Preparation of phosphatidyl[2-3H]inositol from yeast grown in medium containing myo[2-3H]inositol

Phosphatidyl[2-3H]inositol was prepared from Saccharomyces cerevisiae (YSC-2), grown in synthetic medium containing myo[2-3H]inositol. Over 44 microCi (or 81%) of the radiolabeled inositol was taken up by the organism, with 34 microCi incorporated into phosphatidylinositol. Upon purification by silicic acid pressure liquid chromatography (MPLC), a final yield of 24 to 26 microCi of phosphatidyl[2-3H]inositol with a specific radioactivity of 40 X 10(3) dpm/nmole was obtained. The purified phosphatidyl[2-3H]inositol was found to be a suitable for phospholipase C from human platelets.     

Characterization of glycopeptides labelled from D-[2-3H]mannose and L-[6-3H]fucose in intestinal epithelial cell membranes during differentiation.

The labelled glycopeptides obtained by Pronase digestion of rat intestinal epithelial cell membranes were examined by gel filtration after injection of D-[2-3H]mannose and L-[6-3H]fucose. Three labelled fraction were eluted in the following order from Bio-Gel P-6, Fraction I, which was excluded from the gel, was labelled mostly with [3H]fucose and slightly with [3H]mannose. Fraction II contained "complex" asparagine-linked oligosaccharides since it was labelled with [3H]mannose and [3H]fucose, was stable to mild alkali treatment, and resistant to endo-beta-N-acetyl-glucosaminidase H. Fraction III contained "high-mannose" asparagine-linked oligosaccharides, which were labelled with [3H]mannose, but not with [3H]fucose; these were sensitive to endo-beta-N-acetylglucosaminidase H, and were adsorbed on concanavalin A-Sepharose and subsequently eluted with methyl alpha-D-mannopyranoside. The time course of incorporation of [3H]mannose into these glycopeptides in microsomal fractions showed that high-mannose oligosaccharides were precursors of complex oligosaccharides. The rate of this processing was faster in rapidly dividing crypt cells than in differentiated villus cells. The ratio of radioactively labelled complex oligosaccharides to high-mannose oligosaccharides, 3h after [3H]mannose injection, was greater in crypt than in villus-cell lateral membranes. Luminal membranes of both crypt and villus cells were greatly enriched in labelled complex oligosaccharides compared with the labelling in lateral-basal membranes. These studies show that intestinal epithelial cells are polarized with respect to the structure of the asparagine-linked oligosaccharides on their membrane glycoproteins. During differentiation of these cells quantitative differences in labelled membrane glycopeptides, But no major qualitative change, were observed.     

Metabolism of [3-13C]pyruvate in TCA cycle mutants of yeast.

The utilization of pyruvate and acetate by Saccharomyces cerevisiae was examined using 13C and 1H NMR methodology in intact wild-type yeast cells and mutant yeast cells lacking Krebs tricarboxylic acid (TCA) cycle enzymes. These mutant cells lacked either mitochondrial (NAD) isocitrate dehydrogenase (NAD-ICDH1),alpha-ketoglutarate dehydrogenase complex (alpha KGDC), or mitochondrial malate dehydrogenase (MDH1). These mutant strains have the common phenotype of being unable to grow on acetate. [3-13C]-Pyruvate was utilized efficiently by wild-type yeast with the major intermediates being [13C]glutamate, [13C]acetate, and [13C]alanine. Deletion of any one of these Krebs TCA cycle enzymes changed the metabolic pattern such that the major synthetic product was [13C]galactose instead of [13C]glutamate, with some formation of [13C]acetate and [13C]alanine. The fact that glutamate formation did not occur readily in these mutants despite the metabolic capacity to synthesize glutamate from pyruvate is difficult to explain. We discuss the possibility that these data support the metabolon hypothesis of Krebs TCA cycle enzyme organization.     

Metabolism of [U-14C, 2-3H] glucose in rat liver during non-recirculating perfusion: effects of insulin

Livers from fed male rats were perfused in a non-recirculating manner with undiluted blood containing either 6 or 13 mM [U-14C,2-3H] glucose. At the lower concentration there was a small output of glucose which was unaffected by insulin whereas at the high concentration there was a substantial uptake of glucose which was significantly increased by the hormone. The rate of metabolism of [2-3H] glucose was greater than that of [U-14C] glucose in all experiments indicating an active substrate cycle between glucose:glucose 6-phosphate. Cycling was unaffected by insulin at the lower glucose concentration but was increased by perfusion with 13 mM glucose, the latter increase being abolished by insulin. These data show that although the perfused liver acts to autoregulate blood glucose, this is not achieved solely at the substrate cycle glucose:glucose 6-phosphate.     

Underestimation of D-glucose phosphorylation as measured by 3H2O production from D-[2-3H]glucose

In rat pancreatic islets, tumoral islet cells (RINm5F line), parotid gland, and in human erythrocytes, but not in rat hepatocytes, the production of 3H2O from D-[2-3H]glucose is 20-30% lower than from D-[5-3H]glucose. This coincides with the production of tritiated lactic acid from D-[2-3H]glucose and may be attributable to an intramolecular hydrogen transfer in the phosphoglucoisomerase reaction. It is concluded that the production of 3H2O from D-[2-3H]glucose is not a reliable tool to assess the total rate of hexose phosphorylation.     

Transport of 2-deoxy-d-[3H]glucose in microvessels isolated from bovine cerebral cortex

Microvessels were isolated from a bovine cortex and the transport of glucose was investigated by using 2-deoxy-d-[³H]glucose (2-DG). The apparentK _m for 2-DG transport was 118 M and therefore indicates a significant high affinity for the substrate. The inhibition of 2-DG uptake byd-glucose showed an apparentK _i of 222 M. Other sugars, e.g., 3-methyl-d-glucose andd-fructose, also inhibited the 2-DG uptake by 60.6 and 36.0%, respectively. Phloretin (1×10^–3 M) inhibited the 2-DG transport more than phlorizin (83.7 vs. 53.8%). Ouabain (1 and 5×10^–4 M) did not inhibit the uptake of 2-DG but 2,4-dinitrophenol (1×10^–4 M) did (78.0%). The uptake of 2-DG could not be demonstrated in homogenized microvessels. Adenine nucleotides (conc. 2 mM) had various effects on the 2-DG uptake by microvessels. ATP inhibited the uptake by 20.7%, ADP was virtually without effect, and AMP stimulated the uptake of 2-DG by 8.5%. It was also found that the decrease of adenylate energy charge favors the uptake of 2-DG. All these findings suggest that in cerebral microvessels of a bovine cortex, 2-DG is apparently transported by a specific, carrier-mediated transport system.Dedicated to Prof. Dr. R. Sammet on the occasion of his 60th birthday.     

H2 histaminic receptors in rat cerebral cortex. 3. Inhibition of [3H]histamine by H2 agonists

The binding of [3H]histamine to H2 receptors in homogenates of rat cerebral cortex is inhibited by 11 H2 agonists in a characteristic and unique manner. At low concentrations of the radioligand (less than 1.5 nM), the inhibitory profiles of individual agonists (A) are distinctly biphasic; specific binding is well described in most cases by the empirical expression Y = F1K1/(K1 + [A]) + F2K2/(K2 + [A]), in which F1 and F2 sum to 1. Maximal inhibition is the same for all agonists. Since values of F2 vary from 0.42 to 0.90, the agonist appears to determine the equilibrium distribution of receptors between two states of affinity. Ratios of apparent affinity (K2/K1) vary from 204 to 3 090 000, and there is no correlation between values of K1 and K2. Compounds lacking H2 activity, including structural analogues of histamine and dimaprit, reveal a Hill coefficient of 1 and inhibit the radioligand only weakly. For six agonists, values of K2 agree and correlate well (P = 0.00047) with H2 pharmacological potency (EC50) in the guinea pig right atrium; for the others, K2 is less than EC50 by 15-61-fold. Four observations suggest that the inhibition corresponding to F1 is allosteric and cooperative: the dissociation constant of the radioligand appears to vary in the presence of an unlabeled agonist, absolute levels of binding corresponding to F1, as defined by dimaprit, decrease at higher concentrations of [3H]histamine, F1 for dimaprit is reduced from 0.48 to 0.32 by 2-methylhistamine (F1 = 0.27) at a concentration of 20 nM (approximately K1(0.5) K2(0.5) for 2-methylhistamine), but the increase in K1 for dimprit is at least 100-fold less than expected from competitive effects, and 1 equiv of some agonists appears to preclude access of [3H]histamine to more than 1 equiv of receptors, with no evidence that an appreciable fraction of the unlabeled drug is bound. Noncompetitive effects also may account in part for the inhibition corresponding to F2.     

In vivo conversion of [3H]myoinositol to [3H]chiroinositol in rat tissues.

We report here the in vivo conversion of [3H]myoinositol to [3H]chiroinositol. After labeling intraperitoneally with [3H]myoinositol for 3 days to reach radioisotope equilibrium in urine, [3H]chiroinositol was isolated from tissues and purified after 6 N HCl hydrolysis by two sequential paper chromatographies and high performance liquid chromatography (HPLC). Percent conversion of [3H]myoinositol to [3H]chiroinositol was highest in urine (36%), liver (8.8%), muscle (8.8%), and blood (7.6%) with intestine, brain, kidney, spleen, and heart decreasing in percentage from 2.8 to 0.7%. Labeling of other inositol isomers including scyllo-, neo-, and epi-, and mucoinositol was minimal, approximately 0.06% of [3H]myoinositol. Glucose was unlabeled, but glucuronate, the product of myoinositol oxidation, was labeled up to 1.5% of the [3H] myoinositol. Acid hydrolysates of combined inositol-containing phospholipids contain significant labeled chiroinositol. [3H]Phosphatidylinositols and [3H]glycosylphosphatidylinositols were extracted from liver, muscle, and blood, isolated by thin layer chromatography, and inositols purified by HPLC after acid hydrolysis. Percent conversion of [3H]myoinositol to [3H] chiroinositol was highest in blood (60.4%) followed by muscle (7.7%) and liver (2.2%).     

A method for [3H]mannose labeling of Asn-linked oligosaccharides on recombinant glycoproteins synthesized in Xenopus oocytes.

We have developed an efficient method for labeling the Asn-linked oligosaccharides of recombinant glycoproteins synthesized in Xenopus laevis oocytes. By coinjecting GDP-[3,4-(3)H]mannose with mRNA for human cathepsin D, it was possible to incorporate as much as 1800 cpm per oocyte into each of the two Asn-linked oligosaccharides of this glycoprotein. Overall, about 50% of the microinjected GDP-[3,4-(3)H]mannose was incorporated into Asn-linked oligosaccharides, a 10-fold greater value than that obtained when [2-(3)H]mannose was microinjected. Less than 10% of the injected GDP-[3,4-(3)H]mannose was metabolized to water or converted to amino acids. This technique should facilitate studies of Asn-linked oligosaccharide biosynthesis, processing, and structure in recombinant proteins synthesized in Xenopus oocytes.     

Metabolism and actions of 2-deoxy-2-fluoro-D-galactose in vivo

The synthetic D-galactose analog 2-deoxy-2-fluoro-D-galactose (dGalF) offers unique advantages for studies of the D-galactose pathway by non-invasive techniques using 19F-NMR spectroscopy or positron emission from the 18F-labeled compound. The metabolism of 2-deoxy-2-fluoro-D-galactose was studied in rodents using the unlabeled, the 18F-labeled, and the 14C-labeled D-galactose analog. Analyses for the metabolites of 2-deoxy-2-fluoro-D-galactose were performed by HPLC, enzymatic methods, and 19F-NMR spectroscopy in vivo and in vitro. The metabolism of 2-deoxy-2-fluoro-D-galactose was most active in the liver which took up the major part of the administered dose of the 14C-labeled D-galactose analog, but renal excretion was also pronounced. This was confirmed by in vivo scanning of the rat using the 18F-labeled sugar (1.5 microCi/g; 25 nmol/g) and examination by positron-emission tomography and gamma camera. The dose dependence of the levels of the hepatic metabolites of 2-deoxy-2-fluoro-D-galactose was investigated for doses between 25 nmol/g body mass and 1 mumols/g body mass. After 1 h, the major part of the acid-soluble uracil nucleotides consisted of UDP-2-deoxy-2-fluoro-D-hexoses when the dose was at least 0.1 mumols/g. With higher doses, 2-deoxy-2-fluoro-D-galactose 1-phosphate became the predominant initial metabolite. After a dose of 1 mumols/g 2-deoxy-2-fluoro-D-galactose 1-phosphate accumulated rapidly (5.3 +/- 0.4 mumols/g liver after 30 min) followed by the formation of UDP-2-deoxy-2-fluoro-D-galactose and UDP-2-deoxy-2-fluoro-D-glucose (0.7 +/- 0.1 mumols/g and 1.8 +/- 0.1 mumols/g, respectively, after 5 h). The diversion of uridylate, due to the accumulation of UDP-2-deoxy-2-fluoro-D-hexoses, was associated with a rapid depletion of hepatic UTP, UDP-glucose, and UDP-galactose. The UTP content was decreased to 11 +/- 6% of normal within 15 min after administration of 2-deoxy-2-fluoro-D-galactose at a dose of 1 mumols/g. The UTP-depleting action was minimal, however, at a dose of 25 nmols/g or less, indicating that interference in uridylate metabolism would be negligible at the doses required for positron-emission tomography of the liver using the 18F-labeled compound. At higher doses, the UTP deficiency induced by 2-deoxy-2-fluoro-D-galactose could be useful in the chemotherapy of D-galactose-metabolizing tumors such as hepatocellular carcinoma.(ABSTRACT TRUNCATED AT 250 WORDS)     

H2 histaminic receptors in rat cerebral cortex. 2. Inhibition of [3H]histamine by H2 antagonists

Sites labeled by [3H]histamine in homogenates of rat cerebral cortex reveal a pharmacological specificity typical of H2 receptors. Fourteen H2 antagonists inhibit the specific binding of the radioligand to the same level; Hill coefficients are near or equal to one for five compounds and markedly lower for nine. The binding patterns of individual antagonists (A) are well described by the empirical expression Y = F1K1/(K1 + [A]) + F2K2/(K2 + [A]), in which F1 and F2 sum to 1; F2 is 0 for those drugs that reveal a Hill coefficient of 1. Concentrations of A that reduce specific binding by 50% (IC50) correlate well (r = 0.991; P less than 0.00001) and show good numerical agreement with potencies reported for inhibition of the response to histamine in H2-mediated systems. The correlation is poorer when IC50 is replaced by either K1 (r = 0.973) or K2 (r = 0.921) for those antagonists that reveal both; the antihistaminic activity of the drug thus appears not to be associated preferentially with one or other class of sites. Since F2 varies from 0.16 to 0.60 among those antagonists that discern heterogeneity, the antagonist appears to determine the distribution of sites between the two classes. Moreover, a correlation among antagonists between values of K1 and K2 (r = 0.975; P = 0.00001) suggests that the apparent heterogeneity reflects different conformers within an otherwise homogeneous population. H2 antagonists appear to be noncompetitive with respect to each other and to the radioligand: one antagonist has relatively little effect on the values of K1, K2, and F2 revealed by another; also, estimates of K1 and K2 are independent of the concentration of [3H]histamine between 1.3 and 10 nM, although the radioligand exhibits an apparent dissociation constant of 3.9 nM [Steinberg, G. H., Eppel, J. G., Kandel, M., Kandel, S. I., & Wells, J. W. (1985) Biochemistry (preceding paper in this issue)].     

Synthesis of methyl [18-2H3], [17-2H2], [16-2H2], [14-2H2] and [12-2H2] cis-9-octadecenoates

Methyl [17-²H₂]oleate was prepared by stepwise reduction from 17-oxooleate in 24% yield. Methyl [18-²H₃], [16-²H₂], [14-²H₂] and [12-²H₂] oleates were synthesized from appropriately deuterated octylbromides by conversion to deuterated 7-hexadecyn-1-ols and chain extention to deuterated stearolates followed by semihydrogenation; overall yields were about 17%.     

Synthesis of [11-2H2], [8-2H2], [7-2H2], [6-2H2], [5-2H2], [4-2H2] and [3-2H2] cis-9-octadecenoates

Deuterated oleates have been synthesized by semihydrogenation of acetylenic intermediates. [11-²H₂]Oleate was prepared by two-carbon chain extension of the C₁₆ alcohol obtained from [1-²H₂]octyl bromide and 7-octyn-1-ol. [8-²H₂] and [7-²H₂]oleates were both prepared from dimethyl suberate, tetradeutero intermediate C₁₆ alcohols were synthesized from [1,8-²H₄] and [2,7-²H₄]octane diols by monobromination, conversion to deuterated 9-decyn-1-ols and reaction with octyl bromide. Oxidation gave [8-²H₂]-9-octadecynoate and [2,7-²H₂]-9-octadecynoate, after semihydrogenation of the latter, deuterons at C-2 were removed by exchange with aqueous alkali. [6-²H₂] and [5-²H₂]oleates were obtained from methyl 5-tetradecynoate, semihydrogenation, deuterium exchange at C-2 and two malonate extensions gave [6-²H₂]oleate; reduction with lithium aluminum deuteride, two malonate extensions and semihydrogenation gave the [5-²H₂] ester. [4-²H₂] and [3-²H₂]oleates were both obtained from methyl 7-cis-hexadecenoate, exchange of the α protons and chain extension gave the [4-²H₂] ester and reduction with lithium aluminum deuteride and chain extension gave the [3-²H₂] ester.     

Fate of [G-3H]glutamate and [U-14C]glucose in the rat liver in vivo

• 1.1. After injection of a mixture of [G-³H]glutamate and [U-¹⁴C]glucose to rats, the highest amount of ¹⁴C was found in an unidentified compound (glycopeptide?) of the acid soluble extract of the liver at 2 min.
• 2.2. With increasing time after the injection the specific radioactivity of [³H]glutamate decreased and that of [³H]glutamine increased in the liver.
• 3.3. The labelling of the liver protein with ¹⁴C was due to [¹⁴C]glutamate and [¹⁴C]aspartate, and that with ³H was exclusively due to [³H]glutamate.