Conversion of progesterone-1,2-3H to 5β-pregnane-3,20-dione by brain tissue

The presence of a 5β-reductase acting to convert progesterone to 5β-pregnane-3,20-dione is described in the soluble 105,000 × $\text{g}$ fraction of a preparation of dog cerebral cortex. The function of this enzymatic activity is obscure but may be important in regulation of sensorium. 5β-pregnane compounds are potent depressors of the central nervous system.     

Fluorescence studies on the interactions of myelin basic protein in electrolyte solutions.

This paper examines the influence of electrolytes on fluorescence spectral properties of the single tryptophanyl residue, Trp-115, within the 18.5-kDa species of myelin basic protein from bovine brain. Steady-state fluorescence spectra and intensities and time-correlated fluorescence lifetimes increased in the presence of increasing concentrations of mono- and divalent electrolytes (Li+, Na+, K+, Mg2+, Ca2+, Cl-, ClO4-, SO4(2-), and PO4(3-)). In all cases, the increases closely paralleled the ionic strength of the bulk aqueous medium and resembled that observed upon immersion of the protein in solutions of urea. This behavior was therefore concluded to reflect changes in the solution conformation of myelin basic protein. Bimolecular quenching of Trp-115 by acrylamide was rapid (10(9) M-1 s-1), approaching the diffusion limitation, and markedly dependent on the viscosity of the bulk aqueous medium. Rotational depolarization of myelin basic protein was rapid (phi less than or equal to 1 ns), occurring at rates exceeding those predicted for a rigid particle of revolution, and markedly dependent on the viscosity of the surrounding medium. Whereas the bimolecular quenching constants were unaltered in the presence of electrolytes, rotational depolarization of myelin basic protein underwent substantial slowing as indicated by the appearance of an additional decay component characterized by a correlation time of 5-10 ns. These studies indicate that Trp-115 of myelin basic protein is readily accessible to the bulk aqueous medium and is associated with a highly mobile segment of the protein. The slowing of rotational depolarization upon immersion of myelin basic protein in electrolyte solutions is consistent with an electrolyte-induced self-association of myelin basic protein molecules and indicates a relationship between the lability of solution conformation on the one hand and the capacity for self-association on the other.     

Seasonal photosynthetic activity of autotrophic picoplankton in Lake Kinneret, Israel

Autotrophic picoplankton populations in Lake Kinneret are composedof picocyanobacteria and picoeukaryotes. Overall, the ratesof photosynthetic carbon fixed by autotrophic picoplankton duringthis study were low (0.01–1.5 mg Cm^–3 h^–1).The highest chlorophyll photosynthetic activity of the <3µm cell-size fraction was found in spring, when picoeukaryotespredominated and in addition small nanoplankton passed throughthe filters. The maximum cell-specific photosynthetic rate ofcarbon fixation by picocyanobacteria and picoeukaryotes was2.5 and 63 fg C cell^–1 h^–1, respectively. The highestspecific carbon fixation rate of autotrophic picoplankton was11 µg C µg^–1 Chl h^–1 The proportionalcontribution of autotrophic picoplankton to total photosynthesisusually increased with depth. Picocyanobacteria collected fromthe dark, anaerobic hypolimnion were viable and capable of activephotosynthesis when incubated at water depths within the euphoticzone. Maximum rates of photosynthesis (P_max) for picocyanobacteriaranged from 5.4 to 31.4 fg C cell^–1 h^–1 with thehighest values in hypolimnetic samples exposed to irradiance.Photosynthetic efficiency (     

Diffusion chamber studies of carbon flux from living algae to heterotrophic bacteria

The flux of recently photosynthetically fixed, dissolved organic carbon (PDOC) from phototrophs to heterotrophic microorganisms was measured directly in microplankton samples from Lake Kinneret using a diffusion chamber technique. The minimum amounts of PDOC incorporated into small <3 µm heterotrophs during a 12 h light period ranged from 0.8 to 38.4 µg (C) l^-1, or from 0.2% to 6% of net primary productivity. Although quantitatively small in comparison with total photosynthetic fixation, this PDOC flux could yield about 2 to 3 × 10⁶ bacterial cells.ml^-1 daily. This experimental is consistent with previous estimates of algal PDOC release in this lake.     

Kinetics of the inhibition of cotton seeds germination by lucerne saponins

The extent of inhibition of cotton-seed germination by lucernesaponins depends upon the period of pre-immersion in the saponinsolution prior to germination. After 5 hr of pre-immersion ina 0.5% lucerne saponin solution, a 40% drop in germination wasnoted. The respiration rate decreased after 3 hr of pre-immersion.The diffusion of oxygen through the membranes of seeds pre-immersedin saponins also decreased with an increasing pre-immersiontime. Inhibition of germination was irreversible after pre-immersionin saponins for 6 hr or more. The effect of saponins does not appear to be biochemically specificbecause the germination and respiration rates of the cottonseeds which were immersed in aqueous solutions of anionic, nonionicor cationic commercial surfactants were similarly inhibited. (Received February 19, 1975; )     

Property of a heat- and acid-stable inhibitor of serine proteinases from the blood serum to inhibit lymphcyte transformation stimulated by mitogens]

Thermo- and acid-stable serine proteases inhibitor from the rabbit blood serum (TASPI) was shown to inhibit the human peripheral blood lymphocytes transformation stimulated by phytohemagglutinin (PHA) or concanavalin A. The extent of inhibition depended on the concentration of the preparation and its specific activity. The maximal inhibition of lymphocytes proliferation constituted 50 to 70%. TASPI displayed no cytotoxic activity. Considerably more effective inhibition was demonstrated by TASPI addition to the culture medium 24 hours after the addition of PHA. The antiprotease activity of crude human serum and that inactivated under different conditions is described. The results obtained suggest the participation of TASPI in the control of biological activity of the lymphoid tissue cells.     

Peroxidases in the neural retina and pigment epithelium.

Peroxidase activity, assayed with 2 mM-H₂O₂ and suitable hydrogen donors (either p-phenyl-enediamine or diaminobenzidine), was demonstrated in homogenates of neural retina and pigment epithelium of both the dog and the cow. The enzyme is particle-associated in the native state, but is readily extractable by brief sonication or freeze-thawing. At optimum pH, which is between 4.0 and 4.5 for both sources, the specific activity is up to 40 times greater in pigment epithelial cells than in neural retina. Some catalase activity was detected in extracts from both bovine and canine neural retina, but catalase was essentially absent in pigment epithelium. Fractionation of bovine pigment epithelial cells showed that peroxidase activity is associated mainly with heavy organelles sedimenting at low centrifugal forces. Melanosomes, nuclei, melanolysosomes and plasma membranes were the principal organelles identified in these low speed sediments. It was not possible to separate them either by differential centrifugation or on discontinuous sucrose gradients. However, melanosomes were excluded as the only source of peroxidase activity by isolating separately the melanotic and amelanotic cell populations; equal peroxidase was found in both cell types. Since nuclei are not a likely source of this enzyme, it is suggested that most of the peroxidase activity in bovine pigment epithelial cells is localized in either the melanolysosomes, plasma membranes, or both.