Regulation of protein phosphorylation in pancreatic acini by cyclic AMP-mediated secretagogues: interaction with carbamylcholine

The effects on protein phosphorylation in mouse pancreatic acini of cyclic AMP-mediated secretagogues and the Ca2+-mediated agonist carbamylcholine were compared. Under the conditions adopted for the study of protein phosphorylation, carbamylcholine (3 microM) stimulated amylase release from pancreatic acini 6-fold, whereas vasoactive intestinal polypeptide (VIP) (100 nM) and the cyclic AMP analogue 8-bromo-cyclic AMP (1 mM) caused little or no increase in secretion. However, VIP and 8-bromo-cyclic AMP, when added in combination with carbamylcholine, potentiated the stimulation of amylase release to 170-180% of that caused by carbamylcholine alone. As assessed by two-dimensional gel electrophoresis, VIP reproduced four of the ten changes in protein phosphorylation elicited by carbamylcholine, these changes being the increased phosphorylation of one soluble protein and the decreased phosphorylation of three soluble proteins. VIP enhanced the carbamylcholine-induced changes in phosphorylation for three proteins. In addition, VIP increased the phosphorylation of a unique protein of Mr 52,000 and pI 5.66 which was not affected by carbamylcholine. All of the effects on protein phosphorylation exerted by VIP in the presence or absence of carbamylcholine were mimicked by 8-bromo-cyclic AMP. Secretin also reproduced most of the changes in protein phosphorylation caused by VIP, although concentrations of secretin of at least 100-fold higher were required to elicit a maximal response. It is concluded that cyclic AMP-mediated secretagogues alter the phosphorylation of a unique protein as well as of several pancreatic proteins affected by carbamylcholine. Moreover, these effects appear to be mediated primarily by VIP-preferring receptors and may be involved in the synergistic action of VIP to promote carbamylcholine-induced amylase release.     

Reversible dissociation of active octamer of cyanase to inactive dimer promoted by alteration of the sulfhydryl group

Cyanase is an inducible enzyme in Escherichia coli that catalyzes the reaction of cyanate with bicarbonate resulting in the decomposition of cyanate to ammonia and bicarbonate. In this study, the role of the single sulfhydryl group in each of the eight identical subunits of cyanase was investigated. Tetranitromethane, methyl methanethiosulfonate, N-ethylmaleimide, and Hg2+ all reacted with the sulfhydryl group to give derivatives which had reduced activities and which dissociated reversibly to inactive dimer. Association of inactive dimer to active octamer was facilitated by the presence of azide (cyanate analog) and bicarbonate, increased temperature and enzyme concentration, and presence of phosphate. Nitration of tyrosine residues by tetranitromethane occurred only in the absence of azide and bicarbonate, suggesting that at least some of the tyrosine residues become exposed when octamer dissociates to dimer. Site-directed mutagenesis was used to prepare a mutant enzyme in which serine was substituted for cysteine. The mutant enzyme was catalytically active and had properties very similar to native enzyme, except that it was less stable to treatment with urea and to high temperatures. These results establish that in native cyanase the sulfhydryl group per se is not required for catalytic activity, but it may play a role in stabilizing octameric structure, and that octameric structure is required for catalytic activity.     

Membrane protein phosphorylation during stomatocyte-echinocyte transformation of human erythrocytes

The normal, discoid shape of red blood cells represents an equilibrium between two opposing factors, i.e., stomatocytic and echinocytic transformations. Most stomatocytic agents were found to be inhibitors of calmodulin, a regulator of the phosphorylation of membrane proteins. We determined whether red cell shape transformations could be caused by changes in phosphorylation of membrane proteins, specifically the cAMP-dependent phosphorylation of ankyrin and band 4.1. Red blood cells were incubated with 32P and 100 microM chlorpromazine (stomatocytic transformation) or 30 mM sodium salicylate (echinocytic transformation) for various time intervals. Ghost membrane proteins were examined by polyacrylamide gel electrophoresis and autoradiography. Spectrin (beta-chain), ankyrin, band 3, band 4.1 and 4.9 were phosphorylated. No change was found in the degree and pattern of phosphorylation after stomatocytic transformation. Salicylate caused a reversible inhibition of transmembranous phosphate transport in both directions. The results indicate that the stomatocytic transformation induced by chlorpromazine and the echinocytic transformation induced by salicylate do not involve a change in phosphorylation, but that the echinocytic transformation induced by salicylate is associated with an inhibition of transmembranous transport of phosphate. Studies with salicylate suggest that the phosphorylation sites of band 3 are found mainly on the endofacial side of the membrane.     

The effect of papulacandin B on (1----3)-beta-D-glucan synthetases. A possible relationship between inhibition and enzyme conformation

The antibiotic, papulacandin B, inhibited growth or (1----3)-beta-D-glucan synthetase (or both) in the fungi Saccharomyces cerevisiae, Hansenula anomala, Neurospora crassa, Cryptococcus laurentii, Schizophyllum commune and Wangiella dermatitidis. No effect was observed on Achlya ambisexualis. There was no apparent correlation between the inhibition of growth and that of the synthetase. With most of the fungal extracts, the inhibition of glucan synthetase by papulacandin B became less pronounced as the substrate (UDP-glucose) concentration was decreased. At very low levels of UDP-glucose, with the enzymes from S. cerevisiae and W. dermatitidis, the antibiotic stimulated the activity of glucan synthetase. As further studied with the W. dermatitidis enzyme, those low concentrations of UDP-glucose corresponded to a sigmoidal portion of the rate vs. substrate curve. The sigmoid segment of the curve extended to higher concentrations of UDP-glucose as the temperature was increased. Concomitantly, the range of substrate concentrations at which papulacandin B stimulated the reaction or was noninhibitory was broadened. It is tentatively concluded that glucan synthetase may exist in more than one interconvertible form. The stimulatory effect of papulacandin B is possibly due to preferential binding to the active form of the enzyme. The equilibrium between these forms could be shifted by structural changes in the membrane in which the enzyme is embedded. The lack of correlation between the effects of papulacandin B in whole cells and in extracts is discussed in terms of the variations in membrane structure in the two situations.     

Potent and selective anti-HTLV-III/LAV activity of 2',3'-dideoxycytidinene, the 2',3'-unsaturated derivative of 2',3'-dideoxycytidine

2',3'-Dideoxycytidinene (ddeCyd), the 2',3'-unsaturated derivative of 2',3'-dideoxycytidine (ddCyd) is, like ddCyd itself, a potent and selective inhibitor of HTLV-III/LAV in vitro. This conclusion is based on the relatively high ratio of effective antiviral dose (0.3 microM) versus cell growth inhibitory concentration (20-35 microM) and the lack of any appreciable inhibitory activity against a series of non-oncogenic RNA and DNA viruses. Both compounds were considerably more inhibitory to human lymphoid cell lines than human nonlymphoid or murine cell lines. They were highly dependent on prior activation by deoxycytidine kinase to exert their anti-HTLV-III/LAV and cytostatic effects. In contrast with ddCyd, ddeCyd lost part of its anti-retrovirus effect upon prolonged incubation (10 days) with the virus-infected cells in culture.     

Thermodynamic parameters are sequence-dependent for the supercoil-induced B to Z transition in recombinant plasmids

The entropy and enthalpy changes which contribute to the thermodynamics of the B to Z transition were determined for three recombinant plasmids containing a (dC-dG)16 tract and for a plasmid containing a pair of (dT-dG)20 regions. For each base pair which adopts a left-handed conformation in the plasmids with (dC-dG)16 sequences, the delta HBZ and delta SBZ are -2.1 kcal/mol bp and -8.8 cal/K-mol bp, respectively. In the plasmid containing the (dT-dG)20 tracts, however, the delta HBZ and delta SBZ values are 0.58 kcal/mol bp and -0.76 cal/K-mol bp, respectively. Also, these determinations show that for each B-Z junction that forms in the plasmids containing the (dC-dG), the enthalpy and entropy changes are 24 kcal/mol junction and 65 cal/K-mol junction, whereas for the (dT-dG) plasmid, the enthalpy and entropy changes are -1.8 kcal/mol junction and -22 cal/K-mol junction, respectively. Those values for the enthalpy and entropy changes for the formation of a BZ junction in (dC-dG) and (dT-dG) plasmids suggest that the properties and possibly the structures of the junctions are different. Calculations using the enthalpy and entropy changes determined in this study reveal that the B to Z transition in plasmids containing (dC-dG) blocks are more temperature-dependent than the transitions in plasmids with (dT-dG) blocks. Surprisingly, at temperatures above 60 degrees C, calculations indicate that the B to Z transitions in (dT-dG) plasmids should be energetically favored over that transition in (dC-dG) plasmids.     

The kinetics of the first wave of spermatogenesis in the newborn male mouse

A combination of autoradiography and air-dried techniques was used to calculate the duration of the major meiotic stages in the first wave of spermatogenesis in the newborn mouse. The data indicated that the entry into meiosis occurred asynchronously over 2 days, and the time required for each stage and the total cycle was constant. These time intervals were nearly identical with those estimated for adult animals in the present study and by other authors.     

Role of 2-deoxy-D-glucose in the inhibition of phagocytosis by mouse peritoneal macrophage

2-Deoxy-D-glucose inhibits Fc and complement receptor-mediated phagocytosis of mouse peritoneal macrophages. To understand the mechanism of this inhibition, we analyzed the 2-deoxy-D-glucose metabolites in macrophages under phagocytosis inhibition conditions and conditions of phagocytosis reversal caused by glucose, mannose and 5-thio-D-glucose, and compared their accumulations under these conditions. Macrophages metabolized 2-deoxy-D-glucose to form 2-deoxy-D-glucose 6-phosphate, 2-deoxy-D-glucose 1-phosphate, UDP-2-deoxy-D-glucose, 2-deoxy-D-glucose 1, 6-diphosphate, 2-deoxy-D-gluconic acid and 2-deoxy-6-phospho-D-gluconic acid. The level of bulk accumulation as well as the accumulation of any of these 2-deoxy-D-glucose metabolites did not correlate with changes in macrophage phagocytosis capacities caused by the reversing sugars. 2-Deoxy-D-glucose inhibited glycosylation of thioglycolate-elicited macrophage by 70-80%. This inhibition did not cause phagocytosis inhibition, since (1) the reversal of phagocytosis by 5-thio-D-glucose was not followed by increases in the incorporation of radiolabelled galactose, glucosamine, N-acetylgalactosamine or fucose; (2) cycloheximide at a concentration that inhibited glycosylation by 70-80% did not affect macrophage phagocytosis. The inhibition of protein synthesis by 2-deoxy-D-glucose similarly could not account for phagocytosis inhibition, since cycloheximide, when used at a concentration that inhibited protein synthesis by 95%, did not affect phagocytosis. 2-Deoxy-D-glucose lowered cellular nucleoside triphosphates by 70-99%, but their intracellular levels in the presence of different reversing sugars did not correlate with the magnitude of phagocytosis reversal caused by these sugars. The results show that 2-deoxy-D-glucose inhibits phagocytosis by a mechanism distinct from its usual action of inhibiting glycosylation, protein synthesis and depleting energy supplies, mechanisms by which 2-deoxy-D-glucose inhibits other cellular processes.     

Immunohistochemical studies on the localization of cellular retinol-binding protein in rat testis and epididymis

The immunohistochemical localization of cellular retinol-binding protein (CRBP) was studied in rat testis and epididymis. Parallel studies were also carried out on the localization of plasma retinol-binding protein (RBP) and transthyretin (TTR) in testis. The studies employed antibodies purified by immunosorbent affinity chromatography, permitting the specific staining and localization of each antigen by the unlabeled peroxidase-antiperoxidase method. For RBP and TTR, specific immune staining was found in the interstitial spaces between the seminiferous tubules, and not in the tubules themselves. In contrast, strong specific immune staining for CRBP was found in the seminiferous tubules, with a striking localization within Sertoli cells. Moreover, a distinct cyclic variation of specific staining for CRBP within Sertoli cells was observed during the spermatogenic cycle. This cyclic variation was seen with regard to both the intensity of staining and to the anatomic distribution of CRBP within the Sertoli cells. Within the epididymis CRBP was selectively localized to the proximal portion of the caput epididymidis, with variations in intensity of the staining of the epithelium of the ducts in different histological zones. Specific immune staining for CRBP was very weak or absent in the other portions of the epididymis. These results were confirmed by radioimmunoassay. Vitamin A-deficient rats showed markedly reduced specific immune staining for CRBP in both testes and epididymides, and greatly reduced levels of CRBP in these tissues on radioimmunoassay. These studies on the localization of CRBP provide information concerning the specific cells and anatomic loci within the testis and epididymis where retinol may be playing an important role in sperm formation and maturation.     

Vesicular stomatitis virus-infected cells fuse when the intracellular pool of functional M protein is reduced in the presence of G protein.

Five highly cytolytic strains of both Indiana and New Jersey serotypes of vesicular stomatitis virus were shown to induce cell fusion in BHK-21 and R(B77) cells. Inhibition of protein synthesis after the eclipse period of viral replication is a prerequisite for vesicular stomatitis virus-induced cell fusion. Pulse-chase experiments showed that inhibition of protein synthesis would lead to a drastic reduction in the intracellular pool of M protein as compared with other proteins. A temperature-sensitive mutant defective in M protein function (G31) was the only mutant of the five complementation groups to spontaneously induce polykaryocytes at the nonpermissive temperature. Previously, G protein has been shown to play a role in vesicular stomatitis virus-induced cell fusion. These results suggest that the combination of the presence of G protein on the virus-infected cell surface and the absence of functional M protein or a reduced level of intracellular M protein promotes cell fusion. On the basis of this study, we propose that vesicular stomatitis virus infection can induce cell fusion when the functional M protein pool declines to a critical level while G protein remains on the cell surface.