Regulation of deoxyadenosine and nucleoside analog phosphorylation by human placental adenosine kinase

The enzymes responsible for the phosphorylation of deoxyadenosine and nucleoside analogs are important in the pathogenesis of adenosine deaminase deficiency and in the activation of specific anticancer and antiviral drugs. We examined the role of adenosine kinase in catalyzing these reactions using an enzyme purified 4000-fold (2.1 mumol/min/mg) from human placenta. The Km values of deoxyadenosine and ATP are 135 and 4 microM, respectively. Potassium and magnesium are absolute requirements for deoxyadenosine phosphorylation, and 150 mM potassium and 5 mM MgCl2 are critical for linear kinetics. With only 0.4 mM MgCl2 in excess of ATP levels, the Km for deoxyadenosine is increased 10-fold. ADP is a competitive inhibitor with a Ki of 13 microM with variable MgATP2-, while it is a mixed inhibitor with a Ki and Ki' of 600 and 92 microM, respectively, when deoxyadenosine is variable. AMP is a mixed inhibitor with Ki and Ki' of 177 and 15 microM, respectively, with variable deoxyadenosine; it is a non-competitive inhibitor with a Ki of 17 microM and Ki' of 27 microM with variable ATP. Adenosine kinase phosphorylates adenine arabinoside with an apparent Km of 1 mM using deoxyadenosine kinase assay conditions. The Km values for 6-methylmercaptopurine riboside and 5-iodotubercidin, substrates for adenosine kinase, are estimated to be 4.5 microM and 2.6 nM, respectively. Other nucleoside analogs are potent inhibitors of deoxyadenosine phosphorylation, but their status as substrates remains unknown. These data indicate that deoxyadenosine phosphorylation by adenosine kinase is primarily regulated by its Km and the concentrations of Mg2+, ADP, and AMP. The high Km values for phosphorylation of deoxyadenosine and adenine arabinoside suggest that adenosine kinase may be less likely to phosphorylate these nucleosides in vivo than other enzymes with lower Km values. Adenosine kinase appears to be important for adenosine analog phosphorylation where the Michaelis constant is in the low micromolar range.     

Tropomyosin isoforms in chicken embryo fibroblasts: purification, characterization, and changes in Rous sarcoma virus-transformed cells

Seven polypeptides (a, b, c, 1, 2, 3a, and 3b) have been previously identified as tropomyosin isoforms in chicken embryo fibroblasts (CEF) (Lin, J. J.-C., Matsumura, F., and Yamashiro-Matsumura, S., 1984, J. Cell. Biol., 98:116-127). Spots a and c had identical mobility on two-dimensional gels with the slow-migrating and fast-migrating components, respectively, of chicken gizzard tropomyosin. However, the remaining isoforms of CEF tropomyosin were distinct from chicken skeletal and cardiac tropomyosins on two-dimensional gels. The mixture of CEF tropomyosin has been isolated by the combination of Triton/glycerol extraction of monolayer cells, heat treatment, and ammonium sulfate fractionation. The yield of tropomyosin was estimated to be 1.4% of total CEF proteins. The identical set of tropomyosin isoforms could be found in the antitropomyosin immunoprecipitates after the cell-free translation products of total poly(A)+ RNAs isolated from CEF cells. This suggested that at least seven mRNAs coding for these tropomyosin isoforms existed in the cell. Purified tropomyosins (particularly 1, 2, and 3) showed different actin-binding abilities in the presence of 100 mM KCl and no divalent cation. Under this condition, the binding of tropomyosin 3 (3a + 3b) to actin filaments was significantly weaker than that of tropomyosin 1 or 2. CEF tropomyosin 1, and probably 3, could be cross-linked to form homodimers by treatment with 5,5'-dithiobis-(2-nitrobenzoate), whereas tropomyosin a and c formed a heterodimer. These dimer species may reflect the in vivo assembly of tropomyosin isoforms, since dimer formation occurred not only with purified tropomyosin but also with microfilament-associated tropomyosin. The expression of these tropomyosin isoforms in Rous sarcoma virus-transformed CEF cells has also been investigated. In agreement with the previous report by Hendricks and Weintraub (Proc. Natl. Acad. Sci. USA., 78:5633-5637), we found that major tropomyosin 1 was greatly reduced in transformed cells. We have also found that the relative amounts of tropomyosin 3a and 3b were increased in both the total cell lysate and the microfilament fraction of transformed cells. Because of the different actin-binding properties observed for CEF tropomyosins, changes in the expression of these isoforms may, in part, be responsible for the reduction of actin cables and the alteration of cell shape found in transformed cells.     

Preliminary crystallographic data for cross-linked (lysine7-lysine41)-ribonuclease A

A cross-linked derivative of ribonuclease A, N^ε,N^ε′-(2,4-dinitrophenylene-1,5)-(lysine⁷-lysine⁴¹)-RNase A, has been crystallized by dialysis against 30% ($\text{v}\text{v}$) ethanol/water mixtures buffered at high pH. Single crystals belong to the orthorhombic space group P2₁2₁2₁, $\text{a = 37.2}\text{A}\text{?}\text{, b = 41.2}\text{A}\text{?}\text{, b = 41.2}\text{A}\text{?}$, with one molecule in the Crystallographic asymmetric unit.     

Glucagon-stimulated phosphorylation of rat liver glycogen synthase in isolated hepatocytes

Addition of glucagon (20 nM) to the isolated hepatocytes from 24-h starved male rats results in an inactivation of glycogen synthase. The A0.5 for glucose-6-P is increased 2-fold over the control but the S0.5 for UDP-glucose is not significantly affected. The glucagon-stimulated inactivation of glycogen synthase is also accompanied by a 60-120% increase in the phosphorylation of the synthase. Glycogen synthase labeled with 32P by incubation of the hepatocytes with [32P] PO4(3-) was recovered by immunoprecipitation and the resulting immunoprecipitate was subjected to tryptic digestion. Analysis of the 32P-labeled peptides reveals that the sites corresponding to those phosphorylated by cAMP-dependent protein kinase and glycogen synthase (casein) kinase-1 (Itarte, E., and Huang, K.-P. (1979) J. Biol. Chem. 254, 4052-4057) are rapidly phosphorylated in response to glucagon. These results demonstrate that glucagon not only triggers the activation of cAMP-dependent protein kinase through an increase in the intracellular level of cAMP but also, by an unknown mechanism, activates a Ca2+- and cAMP-independent protein kinase.     

Effects of methylglyoxal-bis (guanylhydrazone) and abscisic acid on polyamine metabolism in embryonectomized barley seeds

Incorporation of L-[U-¹⁴C] arginine or L-[U-¹⁴C] ornithine into putrescine (Put), spermidine (Spd) and spermine (Spm) in embryonectomized barley seeds (Hordeum vulgare L. cv. Himalaya) was studied following imbition with methylglyoxal-bis (guanylhydrazone) (MGBG) and abscisic acid (ABA). Both radiolabeled amino acids were incorporated into the amines as a result of active polyamine biosynthesis in the seed during imbibition. In the aleurone layer, the Spd and Spn existed mainly in the free form (acid soluble). However about 50% of Put was recovered in conjugated form(s) (acid insoluble). Imbibition with 5 and 10M ABA for 3 days increased the accumulation of the free form of ¹⁴C-Put, probably as a result of inhibition (70%) of ¹⁴C-Spd accumulation. The ABA treatment showed no significant effect on levels of the conjugated form of Put and Spd. Imbibition with millimolar concentrations of MGBG resulted in (i) abnormal accumulation of the free form of Put and incorporation of ¹⁴C-amino acids into the diamine, (ii) progressive inhibition of the accumulation of the free forms of ¹⁴C-Spd and Spm, and (iii) reduction of the ¹⁴C incorporation into the conjugated forms of Put and Spd. Uptake of ¹⁴C-amino acids was not affected by MGBG treatment. The results indicate that MGBG may inhibit not only the synthesis of Spd and Spm, but the catabolism (e.g. oxidation) of Put in the aleurone layer.This paper is published with the approval of the director of the Kentucky Agricultural Experiment Station.     

The complete amino-acid sequence of the proteinase inhibitor B from the root of the arrowhead (Sagittaria sagittifolia L.)

After reduction and alkylation of the disulfide bonds of the proteinase inhibitor B from the root of the arrowhead (Sagittaria sagittifolia L.) followed by CNBr cleavage three peptide fragments with 68, 62 and 11 amino-acid residues could be separated on DEAE-Sepharose CL-6B. The peptides or the inhibitor itself were further specifically cleaved either by trypsin or by the mixture of (CH3)2SO/HCl/HBr at the arginyl- and the tryptophyl-peptide bond, respectively. The complete amino-acid sequences of the peptides were determined by manual solid phase DABITC/PITC double coupling micro-method and the primary structure of the arrowhead inhibitor B consisting of 141 amino-acid residues was then elucidated. Twenty pairs of amino-acid residues are repeated in the molecule of this inhibitor, three of these pairs even occur three times. The possible locations of the reactive sites are discussed. On the basis of sequence comparisons between this inhibitor and all other serine proteinase inhibitors the arrowhead inhibitor may belong to a new family.     

冬小麦对不同海拔气候条件的反应 Ⅲ.植株光合作用特征

我们在原地测量了840米和2150米两地种植的冬小麦“凤麦13”苗期、拔节期、抽穗期和灌浆初期的光合作用速率的日变化,光合作用速率对光量子通量密度的反应和CO_2补偿点。结果表明生育前期(苗期和拔节期)和后期(抽穗期和灌浆初期)光合作用速率的一日内变化形式相反,而且低地种植的冬小麦其光合速率在前期高于高地种植的,后期高地种植的冬小麦有比低地高的光合速率。光饱和点基本相同。光补偿点在生育前期高地小麦比低地小麦低,而后期低地小麦的的光补偿点减低,并低于高地小麦的。高地小麦的光补偿点比较稳定。CO_2补偿点高地小麦比低地的较低。并就两地气候条件讨论了上述差异。     

Extracellular calcium and microwave enhancement of membrane conductance in snail neurons

Summary Microwave irradiation has been shown to decrease the input resistance of snail neurons. In this study, we examined the role of extracellular calcium in triggering the microwave-induced enhancement of membrane conductance. Two sets of experiments were conducted. In the first set, nerve cells were superfused using Ringer solution with added Cd²⁺ (0.9 mM) which is a known blocker of calcium channels. In the second set, cells were superfused with low Ca²⁺ (0.7 mM) Ringer solution. Microwave irradiation was conducted at 2,450 MHz for 30 min with a specific absorption rate of 13 mW/g. It was found that 7 mM to 0.7 mM lowering of Ca²⁺ in bathing solution as well as blocking of calcium channels in neuronal membrane by means of Cd²⁺ did not influence the fall in membrane resistance induced by microwave radiation. In fact, the observed changed in membrane resistance in these experiments were nearly equal to those observed for neurons superfused by normal Ringer's. Thus, these results rule out the possible contribution of external Ca²⁺ in the observed microwave effect. Experiments with high Ca²⁺ solution also support this conclusion.