High specific radioactivity labeling of oligonucleotides with 3H-succinimidyl propionate

An easy and rapid method for tritium labeling of deprotected oligonucleotides is proposed. The method consists in performing the reaction of commercial 3H-succinimidyl propionate with a terminal amino group of the oligonucleotide in an organic medium. High specific radioactivity labeling can be achieved with minimal radiolysis during long term storage. The synthesis of the nonradioactive congener having an identical structure to the labeled compound is also described.

Turnover of Tubulin in Ciliary Outer Doublet Microtubules

Previous pulse-chase labeling studies have shown that structural proteins incorporate into fully assembled sea urchin embryonic cilia at rates approaching those of full regeneration. When all background ciliogenesis was suppressed by taxol, the turnover of most proteins, including tubulin, continued. The present study utilized chemical dissection to explore the route of tubulin incorporation in the presence of taxol and also in steady-state cilia from prism stage embryos. Surprisingly, in cilia from untreated embryos, the most heavily labeled tubulin was found in the most stable portion of the doublet microtubles, the junctional protofilaments. With taxol, this preferential incorporation was suppressed, although control-level turnover still took place in the remainder of the doublet. This paradoxical result was confirmed by pulse-chase labeling and immediately isolating steady-state cilia, then isolating two additional crops of cilia regenerated, respectively, from pools of high and then decreased label. In each case, the level of label occurring in the tubulin from the junctional protofilaments, compared with that from the remainder of the doublet, correlated with the level of pool label from which it must exchange or assemble. These data indicate that ciliary outer doublet microtubules are dynamic structures and that the junctional region is not inert. Plausible mechanisms of incorporation and turnover of tubulin in fully-assembled, fully-motile cilia can now be assessed with regared to recent discoveries, particularly intraflagellar transport, distal tip incorporation, and treadmilling.

The low-molecular-weight phosphotyrosine protein phosphatase, when overexpressed, reduces the mitogenic response to macrophage colony-stimulating factor and tyrosine phosphorylation of its receptor

The interference of low-molecular-weight phosphotyrosine protein phosphatase with the macrophage response to macrophage colony-stimulating factor was investigated. This paper shows that this phosphatase, already known to be involved in platelet-derived growth factor receptor signaling, is physiologically expressed in murine macrophages and dephosphorylates in vitro macrophage colony-stimulating factor receptor molecules immunoprecipitated from macrophage colony-stimulating factor-stimulated macrophages. We obtained the first demonstration that a phosphotyrosine-specific protein phosphatase dephosphorylates the macrophage colony-stimulating factor receptor in vivo and reduces the mitogenic response to macrophage colony-stimulating factor. The data indicate that low-molecular-weight phosphotyrosine protein phosphatase is a negative regulator of macrophage colony-stimulating factor receptor signaling.

Proton transfer in the mechanism of triosephosphate isomerase

Triosephosphate isomerase (TIM) catalyzes the reversible interconversion of dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (GAP), with Glu-165 removing the pro-R proton from C1 of DHAP and neutral His-95 polarizing the carbonyl group of the substrate. During the TIM reaction, approximately 2% of the pro-R tritium from C1 of DHAP is conserved and appears at C2 of GAP [Nickbarg, E. B., and Knowles, J. R. (1988) Biochemistry 27, 5939]. In the "classical" mechanism, 98% of the pro-R tritium exchanges with solvent from Glu-165 at the intermediate state and the remaining 2% is transferred by Glu-165 to C2 of the same substrate molecule. This intramolecular transfer of tritium is therefore predicted to be independent of DHAP concentration. On the basis of NMR detection of a strong hydrogen bond between Glu-165 and the 1-OH of an analogue of the enediol intermediate [Harris, T. K., Abeygunawardana, C., and Mildvan, A. S. (1997) Biochemistry 36, 14661], we have suggested a "criss-cross" mechanism for TIM in which Glu-165 transfers a proton from C1 of DHAP to O2 of the enediol, and subsequently from O1 of the enediol to C2 of the product GAP. Since the pro-R proton is transferred to O2 instead of C2 in the criss-cross mechanism, no intramolecular transfer of label from substrate to product would be expected to occur. However, intermolecular transfer of label could occur if the label exchanges from O2 into a group on the protein and is transferred to GAP in subsequent turnovers. The extent of intermolecular tritium transfer in the criss-cross mechanism would be predicted to be dependent on DHAP concentration. The extent of tritium transfer was studied as a function of initial DHAP concentration using DHAP highly tritiated at the pro-R position. At 50% conversion to GAP, triphasic tritium transfer behavior was found. For phase 1, between 0.03 and 0.3 mM DHAP, a constant extent of tritium transfer of 1.19 +/- 0.03% occurred. For phase 2, between 0.3 and 1.0 mM DHAP, the extent of transfer progressively increased as a function of DHAP concentration to 2.17 +/- 0.15%. For phase 3, between 1.0 and 7.0 mM DHAP, the extent of transfer slightly decreased to 1.68 +/- 0.17%. In a direct test for intermolecular isotope transfer, doubly labeled [1(R)-D, 13C3]DHAP and 13C-depleted [1(R)-H,12C3]DHAP were synthesized, mixed in equal amounts, and incubated at 1 mM total DHAP with TIM, GAP dehydrogenase, NAD+, and arsenate until 50% conversion to 3-phosphoglycerate occurred. Electrospray ionization mass spectral analysis of the stable 3-phosphoglycerate product detected an extent of 1.4 +/- 0.4% of intramolecular D transfer from [13C3]DHAP to the 13C3 product, but no intermolecular transfer (</=0.02%) of D from [13C3]DHAP to the 12C3 product. Hence, the entire transfer of hydrogen from substrate to product is intramolecular, providing no direct support for the criss-cross mechanism in wild-type TIM. The increase in the extent of intramolecular isotopic transfer with increasing initial DHAP concentration indicates site-site interaction in this dimeric enzyme which either (i) slows proton exchange with solvent from Glu-165 at the intermediate state in the classical mechanism or (ii) alters the partitioning of the abstracted proton between transfer to C2 by the classical mechanism or to O2 by the criss-cross mechanism in which no intermolecular transfer of label occurs.

Alpha-adrenoceptor interaction of tetrandrine and isotetrandrine in the rat: functional and binding assays

The action of 1S,1'S-tetrandrine, a bisbenzyltetrahydroisoquinoline alkaloid, on alpha1-adrenoceptors has been compared with that of its isomer 1R,1'S-isotetrandrine. The work includes binding assays to analyse the affinity of these products for the [3H]prazosin binding site of rat cerebral cortical membranes and functional studies on rat isolated aorta to examine the effects of both alkaloids on intracellular calcium processes related or not to alpha-adrenoceptor activation. A radioligand receptor-binding study showed that both compounds interacted with the alpha1-adrenoceptors displacing [3H]prazosin from the specific binding site. The Ki values (inhibition constants) were 0.69+/-0.12 and 1.6+/-0.4 microM for tetrandrine and isotetrandrine, respectively. The functional studies showed that both alkaloids concentration-dependently inhibited noradrenaline-induced contraction in Ca2+-free solution (IC50 values, i.e. the concentrations needed to induce 50% inhibition, were 252.8 and 174.9 microM for tetrandrine and isotetrandrine, respectively), the spontaneous contractile response elicited by extracellular calcium after depletion of noradrenaline-sensitive intracellular stores (increase in resting tone; IC50 values 11.6 and 19.6 microM for tetrandrine and isotetrandrine, respectively) and the refilling of intracellular Ca2+ stores sensitive to noradrenaline (IC50 values 7.4 and 14.9 microM for tetrandrine and isotetrandrine, respectively). The results show that tetrandrine and isotetrandrine interact with alpha1-adrenoceptors by displacing the [3H]prazosin binding site and that both compounds inhibit mainly the Ca2+-dependent process and have less action on alpha1-adrenoceptors. Tetrandrine is more potent than isotetrandrine.

Ouabain-induced increase in dopamine release from mouse striatal slices is antagonized by riluzole

We have examined the effects of riluzole, a neuroprotective drug which stabilizes voltage-dependent sodium channels in their inactivated state and inhibits the release of glutamate in-vivo and in-vitro, on the release of newly taken up [3H]dopamine induced by ouabain, a potent and selective inhibitor of Na+/K+-ATPase in mouse striatal slices in-vitro. Riluzole potently (IC50 (concentration resulting in 50% inhibition) = 0.9+/-0.3 microM) and dose-dependently antagonized ouabain-stimulated [3H]dopamine release, the effect being observed at low concentrations. Tetrodotoxin (1 microM) and nomifensine (10 microM) also abolished ouabain-induced [3H]dopamine release. Blockade of glutamate receptors with dizocilpine (1 microM) and 6-(1H-imidazol-1-yl)-7-nitro-2,3(1H,4H)-quinoxalinedione (YM-90K; 10 microM), alone or in combination, was without effect. Incubation of striatal slices with 50 microM La3+, which blocks voltage-dependent calcium channels, did not inhibit [3H]dopamine release induced by ouabain. The potent effects of riluzole observed in this model are probably related to its ability to block voltage-dependent sodium channels. The consequences of this activity are critically discussed in relation to the protective action of riluzole previously reported in various models of Parkinson's disease and other neurodegenerative disorders.

Chronic infusion of tritiated thymidine into mice with tumors

A technique for the chronic infusion of mice is presented. Infusion of tritiated thymidine for 3 to 7 days labels most, but not all, of the cells of autochthonous breast tumors. Many of the unlabeled tumor cells are probably nonproliferating, but some may represent cells with exceptionally long intermitotic times.

Stabilization of radical intermediates by an active-site tyrosine residue in methylmalonyl-CoA mutase

The adenosylcobalamin-dependent methylmalonyl-CoA mutase catalyzes the reversible rearrangement of methylmalonyl-CoA into succinyl-CoA by a free-radical mechanism. The recently solved X-ray crystal structure of methylmalonyl-CoA mutase from Propionibacterium shermanii has shown that tyrosine 89 is an active-site residue involved in substrate binding. The role of tyrosine 89, a conserved residue among methylmalonyl-CoA mutases, has been investigated by using site-directed mutagenesis to replace this residue with phenylalanine. The crystal structure of the Tyr89Phe mutant was determined to 2.2 A resolution and was found to be essentially superimposable on that of wild-type. Mutant and wild-type enzyme have very similar KM values, but kcat for the Tyr89Phe mutant is 580-fold lower than for wild-type. The rate of release of tritium from 5'-[3H]adenosylcobalamin during the enzymatic reaction and its rate of appearance in substrate and product were measured. The tritium released was found to partition unequally between methylmalonyl-CoA and succinyl-CoA, in a ratio of 40:60 when the reaction was initiated by addition of methylmalonyl-CoA and in a ratio of 10:90 when the reaction was initiated by addition of succinyl-CoA. The overall release of tritium was four times faster when succinyl-CoA was used as substrate. The tritium isotope effect on the enzyme catalyzed hydrogen transfer, measured with methylmalonyl-CoA as a substrate, was kH/kT = 30, which is within the expected range for a full primary kinetic tritium isotope effect. The different partitioning of tritium, dependent upon which substrate was used, and the normal value for the kinetic tritium isotope effect contrast markedly with the behavior of wild-type mutase. It appears that the loss of a single interaction involving the hydroxyl group of tyrosine 89 both affects the stability of radical intermediates and decreases the rate of interconversion of the substrate- and product-derived radicals.

Inhibition of renal 11beta-hydroxysteroid dehydrogenase in vivo by carbenoxolone in the rat and its relationship to sodium excretion

1. The type 2 isoform of 11beta-hydroxysteroid dehydrogenase, an enzyme which converts cortisol or corticosterone to inactive 11-ketosteroid metabolites, is thought to be responsible for preventing access of endogenous glucocorticoids to mineralocorticoid receptors in the distal nephron; although direct in vivo evidence for this is still lacking. We have examined whether graded inhibition of renal 11beta-hydroxysteroid dehydrogenase activities in vivo results in corresponding changes in urinary electrolyte excretion due to exposure of mineralocorticoid receptors to circulating endogenous glucocorticoids.2. Anaesthetized rats were infused intravenously with vehicle alone or with one of three doses of carbenoxolone: 0.06, 0.6 or 6 mg/h. After measurement of renal electrolyte excretion, the kidneys were snap-frozen in liquid nitrogen and 11beta-hydroxysteroid dehydrogenase activities were measured directly by enzyme assay in the presence of NAD+ or NADP+.3. A dose-dependent inhibition of renal 11beta-hydroxysteroid dehydrogenase activities was observed: the low, intermediate and high doses of carbenoxolone causing approximately 50%, 80% and >90% inhibition respectively. Only with the high dose was an effect on renal function observed (decreased fractional Na+ excretion and urinary Na+/K+ ratio).4. The poor correlation between the extent of inhibition of renal 11beta-hydroxysteroid dehydrogenase and altered urinary Na+ excretion, apparent at the lower doses of carbenoxolone, suggests either that 11beta-hydroxysteroid dehydrogenase has considerable functional reserve, or that it may not be the only mechanism determining mineralocorticoid receptor specificity in the distal nephron.

Presynaptic imidazoline receptors mediate inhibition of noradrenaline release from sympathetic nerves in rat blood vessels

In rat vena cava and aorta preincubated with [3H]noradrenaline the involvement of imidazoline receptors in modulation of [3H]noradrenaline release from sympathetic nerves was investigated. In the vena cava, the guanidine 1,3-di(2-tolyl)guanidine (DTG) inhibited the electrically evoked [3H]noradrenaline release; the inhibitory effect was more pronounced in the presence than in the absence of the alpha 2-adrenoceptor antagonist rauwolscine. The concentration-response curves of BDF 6143 [4-chloro-2-(2-imidazolin-2-ylamino)-isoindoline], and idazoxan for their facilitatory effect on electrically evoked [3H]noradrenaline release was bell-shaped; in the presence of rauwolscine, BDF 6143 inhibited the evoked [3H]noradrenaline release, whereas idazoxan did not. After blockade of alpha 2-autoreceptors by rauwolscine, the electrically evoked [3H]noradrenaline release from vena cava was inhibited not only by DTG and BDF 6143 but also by aganodine, clonidine and cirazoline; the rank order of potency of most of the drugs was similar to that found at the presynaptic imidazoline receptors in the rabbit aorta and pulmonary artery as well as in human atrial appendages. In the presence of rauwolscine, clonidine-induced inhibition of electrically evoked [3H]noradrenaline release was counteracted by 1 microM of the selective CB1 receptor antagonist SR141716A (N-[piperidin-1-yl]-5-[4-chlorophenyl]-1-[2,4-dichlorophenyl] -4-methyl-1H-pyrazole-3-carboxamide). In the aorta, BDF 6143 and cirazoline did not modify [3H]noradrenaline release in the absence of alpha 2-adrenoceptor blockade; in the presence of rauwolscine, the electrically evoked [3H]noradrenaline release from aorta was inhibited by BDF 6143, cirazoline, aganodine and clonidine with a rank order of potency similar to that in the vena cava. SR141716A 1 microM antagonized the inhibitory effect of BDF 6143 and clonidine (in the presence of rauwolscine). In conclusion, noradrenaline release in rat vena cava and aorta is inhibited via presynaptic imidazoline receptors which appear to be related to those previously characterized in rabbit and human cardiovascular tissue.

Short-term insulin-induced glycogen formation in primary hepatocytes as a screening bioassay for insulin action

We describe a novel bioassay to measure specific insulin-like activity in primary cultures of rat hepatocytes by determination of [3H]glycogen from d-[6-3H]glucose. The dose-response curve of insulin in this assay exhibited an EC50 of 0.42 (+/-0.04) nM, which is comparable to the dissociation constant of insulin from its receptor in hepatocytes. We used this assay to examine possible residual insulin-like activity of the four major fragments formed upon insulin degradation by insulin protease. Fragments A1-13B1-9, A1-14B1-9,and A14-21B14-30 showed no measurable activity. Although preparations of fragment A14-21B10-30 displayed dose-dependent agonist activity with an EC50 of 380 (+/-40) nM, we conclude that this was due to an insulin-like impurity since the chemically synthesized fragment showed no such activity. In summary, this bioassay demonstrates the action of insulin on glycogen formation in hepatocytes and provides a rapid and sensitive measurement of insulin-like activity which could facilitate screening studies.

In vitro induction of nitric oxide by fructose-1,6-diphosphate in the cardiovascular system of rats

Nitric oxide (NO) functions as a cellular messenger in a number of organs and cell systems in the cardiovascular system (CVS); it is a significant determinant of basal vascular tone and regulates myocardial contractility and platelet aggregation. The present study focused upon understanding the in vitro effects of fructose-1,6-diphosphate (FDP) on the rat cellular NO pathway. The iNOS activity was measured by monitoring the formation of (3H)-citrulline in 50,000 g soluble fractions of crude homogenates of endothelial (ET) and smooth muscle cells (SMC) from the arteries of rats, and macrophages (MAC) and lymphocytes (LYM) from rat blood. FDP in concentrations of 10-1000 microM stimulated rat cellular iNOS activity in a concentration-dependent manner. FDP-stimulated rat cellular iNOS was found to be completely reversed by 5 microM concentration of NG-monomethyl-L-arginine (L-NMMA), the potent mammalian NOS inhibitor. These studies demonstrated that FDP may induce the formation of NO by stimulating rat cardiovascular iNOS activity.

Detection of dopamine-tissue adducts

Incubation of tritium labeled dopamine (3HDA) with a preparation of rat heart or bovine serum albumin in vitro, or the intravenous injection of 3HDA to rats, gives acid insoluble, tritium labeled material that is slowly converted to soluble labeled material (SLM) by heating in dilute perchloric acid. Determination of SLM may be useful in investigations of DA-tissue adducts formed when the catecholamine is delivered by intravenous infusion.

Evaluation of a novel diffusion cell for in vitro transdermal permeation: effects of injection height, volume and temperature

The objective of this study was to evaluate the performance of a new, compact, dynamic diffusion cell for in vitro transdermal permeation. These so-called Kelder-cells were developed as an automated alternative to the static Franz diffusion cells. The new cells were used in combination with the ASPEC-system (automatic sample preparation with extraction columns) which was initially designed for the automation of solid-phase extractions. Three variables were tested to optimize the performance of the new cell system: injection height into the inlet compartment, volume flowing through the receptor compartment and temperature. Experiments were performed using the tritium labelled anticholinergic [3H]dexetimide permeating through an artificial membrane (Silastic). The injection height of the needle into the inlet compartment of the cell should be programmed at -34 mm to ensure complete air tightness, thus forcing the buffer to flow through the cell. The volume of buffer flow through the receptor compartment is important in maintaining sink conditions: a volume of 117 microliters was chosen to replace the total content of the cell (84 microliters) every 2 min. The temperature was precisely controlled in a thermostatic cabinet to minimize variations in experimental conditions. For [3H]dexetimide, an increase in temperature of 20 degrees C reduced the lag time by a factor of approximately two, however the influence on the flux was negligible. The data for the Kelder-cells were comparable with static Franz diffusion cells at a pseudo-steady state, however Kelder-cells have the advantage of automatic sampling, continuous replacement of the receptor solution, and unattended operation over at least 24 h.

Beta-glucuronidase is not required for transfer of [3H]-estrone-[14C]glucuronide across guinea pig fetal membranes

To understand the means whereby a charged, estrogen conjugate may be transferred across guinea pig amnion and chorion, the permeability to [3H]estrone-[14C]glucuronide was examined at 45 days and near term. No evidence of deconjugation was obtained in either early or late amnion, despite significantly greater transfer near term. Early amnion was virtually impermeable, regardless of ATP depletion. In contrast, early chorion transferred estrone-glucuronide without any requirement for deconjugation or ATP. No effect of tissue orientation was observed in amnion; whereas, incubations from maternal to fetal side of late chorion exhibited beta-glucuronidase activity. Inhibition of the latter demonstrated that hydrolysis was concomitant with but not required for transport. [3H]Estrone produced by deconjugation was enzymatically reduced after pubic symphysis relaxation, although beta-glucuronidase activity began prior to this stage. Transport across late fetal membranes was not saturable and chorion incubations from maternal to fetal side demonstrated a lower transport capacity. In either tissue orientation, late chorion displayed a lower rate of transfer than amnion. These results indicate that fetal membranes possess distinct abilities for transferring intact estrone-glucuronide, depending on stage of development and tissue orientation. The passive nature of transport and its dependence on structural characteristics is consistent with possible regulation of tight junctions.

Transient expression of the vesicular monoamine transporter during development in the rat thalamus and cortex

The postnatal developmental pattern of the central vesicular monoamine transporter-2 (VMAT2) was analyzed in the rat brain by means of quantitative autoradiography with a specific and high affinity ligand [3H]dihydrotetrabenazine ([3H]TBZOH). We show a dense expression of VMAT2 in the cortex (especially area 17) and thalamus (particularly the dorsal lateral geniculate nucleus) at postnatal days 1 and 8. This pattern of VMAT2 distribution was transient since it was no longer observed at day 20 or in the adult rat brain where VMAT2 density was weak and uniform in these regions. These data suggest that monoamine vesicular storage participates in the early postnatal maturation of thalamus and cortex.

Pharmacological characterisation of the recombinant human CRF binding protein using a simple assay

We present the pharmacological characterisation of the recombinant human corticotropin releasing factor binding protein (hCRF-BP) using a simple assay. In this assay we employed [3H]urocortin as the radioligand and, as a means to separate bound and free ligand, adsorption to activated charcoal. Using this method, approximately 60-70% of total binding was specific. Kinetic analysis revealed that association of specific [3H]urocortin binding was monophasic and slow and that the binding was irreversible. Saturation analysis showed a single saturable site of relatively high density (94 fmol per 10 microl of medium from cells transfected with the recombinant CRF binding protein). The apparent Kd for [3H]urocortin binding of 0.25 nM is similar to previously reported affinities of rat urocortin for hCRF-BP. A range of CRF-related peptides potently competed for specific [3H]urocortin binding. The rank order of potency of these agents was human/rat CRF = urotensin 1 > human urocortin > CRF6-33 > sauvagine > ovine CRF. The non-peptide CRF1 receptor antagonists CP 154,526 (N-butyl-N-[2,5-dimethyl-7-(2,4,6-trimethylphenyl)-7H-pyrrolo[2,3-d]p yri midin-4-yl]-N-ethylamine) and SC 241 ([3-(2-bromo-4-isopropyl-phenyl)-5-methyl-3H-[1,2,3]triazo lo[4,5-d]pyrimidin-7-yl]-bis-(2-methoxy-ethyl)-amine) were not active at the highest concentration tested (10(-6) M). We conclude that this is a simple and accurate assay for characterisation of the pharmacology of the recombinant CRF-BP. This assay should assist with further study of the pharmacology and function of the CRF-BP.

Altered turnover of calcium regulatory proteins of the sarcoplasmic reticulum in aged skeletal muscle

We have measured the in vivo protein turnover for the major calcium regulatory proteins of the sarcoplasmic reticulum from the skeletal muscle of young adult (7 months) and aged (28 months) Fischer 344 rats. From the time course of the incorporation and decay of protein-associated radioactivity after a pulse injection of [14C]leucine and correcting for leucine reutilization, in young rats, the apparent half-lives for calsequestrin, the 53-kDa glycoprotein, and ryanodine receptor are 5.4 +/- 0.4, 6.3 +/- 1.3, and 8.3 +/- 1.3 days, respectively. A half-life of 14.5 +/- 2.5 days was estimated for the Ca-ATPase isolated from young muscle. Differences in protein turnover associated with aging were determined using sequential injection of two different isotopic labels ([14C]leucine and [3H]leucine) to provide an estimate of protein synthesis and degradation within the same animal. The Ca-ATPase and ryanodine receptor isolated from aged muscle exhibits 27 +/- 5% and 25 +/- 3% slower protein turnover, respectively, relative to that from young muscle. In contrast, the 53-kDa glycoprotein exhibits a 25 +/- 5% more rapid turnover in aged SR, while calsequestrin exhibits no age-dependent alteration in turnover. Statistical analysis comparing the sensitivity of various methods for discriminating different rates of protein turnover validates the approach used in this study and demonstrates that the use of two isotopic labels provides at least a 6-fold more sensitive means to detect age-related differences in protein turnover relative to other methods.

Direct utilization of mannose for mammalian glycoprotein biosynthesis

Direct utilization of mannose for glycoprotein biosynthesis has not been studied because cellular mannose is assumed to be derived entirely from glucose. However, animal sera contain sufficient mannose to force uptake through glucose-tolerant, mannose-specific transporters. Under physiological conditions this transport system provides 75% of the mannose for protein glycosylation in human hepatoma cells despite a 50- to 100-fold higher concentration of glucose. This suggests that direct use of mannose is more important than conversion from glucose. Consistent with this finding the liver is low in phosphomannose isomerase activity (fructose-6-P<->mannose-6-P), the key enzyme for supplying glucose-derived mannose to the N-glycosylation pathway. [2-3H] Mannose is rapidly absorbed from the intestine of anesthetized rats and cleared from the blood with a t1/2of 30 min. After a 30 min lag, label is incorporated into plasma glycoproteins, and into glycoproteins of all organs during the first hour. Most (87%) of the initial incorporation occurs in the liver, but this decreases as radiolabeled plasma glycoproteins increase. Radiolabel in glycoproteins also increases 2- to 6-fold in other organs between 1-8 h, especially in lung, skeletal muscle, and heart. These organs may take up hepatic-derived radiolabeled plasma glycoproteins. Significantly, the brain, which is not exposed to plasma glycoproteins, shows essentially no increase in radiolabel. These results suggest that mammals use mannose transporters to deliver mannose from blood to the liver and other organs for glycoprotein biosynthesis. Additionally, contrary to expectations, most of the mannose for glycoprotein biosynthesis in cultured hepatoma cells is derived from mannose, not glucose. Extracellular mannose may also make a significant contribution to glycoprotein biosynthesis in the intact organism.

In vitro transesterification of cocaethylene (ethylcocaine) in the presence of ethanol. esterase-mediated ethyl ester exchange esterase-mediated ethyl ester exchange

This study reports that cocaethylene undergoes an esterase-mediated ethyl ester exchange with ethanol, resulting in an increase in the apparent in vitro t1/2, compared with control conditions. Homogenized liver from male Sprague Dawley rats in pH 7.4 phosphate buffer was centrifuged at 9000g, and the resulting supernatant (S9) fraction was collected. Tubes containing the rat S9 fraction and 50 microM cocaethylene plus aqueous buffer (control), 50 mM ethanol, or 51. 3 mM 2H6-ethanol were incubated at 37 degrees C for 4 hr. Samples were collected from the incubation tubes at various times, extracted with a solid-phase extraction system, and assayed for cocaethylene and 2H5-cocaethylene by GC/MS. Concentration-time profiles were constructed and kinetic parameters were determined. The experiment was repeated in the presence of specific and nonspecific esterase inhibitors. Enzyme kinetic parameters were also determined. Cocaethylene underwent ethyl ester exchange, being converted to 2H5-cocaethylene in the presence of 2H6-ethanol. The average apparent in vitro t1/2 value for cocaethylene (13.0 +/- 1.4 min) incubated with the S9 fraction and buffer only was increased approximately 5-fold (67.8 +/- 0.3 min) in the presence of ethanol. Formation of 2H5-cocaethylene was totally blocked with the addition of bis-(p-nitrophenyl)phosphate but was unaffected by physostigmine. The intrinsic metabolite formation clearance of 2H5-cocaethylene from cocaethylene and 2H6-ethanol (1.92 +/- 0.03 microl/min.mg protein) was several times greater than the corresponding value for cocaethylene formation from cocaine and ethanol (0.94 +/- 0.01 microl/min.mg protein) or 2H6-ethanol (0.87 +/- 0.04 microl/min.mg protein).

Short-term effect of dexamethasone on fatty acid and cholesterol synthesis in isolated rat hepatocytes

The short-term effect of dexamethasone, a synthetic glucocorticoid, on both fatty acid and cholesterol synthesis has been investigated in rat hepatocyte cultures. Within 4h following hormone addition to the cultures, a noticeable stimulation of labelled acetate incorporation into fatty acids was observed. Similar behaviour was noticed when [3H]H2O was used as an independent index of the lipogenic activity. In the same cultures, however, cholesterol synthesis from both [14C]acetate or [3H]H2O was significantly reduced by dexamethasone addition. In these conditions, no significative variation of cholesterol synthesis starting from labelled mevalonate was observed.

H2 receptor-mediated facilitation and H3 receptor-mediated inhibition of noradrenaline release in the guinea-pig brain

The effect of histamine and related drugs on the tritium overflow evoked electrically (0.3 Hz) or by introduction of Ca2+ ions into Ca2+-free K+-rich (25 mmol/l) medium containing tetrodotoxin was studied in superfused guinea-pig brain cortex, cerebellum, hippocampus or hypothalamus slices and in mouse brain cortex slices preincubated with 3H-noradrenaline. The electrically evoked tritium overflow in guinea-pig cortex slices was inhibited by histamine; the H3 receptor antagonist clobenpropit reversed the effect of histamine to a slight facilitation. The facilitatory effect of histamine (obtained in the presence of clobenpropit) was not affected by the H1 receptor antagonist mepyramine but abolished by the H2 receptor antagonist ranitidine. In the absence of clobenpropit, ranitidine augmented the inhibitory effect of histamine. In slices superfused in the presence of ranitidine, the evoked overflow was inhibited by histamine and, more potently, by the H3 receptor agonist R-alpha-methylhistamine in a concentration-dependent manner (maximum inhibitory effect obtained for both agonists 30-35%). The concentration-response curve of histamine was shifted to the right by the H3 receptor antagonist thioperamide. R-alpha-methylhistamine inhibited the electrically evoked tritium overflow also in guinea-pig cerebellar, hippocampal and hypothalamic slices. In cortex slices superfused in the presence of clobenpropit, the H2 receptor agonists impromidine and, less potently, R-sopromidine facilitated the evoked overflow in a concentration-dependent manner. S-Sopromidine only tended to increase the evoked overflow. The effect of impromidine was counteracted by the H2 receptor antagonists ranitidine and cimetidine. The extent of the maximum facilitatory effect of impromidine (by 15-20%) was about the same when (i) the Ca2+ concentration in the medium was reduced from 1.3 to 0.98 mmol/l, (ii) the time of exposure to impromidine was reduced from 28 to 8 min or (iii) cerebellar, hippocampal or hypothalamic slices were used instead of cortical slices. The Ca2+-induced tritium overflow in guinea-pig cortex slices was inhibited by histamine (in the presence of ranitidine); this effect was abolished by clobenpropit. In slices superfused in the presence of clobenpropit, impromidine failed to facilitate the Ca2+-evoked tritium overflow. The electrically evoked tritium overflow in mouse brain cortex slices was inhibited by histamine by about 60% (both in the absence or presence of ranitidine). The inhibitory effect of histamine was abolished (but not reversed) by clobenpropit. In conclusion, noradrenaline release in the guinea-pig brain cortex is inhibited via presynaptic H3 receptors and facilitated via H2 receptors not located presynaptically. In the mouse brain cortex, only inhibitory H3 receptors occur. The extent of the H3 receptor-mediated effect is more marked in the mouse than in the guinea-pig brain cortex.