Characterization of HeLa 5'-nucleotidase: a stable plasma membrane marker.

5'-Nucleotidase, assayed as 5'-AMPase, has been extensively characterized and established as a stable, quantitative plasma membrane marker in HeLa S3 cells. The membrane 5'-AMPase has a Km of 7.0 microM. Relative affinities of the other 5'-mononucleotides for the enzyme are 5'-GMP > 5'-TMP > 5'-UMP > 5'-CMP. There are activity optima at pH7 and 10; the latter is Mg(2+)-dependent. The membrane preparations have a small amount of acid phosphatase activity that is distinct from 5'-AMPase activity but no alkaline phosphatase. AOPCP, ADP, and ATP are strongly inhibitory. Mg2+, Ca2+, or Co2+ additions do not affect the pH 7.0 activity; Mn2+ activates slightly, whereas Zn2+, Cu2+, and Ni2+ are inhibitory. EDTA slowly inactivates, but removal of the EDTA without the addition of divalent cations restores activity. The inactivation is also substantially reversed by Co2+ or Mn2+, but reactivability by divalent cations decreases with time in EDTA. ConA strongly inhibits, and alpha-methyl-D-mannoside or glucose (the latter much less efficiently) relieves the inhibition, indicating that the 5'-AMPase is a glycoprotein. Histidine is also inhibitory. Ouabain, phloretin, cytochalasin B, cysteine, phenyl-alanine, MalNEt, and IAA are without effect. 5'-AMPase activity codistributes with pulse-bound [3H]ouabain when either of two cell fractionation procedures are used. The 5'-AMPase activity per cell is constant at different cell densities in exponentially growing cells, and activity per unit cell volume remains constant throughout the cell cycle. These properties, together with its absence in other organelles, its stability to storage, its insensitivity to certain experimental manipulations, and its general insensitivity to inhibitors of specific transport systems, make 5'-AMPase a useful quantitative marker in studies on the regulation of HeLa membrane transport systems. Key Words: HeLa, 5'-nucleotidase, plasma membrane marker, non-specific phosphatases, divalent ions, ConA, AOPCP, cell cycle, mitochondria, transport inhibitors.     

Serine utilization as a precursor of phosphatidylserine and alkenyl-(plasmenyl)-, alkyl-, and acylethanolamine phosphoglycerides in cultured glioma cells

In several tissues and cell lines, serine utilized for phosphatidylserine (PS) synthesis is an eventual precursor of the base moiety of ethanolamine phosphoglycerides (PE). We investigated the biosynthesis and decarboxylation of PS in cultured C6 glioma cells, with particular attention to 1-O-alk-1'-enyl-2-acyl-sn-glycero-3-phosphoethanolamine (plasmenylethanolamine) biosynthesis. Incorporation of [3H]serine into PS reached a maximum within 4-8 h, and label in nonplasmenylethanolamine phosphoglyceride (NP-PE) and plasmenylethanolamine was maximal by 12-24 h and 48 h, respectively. After 8 h, label in PS decreased even though 40-60% of initial label remained in the culture medium. Serial additions of fresh [3H]serine restored PS synthesis to higher levels of labeled PS accumulation followed by a subsequent decrease in 4-8 h. High performance liquid chromatographic analyses confirmed that medium serine was depleted by 8 h, and thereafter metabolites, including acetate and formate, accounted for radioactivity in the medium. The rapid but transient appearance of labeled glycine and ATP inside the cells indicated conversion of serine by hydroxymethyltransferase. 78-85% of label from serine was in headgroup of PS or of PE formed by decarboxylation. A precursor-product relationship was suggested for label from [3H]serine appearing in the headgroup of diacyl, alkylacyl, and alkenylacyl subclasses of PE. By 48 h, a constant specific activity, ratio of approximately 1:1 was reached between plasmenylethanolamine and NP-PE, similar to the molar distribution of these lipids. In contrast, equilibrium was not achieved in cells incubated with [1,2-14C]ethanolamine; plasmenylethanolamine had 2-fold greater specific activity than labeled NP-PE by 72-96 h. These observations indicate that in cultured glioma cells 1) serine serves as a precursor of the head group of PS and of both plasmenyl and non-plasmenyl species of PE; 2) exchange of headgroup between NP-PE and plasmenylethanolamine may involve different donor pools of PE depending on whether the headgroup originates with exogenous serine or ethanolamine; 3) serine is rapidly converted to other metabolites, which limits exogenous serine as a direct phospholipid precursor.     

Inactivation kinetics and pharmacology distinguish two calcium currents in mouse pancreatic B-cells

Summary Voltage-dependent calcium currents were studied in cultured adult mouse pancreatic B-cells using the whole-cell voltage-clamp technique. When calcium currents were elicited with 10-sec depolarizing command pulses, the time course of inactivation was well fit by the sum of two exponentials. The more rapidlyinactivating component had a time constant of 75±5 msec at 0 mV and displayed both calcium influx- and voltage-dependent inactivation, while the more slowly-inanctivating component had a time constant of 2750±280 msec at 0 mV and inactivated primarily via voltage. The fast component was subject to greater steady-state inactivation at holding potentials between –100 and –40 mV and activated at a lower voltage threshold. This component was also significantly reduced by nimodipine (0.5 m) when a holding potential of –100 mV was used, whereas the slow component was unaffected. In contrast, the slow component was greatly increased by replacing external calcium with barium, while the fast component was unchanged. Cadmium (1–10 m) displayed a voltage-dependent block of calcium currents consistent with a greater effect on the high-threshold, more-slowly inactivating component. Taken together, the data suggest that cultured mouse B-cells, as with other insulin-secreting cells we have studied, possess at least two distinct calcium currents. The physiological significance of two calcium currents having distinct kinetic and steady-state inactivation characteristics for B-cell burst firing and insulin secretion is discussed.     

Crystallization and preliminary X-ray investigation of a sarcoplasmic calcium-binding protein from amphioxus

Crystals of a sarcoplasmic Ca(2+)-binding protein from the protochordate amphioxus have been grown from solutions of ammonium sulfate. The crystals are orthorhombic, space group C222(1), with unit cell axes a = 59.6(1) A, b = 81.3(1) A and c = 82.4(1) A. There is one molecule in the asymmetric unit. The crystals diffract beyond 2.5 A and show less than 20% decline in diffraction intensities after a three day exposure to X-rays from a laboratory rotating anode source.     

Use of nitrocellulose membrane to activate and measure insect prophenol oxidase

A method for the activation and measurement of insect prophenol oxidase using nitrocellulose membrane is presented. Using this method we were able to conveniently activate both crude and purified prophenol oxidase from insects belonging to three different orders. This rapid method allows for prophenol oxidase activation, in the absence of a prophenol oxidase-activating system, and in the presence of high ionic strength, protease inhibitors, or chelator.     

Segmentation and the development of the vertebrate nervous system

1. Recent experiments on the development of neural segmentation in chick embryos are reviewed. 2. Segmentation of the spinal peripheral nerves is governed by a subdivision of the somite-derived sclerotome into anterior and posterior halves. Migrating neural crest cells and outgrowing motor axons are confined to the anterior sclerotome as a result, in part, of inhibitory interactions with posterior sclerotome cells. 3. The sclerotomal distribution of certain molecules known to influence growing nerve cells in vitro, namely laminin, fibronectin, N-CAM, N-Cadherin and J1/tenascin/cytotactin, suggest that these molecules play no critical role in determining the preference of nerve cells for anterior sclerotome. 4. Peanut agglutinin (PNA) recognises cell surface-associated components on posterior cells which, when incorporated into liposomes, cause the abrupt collapse of sensory growth cones in vitro. The PNA receptor(s) may be inhibitory for nerve cells in vivo. 5. The chick hindbrain epithelium is segmented early in its development. Each branchiomotor nucleus in the series of cranial nerves V, VII and IX derives from a pair of segments lying in register with an adjacent branchial arch. Neurogenesis of motor and reticular axons begins in alternate segments, suggesting parallels with insect pattern formation.     

Actin organization and fibrin-clot retractile activity of cultured mouse fibroblasts

Summary It is known that human and animal fibroblasts are able to induce the retraction of a fibrin clot. In the present study the correlation between (i) fibrinclot retractile (FCR) activity, (ii) the number of actin stress-lines in mouse fibroblasts during growth in culture, and (iii) the sensitivity of actin stress-lines to a powerful actin-depolymerizing factor (ADF), present in plasma and serum of humans and laboratory animals was investigated. Fibroblasts at early passages (2–4) were tested for these parameters at various intervals after seeding (24, 96, and 168 h). The number of actin stress-lines was progressively higher, while the sensitivity to ADF action was progressively lower in cells cultured from 24 to 168 h; the FCR capacity was significantly decreased at 168 h. These data suggest that cells containing weakly polymerized and/or stabilized actin are more active than those containing highly polymerized and/or stabilized actin in triggering fibroblast contraction.