Differential inhibition of mammalian aminopropyltransferase activities

Rat ventral prostate spermine synthetase was inhibited by 5′-methylthioadenosine and by S-adenosylhomocysteine at concentrations which did not inhibit spermidine synthetase from the same tissue. S-Adenosylethionine inhibited both enzymes to an equal extent. These aminopropyltransferases were also inhibited by diamines not normally present in mammalian cells. All the α,ω-diamines with 3 to 12 C atoms had inhibitory activity, but 1,3-diaminopropane and 1,5-diaminopentane were most active. Spermine synthetase was more sensitive than spermidine synthetase to the effects of these diamines. These results suggest that the relative rates of spermidine and spermine formation $\text{in}$,$\text{vivo}$ might be affected by the intracellular concentration of nucleosides such as S-adenosylhomocysteine. They also raise the possibility that these rates of synthesis could be selectively affected by administration of one or the other of these inhibitors.     

RNA polymerase activity in double-stranded ribonucleic acid virus particles from Aspergillus foetidus.

RNA polymerase activities have been detected in purified particles of $\text{Aspergillus foetidus}$ viruses S and F. Incorporation of [³H]-UTP into acid insoluble RNA was dependent on ATP, GTP, CTP and magnesium ions. No pretreatment of the particles was required and the rate of reaction was proportional to the amount of virus added. In the conditions used RNA synthesis by $\text{A. foetidus}$ virus S was complete in 4 h. The reaction could be stimulated by Triton X-100, but was unaffected by heat shock, dithiothreitol, potassium chloride or ammonium chloride; it was inhibited by ethidium bromide but not by actinomycin D. The major reaction product was single-stranded RNA, as indicated by its sensitivity to degradation by ribonuclease A. This is the first report of synthesis of single-stranded RNA by a double-stranded RNA mycovirus.     

Kinetic properties of pyruvate kinase isolated from rat hepatic tumours.

The results demonstrate the existence of L and M forms of pyruvate kinase in rat hepatomas. Tumours were induced by feeding N-Nitrosodiethylamine. The kinetic properties of the L-type tumour enzyme was markedly different from the L-enzyme form found in normal liver. The L-form of tumour enzyme was purified by DEAE cellulose-Sephadex G200 chromatography (Sp. activity 41 units/mg). $\text{MgADP}{}^{\mathrm{?}}\text{ADP}{}^{\mathrm{2?}}$ of $\text{20}\text{1}$ gave optimum activity for both the intrinsic and F1,6di-P stimulated reactions. ATP did not inhibit the enzyme. Alanine (2.5 nM) caused 60% inhibition at low PEP concentrations (0.25 mM). The homotropic effector (PEP) exhibited a complex allosteric pattern and saturation kinetics were not observed for either the intrinsic or F1,6di-P stimulated reactions with PEP concentrations as high as 10 mM.     

Studies on the in vitro effects of insulin on glycogen synthesis and ultrastructure in isolated rat liver hepatocytes

Rat liver hepatocytes were isolated by collagenase $\text{in vitro}$ perfusion technique and effect of insulin on glycogen synthesis and ultra-structure was studied. Addition of insulin stimulated glycogen synthesis and maintained better cellular structure. Synthesis of glycogen was linear in isolated hepatocytes when incubated with various concentrations of glucose (0–800 mg%) reaching initial levels. Concanavaline A inhibited epinephrine stimulated glycogenolysis but had no effect on glucagon stimulated glycogenolysis. These studies indicate that insulin is required for glycogen synthesis and for maintaining hepatocytes ultrastructure. Furthermore, isolated hepatocytes retain various receptors and that different hormones utilize different receptor sites.     

Characterization of microsomal and cytosolic alpha-1,2-mannosidases from mung bean hypocotyls

Microsomal and cytosolic alpha-mannosidase activities, which hydrolyze alpha-1,2-mannosyl-mannose linkages in the Man5GlcNAc2 oligosaccharide, have been isolated from homogenates of mung bean hypocotyls. The alpha-1,2-mannosidase activities were readily distinguished from previously described aryl alpha-mannosidases by several criteria. They were optimally active in the presence of Ca2+ between pH 5.5 and 6, they were inhibited by Zn2+, and they had essentially no activity with p-nitrophenyl-alpha-mannoside. The microsomal and cytosolic alpha-1,2-mannosidases demonstrated specificity for oligosaccharides with terminal nonreducing alpha-1,2-mannosyl linkages, and they were inhibited by mannosyl-mannose disaccharides, with the inhibition decreasing in the order of alpha-1,2-greater than alpha-1,3-greater than alpha-1,6-mannosyl-mannose. The cytosolic alpha-1,2-mannosidase activity, which was present in the 100,000 g supernatant, was separated from the aryl alpha-mannosidase by ammonium sulfate precipitation. The microsomal alpha-1,2-mannosidase, which was tightly associated with the particulate fraction, was solubilized with Triton X-100 and 0.2 M KCl. The two alpha-1,2-mannosidase activities were readily differentiated by gel-filtration chromatography. The solubilized microsomal enzyme chromatographed in approximately the same position as a Mr 460,000 globular protein whereas the cytosolic enzyme was eluted in a retarded position, indicating a much smaller protein.     

Mn++-specific reactivation of EDTA inactivated alpha-isopropylmalate synthase from Alcaligenes eutrophus H 16.

The α-isopropylmalate synthase (EC 4.1.3.12) from $\text{Alcaligenes}\text{eutrophus}\text{H 16}$ was inactivated by EDTA in a time-dependent reaction. Only the addition of Mn⁺⁺ plus dithiothreitol could restore the activity. The substrate, α-ketoisovalerate, prevented the inactivation; the feedback inhibitor, leucine, and it's antagonist, valine, increased the rate of inactivation. Except for α,α′-bipyridyl, chelating reagents, other than EDTA had no effect on the enzyme stability. It is suggested that the α-isopropylmalate synthase is a metallo enzyme - the evidence points to Mn⁺⁺ as the metal ion - and that this enzyme uses a mechanism of catalysis which differs from that of the analogous malate synthase (EC 4.1.3.2) and citrate synthase (EC 4.1.3.4).     

Kinetic hysteresis for fructose bisphosphatase: a change in substrate configuration specificity.

Kinetic hysteresis for rabbit liver fructose bisphosphatase in the presence of Mg²⁺ (pH 7.6) is exhibited by the varied rates at which product formation is reduced on the addition of different inhibitors under cycling conditions. Two different states of the enzyme are detected: the initial resting state which binds α-, β- and keto analogs of fructose 1,6-bisphosphate; and the active cycling state which binds, and is inhibited by, only the α-analog. Both enzyme states, however, bind the allosteric modifier, AMP, and a product analog, (α+β)methyl-D-fructofuranoside 6-phosphate to the same extent so that the resulting inhibition is state independent. A relatively slow first-order transition (0.13 min^?1) characterizes the reversion of the active enzyme to its resting state. The implications of this phenomenon for regulating fructose bisphosphatase activity $\text{in vivo}$ are discussed.     

Effect of methylglyoxal bis (guanylhydrazone) (MGBG) in vivo on the decarboxylation of s-adenosylmethionine and synthesis of spermidine in the rat and guinea pig.

The rates of decarboxylation of $\text{S}$-adenosylmethionine and synthesis of spermidine have been measured in extracts of liver, kidney and brain of the rat and guinea pig after intraperitoneal injection of MGBG, both before and after dialysis. The rate of decarboxylation of $\text{S}$-adenosylmethionine paralleled that of spermidine synthesis in all of the tissues investigated, even when spermidine synthesis was measured using preformed decarboxylated $\text{S}$-adenosylmethionine as substrate instead of $\text{S}$-adenosylmethionine itself. MGBG inhibited CO₂ production and spermidine synthesis to a similar extent in extracts of liver and kidney of both the rat and the guinea pig. After dialysis, a similar increase in both CO₂ production and spermidine synthesis was noted in these extracts. No effects on CO₂ production or spermidine synthesis were noted on extracts of brain of the rat or guinea pig, either before or after dialysis. When MGBG was injected intracisternally, CO₂ production and spermidine synthesis by extracts of brain were inhibited to the same extent, and after dialysis a similar increase in CO₂ production and spermidine synthesis was observed. These results indicate that the effects of MGBG are essentially the same in brain as they are in liver and kidney, and the MGBG injected intraperitoneally does not pass into the brain.     

Enzymatic cyclization of neryl pyrophosphate to -terpineol by cell-free extracts from peppermint

A cell-free system prepared from peppermint ($\text{Mentha piperita}$ L.) shoot tips catalyzed the cyclization of neryl pyrophosphate to α-terpineol. Cyclization could be demonstrated in the absence of added cofactors, but addition of NaF inhibited competing phosphatase/pyrophosphatase activity, resulting in much higher levels of α-terpineol formation. Under certain conditions cyclization was stimulated by Mg⁺⁺. Similar enzyme preparations were obtained from spearmint ($\text{Mentha spicata}$ L.) leaves and carrot ($\text{Daucus carota}$ L.) storage organ. The cyclization of neryl pyrophosphate to α-terpineol appears to be a key reaction in the biosynthesis of cyclohexanoid monoterpenes.     

1H NMR spectroscopy of carbohydrates from the G glycoprotein of vesicular stomatitis virus grown in parental and Lec4 Chinese hamster ovary cells

Carbohydrate moieties derived from the G glycoprotein of Vesicular Stomatitis Virus (VSV) grown in parental Chinese hamster ovary (CHO) cells and the glycosylation mutant Lec4 have been analyzed by high-field ¹H NMR spectroscopy. The major glycopeptides of $\text{CHO}\text{VSV}$ and $\text{Lec4}\text{VSV}$ were purified by their ability to bind to concanavalin A-Sepharose. The carbohydrates in this fraction are of the biantennary, complex type with heterogeneity in the presence of α(2,3)-linked sialic acid and α(1,6)-linked fucose residues. A minor $\text{CHO}\text{VSV}$ glycopeptide fraction, which does not bind to concanavalin A-Sepharose but which binds to pea lectin-agarose, was also investigated by ¹H NMR spectroscopy. These carbohydrates are complex moieties which appear to contain N-acetylglucosamine in β(1,6) linkage. Their spectral properties are most similar to those of a triantennary complex oligosaccharide containing a 2,6-disubstituted mannose α(1,6) residue. Carbohydrates of this type are not found among the glycopeptides of VSV grown in the Lec4 CHO glycosylation mutant.     

Effect of aphidicolin on viral and human DNA polymerases.

DNA polymerases induced by Herpes simplex and Vaccinia viruses are inhibited by aphidicolin and this inhibition is probably the basis of its antiviral activity $\text{in vivo}$. Its possible clinical use is however hampered by the concomitant effect on human replicative DNA polymerase α. The inhibition of human α-polymerase is reversible both $\text{in}$$\text{vitro}$ and $\text{in vivo}$ and the changes in the rate of incorporation of thymidine into DNA, following treatment with aphidicolin for a generation time, indicate the likely synchronization of the cells due to this agent. DNA polymerase β, which has recently been shown to carry out repair synthesis of damaged nuclear DNA, is not inhibited by aphidicolin either $\text{in vitro}$ on $\text{in vivo}$ suggesting that the drug could allow a rapid and simple evaluation of DNA repair synthesis due to DNA polymerase β.     

Thiol reactivity and the molecular individuality of α- and β-adrenoreceptors in rat liver plasma membranes

Whether or not α- and β-adrenoreceptors are non-identical binding sites on the same protein is still an open question. We investigated the effects of sulfhydryl reagents and dithiothreitol on the binding of [³H]dihydroalprenolol and [³H]dihydroergocryptine to β- and α-adrenoreceptors of rat liver plasma membranes. Dithiothreitol inhibited the binding of [³H]dihydroalprenolol to the β-adrenoreceptor, whereas it had no effect on the specific binding of [³H]dihydroergocryptine to the α-adrenoreceptor. In contrast, mersalyl, a mercurial SH reagent, readily blocked the α-adrenoreceptor and, although to a lesser extent, the β-adrenoreceptor. The interaction of mersalyl with the α-adrenoreceptors was almost instantaneous. In contrast, under the same experimental conditions, the inactivation of the β-adrenoreceptors was much slower ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{:7}\text{min}$). Finally, a marked difference in the accessibility of the SH groups to mersalyl was observed between the α- and β-adrenoceptors. The presence of 15 μM (?)-epinephrine or 1.5 μM phentolamine was sufficient to prevent the blockade of the α-adrenoreceptor by mersalyl, but inactivation of the β-adrenoreceptor by mersalyl was not modified by 500 μM (?)-epinephrine and was only slightly decreased by 50 μM (?)-propranolol. Thus, the α- and β-adrenoreceptors from rat liver plasma membranes exhibited biochemical differences which may be interpreted in favor of their molecular individuality.     

Specific inhibition of the synthesis of putrescine and spermidine by 1,3-diaminopropane in rat liver in vivo

Chronic administration of 1,3-diaminopropane, a compound inhibiting mammalian ornithine decarboxylase (EC 4.1.1.17) in vivo, effectively prevented the large increases in the concentration of putrescine that normally occur during rat liver regeneration. Furthermore, repeated injections of diaminopropane depressed by more than 85% ornithine decarboxylase activtivity in rat kidney.Adminsitration of diaminopropane 60 min before partial hepatectomy only marginally inhibited orthine decarboxylase activity at 4 h after the operation. However, when the compound was given at the time of the operation (4 h before death), or any time thereafter, it virtually abolished the enhancement in ornithine decarboxylase activity in regenerating rat liver remnant.An injection of diaminopropane given 30 to 60 min after operation, but not earlier or later, depressed $\text{S-}\text{adenosyl}\text{-}\text{l}\text{-}\text{methionine}$ decarboxylase activity (EC 4.1.1.50) 4 h after partial hepatectomy.Diaminopropane likewise inhibited ornithine decarboxylase activity during later periods of liver regeneration. In contrast to early regeneration, a total inhibition of the enzyme activity was only achieved when the injection was given not earlier than 2 to 3 h before the death of the animals.Diaminopropane also exerted an acute inhibitory effect on adenosylmethionine decarboxylase activity in 28-h regenerating liver whereas it invariably enhanced the activity of tyrosine aminotransferase (EC 2.6.1.5), used as a standard enzyme of short half-life.Treatment of the rats with diaminopropane entirely abolished the stimulation of spermidien synthesis in vivo from [¹⁴C] methionine 4 h after hepatectomy or after administration of porcine growth hormone.Both partial hepatectomy and the treatment with growth hormone produced a clear stimulation of hepatic RNA synthesis, the extent of which was not altered by injections of diaminopropane in doses sufficient to prevent any enhancement of ornitine decarboxylase activity and spemedicine synthesis.     

Increased levels of rat hepatic nuclear free and engaged RNA polymerase activities during liver regeneration.

Rat liver nuclei, seventeen hours after partial hepatectomy, showed a two to three-fold increase in total RNA synthesis $\text{in vitro}$ over the sham operated controls. When tested with exogenous synthetic template, this was found to be mainly a reflection of increased levels of both the nuclear free and engaged RNA polymerase activities $\text{per se}$. It was also observed that there was a greater stimulation of the species of RNA polymerase that are α-amanitin resistant than sensitive (3.2 μg/ml). This observation was further confirmed by DEAE-Sephadex column chromatography of the solubilized nuclear free and engaged RNA polymerases and found RNA polymerase I and IIIa were the major species greatly stimulated during this period of liver regeneration. These data suggest not only that there exists a sensitive equilibrium between the nuclear free and engaged RNA polymerases; they also suggest the possibility that RNA polymerase itself may play a positive role in the regulation of gene expression.     

Protein synthesis in mitochondrial and microsomal fractions from rat brain and liver after acute or chronic ethanol administration

Incorporation of C¹⁴ Leucine was determined $\text{in vitro}$ or $\text{in vivo}$ in isolated mitochondria and microsomes of rat brain and liver after acute or chronic ethanol administration $\text{in vivo}$.The protein synthesis in mitochondrial and microsomal preparation was inhibited respectively by chloramphenicol and cycloeximide, specific inhibitors for the two systems tested. The experimental data demonstrate that the $\text{in vitro}$ protein synthesis in both systems, mitochondrial and microsomal, is strongly affected only after chronic treatment which produces significant activation at the mitochondrial and microsomal level in the liver and an inhibition on the same systems of the brain.The data for $\text{in vivo}$ protein synthesis instead show strong inhibition after acute administration, except for brain mitochondria, which are practically unaffected, while after chronic treatment no significant alterations are observed.     

Stimulation of intralysosomal proteolysis by cysteinyl-glycine,a product of the action of γ-glutamyl transpeptidase on glutathione

The mechanism of the stimulatory effect of glutathione on proteolysis in mouse kidney lysosomes and a lack of an effect in lysomes from the liver was investigated. The stimulation in kidney lysosomes was inhibited by serine plus borate, a reversible inhibitor of γ-glutamyl transpeptidase. Treatment of mouse kidney lysosome suspensions with $\text{l}\text{-(αS,5S)-α-}\text{amino}\text{-3-}\text{chloro}\text{-4,5-}\text{dihydro}\text{-5-}\text{isoxazoleacetic}$ acid (acivicin), an irreversible inhibitor of the transpeptidase, also inhibited the effect of glutathione, but this inhibition was completely relieved by washing and addition of freshly prepated kidney membranes or purified γ-glutamyl transpeptidase to the incubation mixtures. Cysteinyl-glycine, a product of the action of γ-glutamyl transpeptidase, stimulated proteolysis in acivicin-inhibited kidney lysosome preparations similarly to glutathione, and cysteine had no effect at equivalent concentrations. Glutathione also stimulated proteolysis in liver lysosomes in the presence of washed kidney membranes or γ-glutamyl transpeptidase, but the effect was similar to that produced by equivalent concentrations of cysteine. These results suggest that the stimulatory effect of glutathione was mediated by the action of γ-glutamyl transpeptidase present in contaminating cell membrane fragments in the lysosome preparations, and that glutathione does not take part in intralysosomal proteolysis. However, the possibility that cysteinyl-glycine is a physiological intralysosomal disulfide reductant in kidney lysosomes has not been excluded.     

1-Aminoglycosides, a new class of specific inhibitors of glycosidases

1-Aminoglycosides represent a new class of specific and relatively potent inhibitors of glycosidases. These compounds are specific against those enzymes which act upon glycosides that correspond to glycone of the inhibitor. Thus α- and β-$\text{D}\text{=}$- are inhibited by $\text{D}\text{=}$-glucosidases but not by $\text{D}\text{=}$-galactosylamine and $\text{D}\text{=}$-mannosylamine. α- and $\text{D}\text{=}$-galactosidases are inhibited by $\text{D}\text{=}$-galactosylamine but not by the other two glycosylamines. $\text{D}\text{=}$-Mannosylamine inhibits mannosidase.     

Corticotropin releasing factor stimulates adrenocorticotropin and beta-endorphin release from AtT-20 mouse pituitary tumor cells

Corticotropin releasing factor (CRF) was tested for its ability to stimulate ACTH and β-endorphin secretion from clonal $\text{AtT-20}\text{D16-16}$ mouse pituitary tumor cells. Release of both hormones was stimulated 4 to 5-fold over the basal release at nanomolar concentrations of synthetic CRF. CRF analogues stimulated $\text{ACTH}\text{β-endorphin}$ release with the same order of potency in the tumor cells as in primary cultures of anterior pituitary cells. A 90-min exposure to CRF elicited a 29–35% increase in total ACTH and β-endorphin immunoreactivity in tumor cell cultures. Dexamethasone markedly inhibited CRF-stimulated and basal ACTH and β-endorphin release. $\text{AtT-20}\text{D16-16}$ cells may serve as a good model system for studying the biochemistry of CRF receptor-mediated events involved in $\text{ACTH}\text{β-endorphin}$ release and synthesis.     

S-Adenosylmethionine synthetase isozyme patterns from rat hepatoma induced by N-2-fluorenylacetamide

Isozyme patterns of $\text{S}$-adenosylmethionine synthetase have been measured with and without dimethylsulfoxide in hepatoma of rats induced by $\text{N}$-2-fluorenylacetamide. The isozymes of α- and β-types existing in normal rat liver gradually decreased with the progress of hepatocarcinoma, and the kidney type γ-enzyme appears along with disappearance of both α- and β-enzymes. The liver from rat fetus contains a greater part of γ-type enzyme.