molecules that have their β-sheet unfolded in initial conditions. It is observed that a unique dependence of these two folding rates exists for RNAase A, RNAase S and S-protein as function of t_m, the temperature of half-completion of thermal denaturation as measured by unfolding of the β-sheet in the respective compound in final conditions. The t_m value varies with changing pH, with GuHCl concentration and (for RNAase S) with changing fragment concentration. The findings are interpreted to argue in favor of a sequential mechanism of folding, where the stability of a structural precursor determines the rate of folding.