Biochemical and proton NMR characterization of the isolated functional beta-subunit of coupling factor one from spinach chloroplasts

Beta subunits have been dissociated from CF1 of spinach chloroplasts, purified by HPLC and characterized by two-dimensional electrophoresis and fluorescence emission. The solutions of isolated beta subunits are able to hydrolyze MgATP; this ATPase activity is an intrinsic property of the beta molecule. From proton NMR at 300 and 500 MHz, it is shown that the preparations are fully reproducible and that beta subunits remain monomeric with 75% aliphatic protons associated with rigid parts of the molecule. The other 25% give rise to separate resonances and belong to mobile side-chains and/or to flexible regions. The measurement of the transverse relaxation times T2 has permitted a detailed characterization of the molecular dynamics of the isolated beta subunits.     

Reactivity of HDL subfractions towards lecithin-cholesterol acyltransferase. Modulation by their content in free cholesterol

(1) Human HDL2 (d 1.070-1.125) and HDL3 (d 1.125-1.21) labelled with unesterified [14C]cholesterol, were incubated with a source of lecithin-cholesterol acyltransferase. For optimal activity, the reaction required the addition of albumin in excess, at least 3-times greater than the concentration of HDL-free cholesterol. Under such conditions, the reaction appeared saturable. HDL3 was found the most efficient substrate and the Vmax values expressed for 1.5 IU LCAT/ml and with an albumin/free cholesterol ratio of 3, were 8.3 nmol free cholesterol esterified/ml per h and 4.1 nmol/ml per h for HDL3 and HDL2, respectively. (2) HDL3 were modified in the presence of VLDL by inducing triacylglycerol lipolysis with a semipurified lipoprotein lipase from bovine milk. The newly formed HDL had gained free cholesterol and phospholipids, so that about 50% of these modified HDL, referred to as light-LIP-HDL3, were reisolated in the HDL2 density range. Light-LIP-HDL3 were enriched mostly in free cholesterol (+ 160%) and in phospholipid (+ 40%). Their reactivity towards LCAT was half-reduced compared to parent HDL3, which correlated well with a decrease in their phospholipid/free cholesterol molar ratio. Moreover, HDL3 artificially enriched in free cholesterol and exhibiting a comparable PL/FC behaved like lipolysis-modified HDL in their reactivity towards LCAT. (3) HDL3 were also modified by co-incubation with VLDL (post-VLDL-HDL3), or with VLDL and a source of lipid transfer protein (CET-HDL3). The latter treatment greatly affected the lipid composition of the core particle (-25% esterified cholesterol, +190% TG). In both cases, the moderate decreasing LCAT reactivity observed could be related to the phospholipid/free cholesterol ratio. Thus, like in artificial substrates, the lipid composition of the HDL surface may control the rate of LCAT-mediated cholesterol esterification.     

Preparation and characterization of envelope membranes from nongreen plastids

We have developed a reliable procedure for the purification of envelope membranes from cauliflower (Brassica oleracea L.) bud plastids and sycamore (Acer pseudoplatanus L.) cell amyloplasts. After disruption of purified intact plastids, separation of envelope membranes was achieved by centrifugation on a linear sucrose gradient. A membrane fraction, having a density of 1.122 grams per cubic centimeter and containing carotenoids, was identified as the plastid envelope by the presence of monogalactosyldiacylglycerol synthase. Using antibodies raised against spinach chloroplast envelope polypeptides E24 and E30, we have demonstrated that both the outer and the inner envelope membranes were present in this envelope fraction. The major polypeptide in the envelope fractions from sycamore and cauliflower plastids was identified immunologically as the phosphate translocator. In the envelope membranes from cauliflower and sycamore plastids, the major glycerolipids were monogalactosyldiacylglycerol, digalactosyldiacylglycerol, and phosphatidylcholine. Purified envelope membranes from cauliflower bud plastids and sycamore amyloplasts also contained a galactolipid:galactolipid galactosyltransferase, enzymes for phosphatidic acid and diacylglycerol biosynthesis, acyl-coenzyme A thioesterase, and acyl-coenzyme A synthetase. These results demonstrate that envelope membranes from nongreen plastids present a high level of homology with chloroplasts envelope membranes.     

Studies on well-coupled Photosystem I-enriched subchloroplast vesicles — energy-dependent switching between two different active states of the proton-translocation adenosine triphosphatase

Single-turnover flash-induced ATP synthesis coupled to natural cyclic electron flow in Photosystem I-enriched subchloroplast vesicles (from spinach) was continuously followed by the luciferin-luciferase luminescence. The ATP yield per flash was maximal (1 ATP per s per 1000 Chl) around a flash frequency of 0.5–2 Hz. It decreased both at lower and higher flash frequencies. The decrease at high flash frequency was due to limitation by the electron-transfer rate, while the decrease at low flash frequency was directly due to intrinsic properties of the ATPase itself. Carbonylcyanide-p-trifluoromethoxyphenylhydrazone (FCCP) decreased the yield at low frequency more than at high frequency. The same behaviour was observed if electron transfer was artificially mediated by pyocyanin. If the ADP concentration was increased from 40 to at least 80 μM, or if the vesicles were preincubated with 5 mM dithiothreitol (DTT), the decrease of the yield at flash frequencies below 0.5 Hz was no longer observed. Incubation with DTT increased the rates of ATP hydrolysis and synthesis at any flash frequency. The decrease of the yield could be elicited again by addition of 50 nM FCCP. It is concluded that at low levels of the protonmotive force (Δ gm_H⁺), the ATPase is converted into an active ATP-hydrolyzing state in which ATP synthesis activity is decreased due to a decreased affinity towards ADP and/or to a decreased release of newly synthesized ATP, that can be cancelled by increasing the ADP concentration or by addition of DTT in the absence of uncoupler.     

Charge effects on phospholipid monolayers in relation to cell motility

A new sensitive method for the assay of retinyl ester hydrolase in vitro was developed and applied to liver homogenates of 18 young pigs with depleted-to-adequate liver vitamin A reserves. Radioactive substrate was not required, because the formation of retinol could be adequately quantitated by reversed-phase high-performance liquid chromatography. Optimal hydrolase activity was observed with 500 microM retinyl palmitate, 100 mM 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate, and 2 mg/ml Triton X-100 at pH 8.0. The relative rates of hydrolysis of six different retinyl esters by liver homogenate were: retinyl linolenate (100%), myristate (99%), palmitate (47%), oleate (38%), linoleate (31%), and stearate (29%). The enzyme was found primarily in the membrane-containing fractions of liver (59 +/- 3%, S.E.) and kidney (76 +/- 3%), with considerably lower overall activity in kidney (57-375 nmol/h per g of tissue) than in liver (394-1040 nmol/h per g). Retinyl ester hydrolase activity in these pigs was independent of serum retinol values, which ranged from 3 to 24 micrograms/dl, and of liver vitamin A concentrations from 0 to 32 micrograms/g. Pig liver retinyl ester hydrolase differs from the rat liver enzyme in its substrate specificity, bile acid stimulation, and interanimal variability.     

Synthesis and characterization of a d(ApG) platinated nonanucleotide duplex.

The nonamer 5'd(CTCAGCCTC) 3' 1 has been reacted with cis-diamminediaquaplatinum(II) in water at pH 4.2. The major reaction product was shown by enzymatic digestion and 1H NMR to be the d(ApG)cis-Pt(NH3)2 chelate [cis-Pt(NH3)2[d(CTCAGCCTC)-N7(4),N7(5)]] 1-Pt. When mixed with its complementary strand 2, 1-Pt forms a B DNA type duplex 3-Pt with a Tm of 35 degrees C (versus 58 degrees C for the unplatinated duplex). The NMR study of the exchangeable protons of 3-Pt revealed that the helix distortion is localized on the CA*G*-CTG moiety (the asterisks indicating the platinum chelation sites) with a strong perturbation of the A*(4)T(15) base pair related to a large tilt of A*(4).     

The role of Mg2+ in the hydrolytic activity of the isolated chloroplast ATPase: Study by high-performance liquid chromatography

The influences of total magnesium ion concentration at different total ATP concentrations, and of total ATP concentration, for different total magnesium ion concentrations, on the enzymatic rate of the isolated chloroplast F₁ ATPase, have been followed by a chromatographic method consisting in the separation and determination of ADP. From the various series of curves, it is concluded that the experimental results (position of the maxima,K _m values) are better fitted by a mechanism involving the activation of the enzyme by magnesium ion and hydrolysis of free ATP, rather than by the classical mechanism, for which the enzyme hydrolyzes the MgATP complex and is inhibited by Mg²⁺. Although the equations giving the reaction rate are similar in the two cases, the calculated values ofK _m are widely different. The value obtained from the classical mechanism does not agree withK _D , the dissociation constant of the enzyme-substrate complex, measured by the Hummel and Dreyer method. Moreover, when the total ATP concentration tends toward the total magnesium ion concentration, the nucleotide binding to the enzyme tends toward zero, although it should be maximum if MgATP were the true substrate. Finally, the inhibitory effect of Na⁺ is more easily explained as a competition between this ion and the activating Mg²⁺, than by the classical mechanism.     

Depolarization and Neurotransmitters Increase Neuronal Protein Tyrosine Phosphorylation

Abstract: In rat hippocampal slices and in neurons in primary culture, K⁺-induced depolarization increased markedly and rapidly tyrosine phosphorylation of a 110-kDa protein (pp110) and, to a lesser degree, of a 120-kDa protein (pp120), in a calcium-dependent fashion. Qlutamate, 1-aminocyclopentane- trans -1,3-dicarboxylic acid (an agonist of metabotropic glutamate receptors), and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (an agonist of ionotropic glutamate receptors) stimulated also tyrosine phosphorylation of pp110 and pp120. These effects were not observed in astrocytes in primary culture. In hippocampal slices tyrosine phosphorylation of pp110 and pp120 was stimulated by Ca²⁺-ionophores and by phorbol esters and antagonized by a chelator of intracellular Ca²⁺and by drugs that inhibit protein kinase C. Stimulation of muscarinic and α₁,-adrenergic receptors increased also tyrosine phosphorylation of pp110 and pp120. These results demonstrate that membrane depolarization and stimulation of neurotransmitter receptors activate a tyrosine phosphorylation pathway in neurons. This pathway involves an increase in intracellular Ca²⁺ concentrations and the activation of protein kinase C. It may provide a biochemical basis for some neurotrophic effects of electrical activity and neurotransmitters and may contribute to the role of tyrosine phosphorylation in long-term potentiation.     

Ozonolysis of Thymidine: Isolation and Identification of the Main Oxidation Products

The ozone-mediated oxidation of thymidine was investigated on the basis of final product identification. The oxidation reaction gave rise to five major modified nucleosides which were isolated and characterised from extensive H NMR and mass spectrometry studies. The comparison with the current knowledge of the hydroxyl radical-mediated oxidation reactions of thymidine in aerated aqueous solution indicates that the formation of ozone oxidation products may be mostly explained in terms of initial generation of an ozonide. Indeed, the identified products obtained by ozonolysis of thymidine resulted from the opening of the pyrimidine C5-C5 bond.