Binding of chloride and a disulfonic stilbene transport inhibitor to red cell band 3

Summary The effect of chloride on 4,4-dibenzamido-2,2-disulfonic stilbene (DBDS) binding to band 3 in unsealed red cell ghost membranes was studied in buffer [NaCl (0 to 500mm) + Na citrate] at constant ionic strength (160 or 600mm). pH 7.4, 25°C. In the presence of chloride, DBDS binds to a single class of sites on band 3. At 160mm ionic strength, the dissociation constant of DBDS increases linearly with chloride concentration in the range [Cl]=450mm. The observed rate of DBDS binding to ghost membranes, as measured by fluorescence stopped-flow kinetic experiments, increases with chloride concentration at both 160 and 600mm ionic strength. The equilibrium and kinetic results have been incorporated into the following model of the DBDS-band 3 interaction: The equilibrium and rate constants of the model at 600mm ionic strength areK ₁=0.67±0.16 m,k ₂=1.6±0.7 sec^–1,k _–2=0.17±0.09 sec^–1,K ₁=6.3±1.7 m,k ₂=9±4 sec^–1 andk _–2=7±3 sec^–1. The apparent dissociation constants of chloride from band 3,K _Cl, are 40±4mm (160mm ionic strength) and 11±3mm (600mm ionic strength). Our results indicate that chloride and DBDS have distinct, interacting binding sites on band 3.     

Growth,fecundity, and egg size of least brook lamprey,Lampetra aepyptera

Synopsis Eight geographically-diverse (33°11N to 39°14N) populations of least brook lamprey varied significantly in density of larvae and mean length of metamorphosing and adult lampreys (101–152 mm). Absolute fecundity (503–5900 eggs) did not change significantly with stage of post-larval development, suggesting little post-metamorphic atresia; egg dry weight increased to stage 6, the penultimate stage of metamorphosis. Absolute fecundity increased with the cubic power of total length. Relative fecundity (250–1124 eggs per gram total body weight) decreased significantly with increasing length within two streams. Relative fecundity, adjusted to a common length, increased with mean length of the population. Conversely, mean egg dry weight (0.09–0.24 mg, stages 6–8) increased with maternal length within three streams, and decreased with length among streams. In populations where spawning occurs at a small body size, females produce comparatively fewer but larger eggs. Large eggs may be required to counter the effects of an unproductive environment and the accompanying reduction in relative fecundity is compensatory. No relationship was evident between latitude, annual thermal units, or density and relative fecundity or egg size. Mean length of a population was also unrelated to these environmental factors.     

Fish populations in the Paraná River: Lentic environments of Diamante and San Pedro Areas (Argentine Republic)

Data on the fish populations of lentic water bodies of the Lower Paraná River in the Diamante and San Pedro areas (33°40–31°55 S, 59°40'–60°40 W), sampled during 1971 are given. Limnological characteristics such as surface area and depth of ponds, water turbidity, and plant cover are included. Differences in fish population size, structure, specific diversity, dominance, and biomass of particular species are established between two areas in a dry season.     

Structural phase diagrams of animal communities in assessment of freshwater ecosystem conditions

A phase diagram method, using indices of biological diversity, was applied to describe the dynamics of zooplankton and fish community structure in natural and experimental conditions. The phase diagram is constructed in the coordinates H and dH × dt^–1, where H is the index of diversity of the community. The method reveals zones of steady state of a community and demonstrates community dynamics in undisturbed and disturbed conditions. Analysis of zooplankton dynamics in a fishless pond showed that in the absence of disturbances the community enters a zone of steady state (around dH × dt^–1=0) and stays in it for a long time. Under the action of disturbances (e.g. oxygen depletion for fish, influence of heavy metals for zooplankton) the community leaves the steady state zone. The greater the disturbance, the more the system deviates from steady state.     

Observation of a distinct transition in the mode of interconversion of ring pucker conformers in non-crystalline d-ribose-2'-d from 2H NMR spin-alignment

Internal motions of d-ribose selectively ²H-labeled at the 2 position were measured using solid state ²H NMR experiments. A sample of d-ribose-2 -d was prepared in a hydrated, non-crystalline state to eliminate effects of crystal-packing. Between temperatures of –74 and –60°C the C2–H2 bond was observed to undergo two kinds of motions which were similar to those of C2–H2/H2 found previously in crystalline deoxythymidine (Hiyama et al. (1989) J. Am. Chem. Soc., 111, 8609–8613): (1) Nanosecond motion of small angular displacement with an apparent activation energy of 3.6 ± 0.7 kcal mol^–1, and (2) millisecond to microsecond motion of large amplitude with an apparent activation energy 4 kcal mol^–1. At –74°C, the slow, large-amplitude motion was best characterized as a two-site jump with a correlation time on the millisecond time scale, whereas at –60°C it was diffusive on the microsecond time scale. The slow, large-amplitude motions of the C2–H2 bond are most likely from interconversions between C2-endo and C3-endo by way of the O4-endo conformation, whereas the fast, small-amplitude motions are probably librations of the C2–H2 bond within the C2-endo and C3-endo potential energy minima.     

Structure and variation of the Paraguay River phytoplankton in two periods of its hydrological cycle

The structure and distribution of the Paraguay River phytoplankton (between 16° 03 and 26° 53 S and between 57° 13 and 58° 23 W was studied during June–July/95 and December/95–January/96. Phytoplankton density values were lower in winter (mean value between 731 and 878 ind. ml^–1) in summer (between 1113 and 1876 ind. ml^–1), and mean biomass (biovolume) ranged between 0.215 and 0.372 mm³ l^–1 and between 0.586 and 1.223 mm³ respectively. In all, 332 algal taxa were identified: 298 in the upper section (Pantanal) and 143 in the middle and lower sections. They pertained to the Cyanophyta, Chlorophyta, Euglenophyta, Cryptophyta, Pyrrhophyta, Bacillariophyceae, Chrysophyceae and Xanthophyceae. Small Chlorophyta (Chloromonas gracilis, Choricystis minor, Crucigenia quadrata, Scenedesmus ecornis, Monoraphidium contortum and M. minutum) and Cryptophyta (Cryptomonas marssonii, C. ovata and Rhodomonas minuta) dominated. Bacillariophyceae (genera Aulacoseira and Cyclotella) increased in the lower section of the river. Biomass was dominated by centric forms of the genus Aulacoseira (A. granulata and its morphotypes, and A. herzogii). The diversity index varied between 1.99 and 5.0 bits ind.^–1 in winter and between 1.49 and 4.87 bits ind.^–1 in summer. Species richness (between 4 and 105 taxa per sample) showed a decrease in north–south direction. The presence of one of the largest wetlands of the world may explain the high algal diversity in the upper section. The Bermejo River, with highly mineralized waters and a high content of suspended solids, causes a strong discontinuity in the lower section of the main course, viz, a decrease in water transparency and in the density, biomass, diversity and species richness values of phytoplankton, before its waters meet the Paraná River.     

AZT toxicity and AIDS prophylaxis: Is AZT beneficial for HIV+ asymptomatic persons with 500 or more T4 cells per cubic millimeter?

Analyses of the effects of prophylactic use of zidovudine (AZT) on progression to acquired immune deficiency syndrome (AIDS) in human immunodeficiency virus seropositive (HIV+) asymptomatic persons with T4 lymphocyte (CD4+) cell counts 500 mm³ is reported for data obtained from two studies, the Australian European Group Collaborative Study, a multi-centered double-blind placebo-controlled clinical trial of the effects of AZT on progression to AIDS and other clinical endpoints, and the San Francisco Men's Health Study, an observational cohort. The analyses of the data of both studies demonstrate no benefit from AZT treatment in terms of progression to AIDS for those who are asymptomatic with CD4+ cell counts 500 mm³. The analysis of the San Francisco study, performed with Kaplan-Meier survivorship estimates, indicates a heterogeneity in the efficacy of AZT between baseline CD4+ cell count strata, 200–499 mm³ and 500–800 mm³. Within the 200–499 stratum, 47% of those receiving AZT therapy and 62% of those not receiving AZT therapy progressed to AIDS during the study period. By contrast, within the 500–800 stratum 41% of those receiving AZT therapy and 27% of those not receiving AZT therapy progressed to AIDS during the same period. Application of the Cox proportional hazards survivorship regression model for the relative risk of progression to AIDS to these same data accounts for this heterogeneity. The model includes an interaction between AZT treatment and baseline CD4+ cell counts. The hematological toxicity of AZT, demonstrated in clinical studies and laboratory investigations, indicates a biological correlate for this interaction: the toxic effects of AZT on the more intact immune system of those with CD4+ cell counts in the 500–800 mm³ range.Abbreviations ACTG AIDS Clinical Trials Group - AIDS Acquired immune deficiency syndrome - AMT 3-amino-3-deoxythymidine - ARC AIDS-related-complex - AZT 3-azido-3-deoxythymidine, zidovudine - BFU-E Erythrocyte burst-forming units - CD4+ T4 lymphocyte - CFU-GM Granulocyte macrophage colony-forming units - EAGCS European-Australian Group Collaborative Study - HIV Human immunodeficiency virus - SFMHS San Francisco Men's Health Study     

Interaction of bovine rhodopsin with calcium ions

The formation of metarhodopsin II in various bovine rhodopsin preparations (rod outer segment (ROS) suspensions and rhodopsin-detergent solutions) was measured by means of flash spectrophotometry. The half-lifetime and formation of metarhodopsin II in ROS did not depend on the calcium concentration in the range of less than 10^–9 M (using EGTA or EDTA) to 15×10^–3 M calcium at pH values of 5.0, 7.1, and 9.0 (Table 1).The regeneration of rhodopsin from opsin by adding 11-cis retinal to ROS-suspensions and rhodopsin digitonin solutions was measured spectrophotometrically. It was not substantially different in either saline, one containing less than 10^–7 M calcium (by adding EGTA), the other containing 10^–3 M calcium (Table 2).Abbreviations A absorption - A absorption change - CTAB N-Cetyl-N,N,N-trimethylammoniumbromide - E₇₀₀ extinction at =700 nm - EDTA ethylenediamine-NNNN-tetraacetic acid - EGTA 2,2-ethylenedioxybis [ethyliminodi (acetic acid)] - MI metarhodopsin I - MII metarhodopsin II - Rh rhodopsin - ROS rod outer segment This work is based upon a Ph. D. dissertation (Nöll, 1974) and was presented in part at the Jahrestagung der Deutschen Gesellschaft für Biophysik, Freiburg, Germany, October 1974     

The life cycle of Laminaria abyssalis (Laminariales,Phaeophyta) in culture

Laminaria abyssalis occurs in deep water in tropical latitudes of the Brazilian coast (19° 23 S, 38° 28 W to 22° 54 S, 42° 13 09 W). Its life cycle has been completed in the laboratory in seven months using different conditions of light and temperature. The gametophytic stage required for growth the low photon flux density of 1.2 ± 0.3 µmol m^–2 s^–1 and 18 °C, while the juvenile and adult sporophytes needed 15 µmol m^–2 s^–1 and 18 °C. The sporophytes became fertile at 23 °C. Our results showed that light and temperature are the main factors regulating the growth and life history of this species under the culture conditions tested.     

Annual primary productivity of an antarctic continental lake: Phytoplankton and benthic algal mat production strategies

Primary production in Watts Lake, Vestfold Hills, Antarctica (68°36S, 78°13E), was measured from March 1981 to February 1982. Phytoplankton production peaked in autumn and spring, with a September maximum (340 mgC m^–2 d^–1), then declined in summer and was not detectable in winter. Benthic algal production peaked in summer at 74 mgC m^–2 d^–1), Production strategies differed, with the more efficient phytoplankton adapted to growth at low light, while benthic production increased with increasing light in summer. Estimation of annual production was 10.1 gC m^–2 and 5.5 gC m^–2 for the phytoplankton and benthos respectively.     

The effects of temperature on feeding in zooplanktivorous walleye,Stizostedion vitreum,larvae

Synopsis We examined the prey capture process in walleye larvae (9.8–18.0 mm mean length) feeding on zooplankton (density 1501^–1) in laboratory aquaria at 15,18.5 and 22°C. Larvae were starved for 8 h prior to the experiment and only allowed to feed for 10 min during the experiment in order to minimize the influence of gut processing or satiation on feeding behaviour. Prey consumption (g min^–1) increased exponentially and prey capture success (%) increased logarithmically with mean walleye length. Prey consumption and attack rate (strikes min^–1) increased significantly with increasing temperature. The effect of temperature on capture success was slight (2% increase from 15 to 22°C) and not significant. Walleye showed positive electivity for medium-sized prey (0.3–0.6 mm body width, mostlyCeriodaphnia quadrangula), negative electivity for small prey (0.3 mm, mostly cyclopoid copepods) and neutral electivity for large prey (0.6 mm, mostlyDaphnia sp.). Neither prey species electivity nor prey size electivity were significantly affected by temperature. However, prey size electivity did show a fairly large effect size with respect to temperature and we suggest that this relationship should be examined further. These results indicate that temperature affects prey capture in walleye larvae primarily by influencing attack rate.     

δ andk opiate receptors in primary astroglial cultures part II: Receptor sets in cultures from various brain regions and interactions with ß-receptor activated cyclic AMP

In a previous paper, and opiate receptors were shown to be co-localized on the same cell in enriched primary cultures of astroglia from neonatal rat cerebral cortex. Activation of the receptors inhibited adenylate cyclase. In this work, the presence of opiate receptors was investigated in astroglial primary cultures from neonatal rat striatum and brain stem. Cyclic adenosine 3, 5-monophosphate accumulation was quantified in the presence of different opioid receptor ligands after stimulation of the cyclic adenosine 3,5-monophosphate system with forskolin. Morphine was used as a receptor agonist. [d-Ala², D-Leu⁵]-enkephalin or[d-Pen²,d-Pen⁵]-enkephalin were used as receptor agonists and dynorphin 1–13 or U-50,488H were used as receptor agonists. Specific antagonists for the respective receptors were used. After striatum or brain stem cultures had been incubated in 10^–9–10^–5M of each [d-Ala²,d-Leu⁵]-enkephalin, [d-Pen², D-Pen⁵]-enkephalin and Dynorphin 1–13 or U-50,488H, dose related inhibitions of the 10^–5M rorskolin stimulated cyclic adenosine 3,5-monophosphate accumulation were observed. The changes were reversed to the forskolin-induced control level in the presence of the respective antagonists. 10^–9–10^–5M morphine did not significantly change the forskolin-induced accumulation of cyclic adenosine 3,5-monophosphate in the cultures studied. Furthermore, cultures from cerebral cortex, striatum or brain stem were incubated with isoproterenol alone or together with morphine or [d-Ala²,d-Leu⁵]-enkephalin. Isoproterenol stimulated cyclic adenosine 3,5-monophosphate accumulation more prominently in the cerebral cortex and striatum cultures than in the brain stem cultures. Morphine did not influence isoproterenol-induced cyclic adenosine 3,5-monophosphate accumulation, while [d-Ala²,d-Leu⁵]-enkephalin inhibited the accumulation. The results indicate that astroglial cells in primary cultures from striatum, brain stem and cerebral cortex express andk opioid receptors linked to the adenylate cyclase/cyclic adenosine 3,5-monophosphate system. No receptors were detected, however, in the present model. Aspects of the relation between the expression of opioid peptides and opioid receptors are discussed, while speculations are also made on the functional aspects of opioid receptors on astroglia.     

Direct Development of the Visual System of the Coral Reef Teleost, The Spiny Damsel, Acanthochromis polyacanthus

Unlike most marine teleosts, the coral reef-dwelling spiny damsel, Acanthochromis polyacanthus, lacks a pelagic larva dispersal phase and represents one of few examples of self recruitment onto a natal reef by a marine teleost immediately after hatching. Benthic eggs are protected by the parents, and upon hatching the young remain under parental care for several months. Visual morphogenesis of spiny damsel embryos and juveniles was examined to evaluate the potential visual capabilities of the young after emergence onto the reef. The optic primordia were visible in the embryo as hollow spheres of undifferentiated neuroblasts 2 days after fertilization (daf). Visual morphogenesis proceeded rapidly thereafter in the embryo such that at hatching (between 10 and 12daf) gross visual morphology was consistent with that reported in the majority of juvenile marine teleosts, reflecting direct development of the retina of the spiny damsel within the egg. At hatching, the outer nuclear layer comprised 2 classes of photoreceptors; cones and rods. Tangential sections of the retina revealed a square cone mosaic in which 4 double cones surrounded a single cone. This arrangement remained unchanged in all later life history intervals examined. Absolute eye size was large compared to larvae of marine pelagic spawners. Eye and lens diameters increased from 0.69 and 0.23mm, respectively, on the day of hatching (12daf), to 3.77 and 1.52mm, respectively, in a fish 131daf. Angular density of cones increased from 0.25 cones 10 visual arc^–1 in an embryo 8daf, to 1.14 cones 10 visual arc^–1 in a fish 131daf, demonstrating the potential for significant increase in spatial resolution with increasing eye size. Convergence ratios of cones to ganglion cells remained relatively constant from the time of hatching, suggesting that the determinate ganglion cell photopic receptive field was established early in development. The increase in the convergence ratios of rods: ganglion cells from 1.4 in the late stages of embryogenesis (10daf; 2 days prior to hatching) to 4.9 in a fish 103daf, demonstrated increasing scotopic ganglion cell receptive field size, with increasing age. This was a result of rod cell addition with growth. An increase in the angular density of rods from 0.18 rods 10 visual arc^–1 in an embryo 8daf, to 4.07 rods 10 visual arc^–1 in a fish 131daf, and the increase in mean scotopic light path-length from 13.3±1.1m in an embryo 8dpf, to 55±5.2m a fish 22dpf, collectively indicate the potential for increasing scotopic sensitivity during growth. On the basis of visual morphology it is predicted that newly hatched spiny damsel juveniles have substantially greater visual capabilities than first feeding larvae with a pelagic dispersal phase. In addition, we propose that the developmental trajectory of the spiny damsel is different from that of pelagic dispersing larvae and does not simply reflect displacement along a common developmental continuum by an extended embryonic duration.     

Plankton and hydrochemistry of Lake Futalaufquen (Patagonia,Argentina) during the growing season

Plankton communities and hydrochemistry of an oligotrophic lake occupying a glacial valley in Argentinian Patagonia (42 °49S; 71 °43W) were studied. Monthly samples at three stations integrated from 0 to 50 m and stratified samples at the site of maximum depth, were taken during the growing season. Transparency was always controlled by glacial silt, and not by phytoplankton. Lake water belongs to the calcium-bicarbonate type, with low conductivity (24 µS cm^–1), and poor buffering capacity. Forty-five phytoplankton taxa were found. Mean phytoplankton density was 49 cells ml^–1 and mean biomass 69 µg l^–1. N:P relationships, inorganic nitrogen exhaustion in the photic layer, and correlations between nutrients and phytoplankton density suggests nitrogen as the main limiting factor. Fifteen zooplankton species were found. Mean zooplankton density was 12.2 ind. l^–1 and mean biomass 22.9 µg l^–1. Diatoms and Boeckellidae were the dominant planktonic groups. Morphometry and hydrological factors were responsible for horizontal heterogeneity in phytoplankton and chemical variables.     

Population structure and dynamics of a neotropical palm in fire-impacted fragments of the Brazilian Atlantic Forest

The demography of the acaulescent palm Attalea humilis was studied from 1996 to 1999 in three forest fragments (1.6, 6.4, and 9.9ha) in southeastern Brazil (22°30–22°33 S, 42°15–42°19 W). A human-induced fire damaged the populations in August–September 1997. Palm density was higher in the medium-sized fragment, where it was concentrated in a single disturbed sector. Population ontogenetic stage structure was dependent on fragment and year, but in general later stages were more abundant than initial ones. Population finite growth rates projected from linear, stage-structured demographic models indicated that the populations were increasing before fire. Population growth rate in the smallest fragment was larger due to a single recruitment event, being most sensitive to changes in survival and growth of seedlings, virgins and the immature, and relatively insensitive to changes in shrinkage and fecundity. Observed growth rates for the entire study period showed that populations remained in equilibrium after fire, but the population in the medium-sized fragment showed a declining tendency. Population structure was resilient to fire, which caused a transitory concentration of individuals in the medium-sized stage. The self-sustainability of the population in the largest, most preserved fragment suggests that habitat-protection measures may suffice to ensure the persistence of populations in small fragments. However, population fluctuations and exclusion from hyper-disturbed sectors in the smallest fragment suggest that reduced fragment area, habitat degradation and human-induced fires can interact to threaten the persistence of even abundant, preadapted to fire, species like A. humilis.     

Polychlorobiphenyl (PCB) metabolism by plant cells

Summary Suspension cultures of Paul's Scarlet rose provided with 0.3 ppm of 2,2, 4,4-tetrachlorobiphenyl metabolized the compound at a rate of 45 proles· n^–1·g dry wt^–1. The environmental and technological significance of plants to metabolize environmentally persistent compounds is discussed.     

Studies on the cation permeability of human red cell ghosts

Summary Net K movements in reconstituted human red cell ghosts and the resealing of ghosts to cations after osmotic hemolysis of red cells have been studied as functions of the free Ca ion concentration. The Ca-dependent specific increase in K permeability was shown to be mediated by a site close to the internal surface of the membrane with an apparent dissociation constant at pH 7.2 for Ca (K _D1) of 3–5×10^–7 m, for Sr of 7×10^–6 m. Ba and Mg did not increase the K-permeability of the membrane but inhibited the Ca-mediated permeability changes.K _D1 decreased in a nonlinear fashion when the pH was increased from 6.0 to 8.5. Two different pK values of this membrane site were found at pH 8.3 and 6.3. The Ca-activated net K efflux into a K-free medium was almost completely inhibited by an increase in intracellular Na from 4 to 70mm. Extracellular K antagonized this Na effect. Changes in the extracellular Na (0.1–140mm) or K(0.1–6mm) concentrations had little effect and did not changeK _D1. The Ca-stimulated recovery of a low cation permeability in ghost cells appeared to be mediated by a second membrane site which was accessible to divalent cations only during the process of hemolysis in media of low ionic strength. The apparent dissociation constant for Ca at this site (K _D2) varied between 6×10^–7 and 4×10^–6 m at pH 7.2. Mg, Sr, and Ba could replace Ca functionally. The selectivity sequence was Ca>Sr>Ba>Mg.K _D2 was independt on the pH value in the range between 6.0 and 8.0. Hill coefficients of 2 were observed for the interaction of Ca with both membrane sites suggesting that more than one Ca ion is bound per site. The Hill coefficients were affected neither by the ion composition nor by the pH values of the intra- and extracellular media. It is concluded that two different pathways for the permeation of cations across the membrane are controlled by membrane sites with high affinities for Ca: One specific for K, one unspecific with respect to cations. The K-specific channel has properties similar to the K channel in excitable tissues.     

Single chloride channels in endosomal vesicle preparations from rat kidney cortex

Summary Endocytotic vesicles from rat kidney cortex, isolated by differential centrifugation and enriched on a Percoll gradient, contain both an electrogenic H⁺ translocation system and a conductive chloride pathway. Using the dehydration/rehydration method, we fused vesicles of enriched endosomal vesicle preparations and thereby made them accessible to the patch-clamp technique. In the fused vesicles, we observed Cl^– channels with a single-channel conductance of 73±2 pS in symmetrical 140mm KCl solution (n=25). The current-voltage relationship was linear in the range of –60 to +80 mV, but channel kinetic properties dependended on the clamp potential. At positive potentials, two sublevels of conductance were discernible and the mean open time of the channel was 10–15 msec. At negative voltages, only one substate could be resolved and the mean open time decreased to 2–6 msec. Clamp voltages more negative than –50 mV caused reversible channel inactivation. The channel was selective for anions over cations. Ion substitution experiments revealed an anion permeability sequence of Cl^–=Br^–=I^–>SO ₄ ^2– F^–. Gluconate, methanesulfonate and cyclamate were impermeable. The anion channel blockers 4,4-diisothiocyanatostilbene-2,2-disulfonic acid (DIDS, 1.0mm) and 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB, 0.1mm) totally inhibited channel activity. Comparisons with data obtained from radiolabeled Cl^–-flux measurements and studies on the H⁺ pump activity in endocytotic vesicle suspensions suggest that the channel described here is involved in maintenance of electroneutrality during ATP-driven H⁺ uptake into the endosomes.     

Distribution of selected phospholipid modifying enzymes in rat brain microsomal subfractions prepared by density gradient zonal rotor centrifugation

A rat brain P₃ fraction enriched in ER derived microsomes was centrifuged through a 20–40% linear sucrose gradient in a Beckman Ti-14 Zonal rotor and 11 fractions were obtained. The distribution of marker enzyme activities and protein were determined in these 11 subfractions. NADPH-Cytochrome C reductase, choline phosphotransferase were employed for endoplasmic reticulum, Na⁺, K⁺-ATPase, 5-nucleotidase, and acetylcholinesterase were employed for plasma membrane, 2, 3-cyclic nucleotide phosphohydrolase was employed for myelin. The bulk of the protein was recovered in the 24–34% sucrose fractions, Na⁺, K⁺-ATPase, 5-nucleotidase, and acetylcholinesterase were in the 22–38% sucrose fractions while NADPH-cytochrome C reductase and CNPase were enriched in the 20–22% sucrose fractions. The ethanolamine and the serine base exchange activities had a bimodal distribution, with highest specific activities in sucrose fractions 32–34% and 20–24%. Choline base exchange activity was nearly undetectable in all the fractions. The specific activities of CDP-choline phosphotransferase, and phospholipid-N-methyltransferase were highest in the 20–22% sucrose fraction. Phospholipid-N-methyltransferase activity was significantly stimulated in the presence of exogenous phospholipid acceptors as phosphatidylethanolamine or phosphatidylmonomethylethanolamine or phosphatidyldimethylethanolamine, however, the greatest response was with phosphatidylmonomethylethanolamine. The rat brain P₃ fraction yielded a population of a membrane at the light end of the sucrose gradient which has a buoyant density similar to myelin but seemed to be enriched with NADPN cytochrome C reductase and phospholipid modifying enzymes. This is in contrast to liver microsomes submitted to a similar fractionation.