The mode of condensation of aspartic acid and dihydroxyacetone phosphate in quinolinate synthesis in escherichia coli

Dihydroxy[3-¹⁴C]acetone phosphate was prepared enzymatically from [1-¹⁴C]glucose and used as a substrate in a partially purified quinolinate synthetase system prepared from Escherichia coli mutants. Carbon-by-carbon degradation of the resulting [¹⁴C]quinolinate showed that 96% of the ¹⁴C was located in carbon-4, indicating that carbon-3 of dihydroxyacetone phosphate condenses with carbon-3 of aspartate in quinolinate synthesis in E. coli.     

Utilization of an Escherichia coli mutant for carbon-13 enrichment of tRNA for NMR studies.

The enrichment of tRNA at specific sites with carbon-13 has been accomplished in vivo using a mutant of Escherichia coli. A relaxed strain of E. coli auxotrophic for methionine was grown in a specifically defined medium supplemented with either [14C] or [13C]-methyl labeled methionine. Cells were collected at the end of the log-phase of growth and tRNA was extracted. Analysis of the radioactivity of the [14C]-labeled tRNA established an incorporation ratio of three labeled carbons per tRNA molecule. Incorporation of the [14C]-label in vivo was confined to the methylation of nucleotides as determined by thin layer chromatography of nucleotides resulting from a ribonuclease digestion of [14C]-labeled tRNA. The carbon-13 NMR spectrum of [13C]-enriched tRNA indicated a similar degree of incorporation into the methylated nucleotides by the substantial enhancement of [13C]-methyl NMR signals only. Assignment of signals has been made for the methyl groups of ribothymidine and N7-methylguanosine in E. coli tRNA.     

Carbon-13-enriched carbohydrates: preparation of triose, tetrose, and pentose phosphates.

Three-, four-, and five-carbon aldononitrile phosphates were prepared, purified, and catalyticlly reduced with palladium--barium sulfate (5%) to the corresponding aldose phosphates in high yields at pH 1.7 +/- 0.1 and atmopsheric pressure. DL-Glyceraldehyde 3-phosphate and the tetrose 4-phosphates were prepared with carbon-13 enrichment at C-1, while the pentose 5-phosphates were prepared with enrichment at C-1 and C-2. Preparations of glycolaldehyde phosphate and d-glyceraldehyde 3-phosphate by lead tetra-acetate oxidation of glycerol phosphate and fructose 6-phosphate, respectively, are described. The proportions of cyclic hemiacetals and linear gem-diol forms of the two- to five-carbon aldose phosphates in aqueous solution are reported. Carbon-13 chemical shifts and carbon--phosphorus and carbon--hydrogen coupling constants for the furanose phosphate ring and linear gem-diol phosphates are reported and discussed. d-[2(-13)C]Ribulose 1,5-bisphosphate and L-[3,4(-13)C]sorbose 1,6-bisphosphate were prepared enzymatically from D-[2(-13)C]ribose 5-phosphate and dl-[1(-13)C]glyceraldehyde 3-phosphate, respectively.     

Structure and functions of linkage unit intermediates in the biosynthesis of ribitol teichoic acids in Staphylococcus aureus H and Bacillus subtilis W23

The stepwise formation and characterization of linkage unit intermediates and their functions in ribitol teichoic acid biosynthesis were studied with membranes obtained from Staphylococcus aureus H and Bacillus subtilis W23. The formation of labeled polymer from CDP-[14C]ribitol and CDP-glycerol in each membrane system was markedly stimulated by the addition of N-acetylmannosaminyl(beta 1----4)N-acetylglucosamine (ManNAc-GlcNAc) linked to pyrophosphorylyisoprenol. Whereas incubation of S. aureus membranes with CDP-glycerol and ManNAc-[14C]GlcNAc-PP-prenol led to synthesis of (glycerol phosphate) 1-3-ManNAc-[14C]GlcNAc-PP-prenol, incubation of B. subtilis membranes with the same substrates yielded (glycerol phosphate)1-2-ManNAc-[14C]GlcNAc-PP-prenol. In S. aureus membranes, (glycerol phosphate)2-ManNAc-[14C]GlcNAc-PP-prenol as well as (glycerol phosphate)3-ManNAc-[14C]GlcNAc-PP-prenol served as an acceptor for ribitol phosphate units, but (glycerol phosphate)-ManNAc-[14C]GlcNAc-PP-prenol did not. In B. subtilis W23 membranes, (glycerol phosphate)-ManNAc-[14C]GlcNAc-PP-prenol served as a better acceptor for ribitol phosphate units than (glycerol phosphate)2-ManNAc-[14C]GlcNAc-PP-prenol. In this membrane system (ribitol phosphate)-(glycerol phosphate)-ManNAc-[14C]GlcNAc-PP-prenol was formed from ManNAc-[14C]GlcNAc-PP-prenol, CDP-glycerol and CDP-ribitol. The results indicate that (glycerol phosphate)1-3-ManNAc-GlcNAc-PP-prenol and (glycerol phosphate)1-2-ManNac-GlcNAc-PP-prenol are involved in the pathway for the synthesis of wall ribitol teichoic acids in S. aureus H and B. subtilis W23 respectively.     

Metabolism of propionate to acetate in the cockroach Periplaneta americana

Carbon-13 NMR and radiotracer studies were used to determine the precursor to methylmalonate and to study the metabolism of propionate in the cockroach Periplaneta americana. [3,4,5-13C3]Valine labeled carbons 3, 4, and 26 of 3-methylpentacosane, indicating that valine was metabolized via propionyl-CoA to methylmalonyl-CoA and served as the methyl branch unit precursor. Potassium [2-13C]propionate labeled the odd-numbered carbons of hydrocarbons and potassium [3-13C]propionate labeled the even-numbered carbons of hydrocarbons in this insect. This labeling pattern indicates that propionate is metabolized to acetate, with carbon-2 of propionate becoming the methyl carbon of acetate and carbon-3 of propionate becoming the carboxyl carbon of acetate. In vivo studies in which products were separated by HPLC showed that [2-14C]propionate was readily metabolized to acetate. The radioactivity from sodium [1-14C]propionate was not incorporated into succinate nor into any other tricarboxylic acid cycle intermediate, indicating that propionate was not metabolized via methylmalonate to succinate. Similarly, [1-14C]propionate did not label acetate. An experiment designed to determine the subcellular localization of the enzymes involved in converting propionate to acetate showed that they were located in the mitochondrial fraction. Data from both in vivo and in vitro studies as a function of time indicated that propionate was converted directly to acetate and did not first go through tricarboxylic acid cycle intermediates. These data demonstrate a novel pathway of propionate metabolism in insects.     

Biosynthesis of riboflavin. Enzymatic formation of 6,7-dimethyl-8-ribityllumazine from pentose phosphates

The xylene ring of riboflavin originates by dismutation of the precursor, 6,7-dimethyl-8-ribityllumazine. The formation of the latter compound requires a 4-carbon unit as the precursor of carbon atoms 6 alpha, 6, 7, and 7 alpha of the pyrazine ring. The formation of riboflavin from GTP and ribose phosphate by cell extract from Candida guilliermondii has been observed by Logvinenko et al. (Logvinenko, E. M., Shavlovsky, G. M., Zakal'sky, A. E., and Zakhodylo, I. V. (1982) Biokhimiya 47, 931-936). We have studied this enzyme reaction in closer detail using carbohydrate phosphates as substrates and synthetic 5-amino-6-ribitylamino-2,4-(1H,3H)-pyrimidinedione or its 5'-phosphate as cosubstrates. Several pentose phosphates and pentulose phosphates can serve as substrate for the formation of riboflavin with similar efficiency. The reaction requires Mg2+. Various samples of ribulose phosphate labeled with 14C or 13C have been prepared and used as enzyme substrates. Radioactivity was efficiently incorporated into riboflavin from [1-14C]ribulose phosphate, [3,5-14C]ribulose phosphate, and [5-14C]ribulose phosphate, but not from [4-14C]ribulose phosphate. Label from [1-13C]ribose 5-phosphate was incorporated into C6 and C8 alpha of riboflavin. [2,3,5-13C]Ribose 5-phosphate yielded riboflavin containing two contiguously labeled segments of three carbon atoms, namely 5a, 9a, 9 and 8, 7, 7 alpha. 5-Amino-6-[1'-14C] ribitylamino-2,4 (1H,3H)-pyrimidinedione transferred radioactivity exclusively to the ribityl side chain of riboflavin in the enzymatic reaction. It follows that the 4-carbon unit used for the biosynthesis of 6,7-dimethyl-8-ribityllumazine consists of the pentose carbon atoms 1, 2, 3, and 5 in agreement with earlier in vivo studies.     

alpha-Dihydrodecaprenyl phosphate as a sugar carrier in the membrane of AH 70Btc hepatoma cells

The effect of alpha-dihydrodecaprenyl phosphate, dolichyl phosphate and solanesyl phosphate on the lipid intermediate pathway for protein glycosylation was studied with crude membrane fraction prepared from AH 70Btc hepatoma cells. alpha-Dihydrodecaprenyl phosphate increased the incorporations of [14C]mannose from GDP-[14C]mannose into CHCl3-CH3OH (2:1, v/v) extract, oligosaccharide-lipid and proteins. The above and the other data showed that alpha-dihydrodecaprenyl phosphate may function as a mannose carrier in the lipid intermediate pathway.     

Oxalic acid biosynthesis and oxalacetate acetylhydrolase activity in Streptomyces cattleya

In addition to producing the antibiotic thienamycin, Streptomyces cattleya accumulates large amounts of oxalic acid during the course of a fermentation. Washed cell suspensions were utilized to determine the specific incorporation of carbon-14 into oxalate from a number of labeled organic and amino acids. L-[U-14C]aspartate proved to be the best precursor, whereas only a small percentage of label from [1,5-14C]citrate was found in oxalate. Cell-free extracts catalyzed the formation of [14C]oxalate and [14C]acetate from L-[U-14C]aspartate. When L-[4-14C]aspartate was the substrate only [14C]acetate was formed. The cell-free extracts were found to contain oxalacetate acetylhydrolase (EC 3.7.1.1), the enzyme that catalyzes the hydrolysis of oxalacetate to oxalate and acetate. The enzyme is constitutive and is analogous to enzymes in fungi that produce oxalate from oxalacetate. Properties of the crude enzyme were examined.     

Characterization of quinolinate synthases from Escherichia coli, Mycobacterium tuberculosis, and Pyrococcus horikoshii indicates that [4Fe-4S] clusters are common cofactors throughout this class of enzymes

Quinolinate synthase (NadA) catalyzes a unique condensation reaction between iminoaspartate and dihydroxyacetone phosphate, affording quinolinic acid, a central intermediate in the biosynthesis of nicotinamide adenine dinucleotide (NAD). Iminoaspartate is generated via the action of l-aspartate oxidase (NadB), which catalyzes the first step in the biosynthesis of NAD in most prokaryotes. NadA from Escherichia coli was hypothesized to contain an iron-sulfur cluster as early as 1991, because of its observed labile activity, especially in the presence of hyperbaric oxygen, and because its primary structure contained a CXXCXXC motif, which is commonly found in the [4Fe-4S] ferredoxin class of iron-sulfur (Fe/S) proteins. Indeed, using analytical methods in concert with Mossbauer and electron paramagnetic resonance spectroscopies, the protein was later shown to harbor a [4Fe-4S] cluster. Recently, the X-ray structure of NadA from Pyrococcus horikoshii was solved to 2.0 A resolution [Sakuraba, H., Tsuge, H.,Yoneda, K., Katunuma, N., and Ohshima, T. (2005) J. Biol. Chem. 280, 26645-26648]. This protein does not contain a CXXCXXC motif, and no Fe/S cluster was observed in the structure or even mentioned in the report. Moreover, rates of quinolinic acid production were reported to be 2.2 micromol min (-1) mg (-1), significantly greater than that of E. coli NadA containing an Fe/S cluster (0.10 micromol min (-1) mg (-1)), suggesting that the [4Fe-4S] cluster of E. coli NadA may not be necessary for catalysis. In the study described herein, nadA genes from both Mycobacterium tuberculosis and Pyrococcus horikoshii were cloned, and their protein products shown to contain [4Fe-4S] clusters that are absolutely required for activity despite the absence of a CXXCXXC motif in their primary structures. Moreover, E. coli NadA, which contains nine cysteine residues, is shown to require only three for turnover (C113, C200, and C297), of which only C297 resides in the CXXCXXC motif. These results are consistent with a bioinformatics analysis of NadA sequences, which indicates that three cysteines are strictly conserved across all species. This study concludes that all currently annotated quinolinate synthases harbor a [4Fe-4S] cluster, that the crystal structure reported by Sakuraba et al. does not accurately represent the active site of the protein, and that the "activity" reported does not correspond to quinolinate formation.     

The tritium isotope effect of sn-glycerol 3-phosphate oxidase and the effects of clofenapate and N-(2-benzoyloxyethyl)norfenfluramine on the esterification of glycerol phosphate and dihydroxyacetone phosphate by rat liver mitochondria

1. Owing to a (3)H isotope effect, the mitochondrial sn-glycerol 3-phosphate oxidase (EC 1.1.99.5) had a mean activity which was 8.4 times less with sn-[2-(3)H]-rather than with sn-[1-(14)C]glycerol 3-phosphate as a substrate. 2. A method for measuring the simultaneous synthesis of lipid from glycerol phosphate and dihydroxyacetone phosphate in rat liver mitochondria is described. 3. The lipid synthesized by rat liver mitochondria from sn-[1-(14)C]glycerol 3-phosphate was mainly phosphatidate and lysophosphatidate, whereas that synthesized from dihydroxy[1-(14)C]acetone phosphate was mainly acyldihydroxyacetone phosphate. 4. Additions of NADPH facilitated the conversion of acyldihydroxyacetone phosphate into lysophosphatidate and phosphatidate. 5. Hydrazine (1.4mm) or KCN (1.4mm) inhibited the synthesis of lipids from dihydroxyacetone phosphate but not from glycerol phosphate. 6. Clofenapate (1-2.5mm) inhibited the synthesis of lipids from dihydroxyacetone phosphate but slightly stimulated synthesis from glycerol phosphate. 7. The methanesulphonate of N-(2-benzoyloxyethyl)norfenfluramine, at 0.25-0.75mm, inhibited lipid synthesis from both glycerol phosphate and dihydroxyacetone phosphate.     

Synthesis of quinolinate from D-aspartate in the mammalian liver-Escherichia coli quinolinate synthetase system.

Two proteins (A and B) from $\text{Escherichia coli}$ are required for the synthesis of the NAD precursor quinolinate from aspartate and dihydroxyacetone phosphate. Mammalian liver contains a FAD linked protein which replaces $\text{E. coli}$ B protein for quinolinate synthesis. D-aspartic acid but not L-aspartic acid is a substrate for quinolinic acid synthesis in a system composed of the B protein replacing activity of mammalian liver and $\text{E. coli}$ A protein. In contrast the $\text{E. coli}$ B protein-$\text{E. coli}$ A protein quinolinate synthetase system requires L-aspartic acid as substrate. The previous report that L-aspartate was a substrate in the liver-$\text{E. coli}$ system was due to contamination of commercially available [¹⁴C]L-aspartate with [¹⁴C]D-aspartate. These and other observations suggest that liver B protein is D-aspartate oxidase and $\text{E. coli}$ B protein is L-aspartate oxidase.     

Nicotinate, quinolinate and nicotinamide as precursors in the biosynthesis of nicotinamide–adenine dinucleotide in barley

1. The relative efficiencies of nicotinate, quinolinate and nicotinamide as precursors of NAD(+) were measured in the first leaf of barley seedlings. 2. In small amounts, both [(14)C]nicotinate and [(14)C]quinolinate were quickly and efficiently incorporated into NAD(+) and some evidence is presented suggesting that NAD(+) is formed from each via nicotinic acid mononucleotide and deamido-NAD. 3. [(14)C]Nicotinamide served equally well as a precursor of NAD(+) and although significant amounts of [(14)C]NMN were detected, most of the [(14)C]NAD(+) was derived from nicotinate intermediates formed by deamination of [(14)C]nicotinamide. 4. Radioactive NMN was also a product of the metabolism of [(14)C]nicotinate and [(14)C]quinolinate but most probably it arose from the breakdown of [(14)C]NAD(+). 5. In barley leaves where the concentration of NAD(+) is markedly increased by infection with Erysiphe graminis, the pathways of NAD(+) biosynthesis did not appear to be altered after infection. A comparison of the rates of [(14)C]NAD(+) formation in infected and non-infected leaves indicated that the increase in NAD(+) content was not due to an increased rate of synthesis.     

Ethylene production by wounded tissue of citrus fruit

Segments cut from young immature fruits and albedo discs excisedfrom both immature and mature fruits of Satsuma mandarin ormature fruits of Natsudaidai produced much ethylene during incubationat 26?C in the dark. Ethylene formation was markedly acceleratedby the application of abscisic acid but markedly delayed by3,5-dibromo-4-hydroxybenzoic acid. Both the stimulation andretardation decreased greatly during the course of incubation.Both compounds seem to be associated with the early stages ofethylene formation by wounded citrus fruit tissues. Albedo discs were fed ¹⁴C methionine labeled at one of threedifferent positions. Of the three radioanalogs (carbon-2, carbon-3and methyl carbon), the label at the 3 position was preferentiallyincorporated into ethylene. This agrees with the former observationthat ethylene is derived from carbon-3 and -4 of methionine.Incorporation of label into ethylene from L-[3-¹⁴C] methioninewas strongly inhibited by L-canaline, L-ethionine, 2,4-dinitrophenoland cycloheximide. Ethylene evolution was also strongly inhibitedby 2,4-dinitrophenol, KCN, NaN₃ and cycloheximide, but lesscompletely by L-canaline and L-ethionine. These results supportthe view that ATP and pyridoxal phosphate are utilized in activationof methionine to form ethylene. (Received October 25, 1977; )     

Biosynthesis of 3'-deoxyadenosine by Cordyceps militaris. Mechanism of reduction.

The biosynthesis of 3'-deoxyadenosine (cordycepin) by Cordyceps militaris has been investigated using [U-14C]adenosine and [3-3H]ribose. Crystallization of the resulting radioactive 3'-deoxyadenosine to a constant specific activity showed incorporation of both labeled compounds. A control showed that the 3H:14C ratio of the AMP isolated from the RNA was the same as the 3H:14C ratio in the 3'-deoxyadenosine. The 14C ratio in the adenine: ribose of the [U-14C]adenosine added to the 3'-deoxyadenosine producing cultures of C. militaris and of the isolated 3'-deoxyadenosine was the same, e.g. 50:50. These data provide strong evidence that adenosine in converted to 3'-deoxyadenosine without hydrolysis of the N-riboside bond. Degradation of the 3-deoxyribose from 3'-deoxyadenosine showed that the 3H was retained on carbon-3. These results suggest that the formation of 3'-deoxyadenosine may proceed by a reductive mechanism similar to that for the formation of 2'-deoxynucleotides.     

Metabolism of radiolabeled glucose by mouse oocytes and oocyte-cumulus cell complexes.

This study was carried out to examine the metabolism of [1-14C]-, [6-14C]-, and [5-3H]glucose by oocyte-cumulus cell complexes (OCC) and denuded oocytes (DO) and to test the hypothesis that metabolism of glucose through the pentose phosphate pathway is associated with meiotic induction. OCC or DO were cultured in hanging drops suspended from the cap of a microfuge tube, with NaOH serving as a trap to collect released 3H2O or 14CO2. Preliminary experiments established that this culture system supports both spontaneous and ligand-induced meiotic maturation. An initial time course experiment (1.5-6 h) showed that hypoxanthine-treated OCC from eCG-primed animals metabolized glucose principally via glycolysis, with an increase to 2.7-fold in response to FSH. Though more [1-14C]glucose was oxidized than [6-14C]glucose, its metabolism was about two orders of magnitude less than that of [5-3H]glucose. Also, FSH significantly increased oxidation of [1-14C]glucose but not [6-14C]glucose, indicating a preferential activation of the pentose phosphate pathway. Pyrroline carboxylate, an activator of the pentose phosphate pathway, increased the activity of this pathway to over 2-fold but failed to affect glucose oxidation through the tricarboxylic acid cycle. Glycolytic metabolism was increased by 25%. The addition of pyruvate to pyruvate-free medium resulted in significant reduction in the metabolism of all three glucose analogues. In OCC retrieved from hCG-injected, primed mice and cultured under hormone-free conditions, metabolic responses were similar to those in FSH-treated complexes cultured in hypoxanthine. DO metabolized glucose, but at a much reduced rate when compared to OCC. Pyruvate reduced the consumption of all three glucose analogues by DO. Pyrroline carboxylate reduced [5-3H]glucose metabolism by DO but had little effect on [1-14C]- and [6-14C]glucose oxidation. These data demonstrate metabolism of glucose by both DO and OCC, but reveal that cumulus cells are more active than the oocyte in this regard. In addition, induction of maturation by FSH, hCG, or pyrroline carboxylate was accompanied by a significant increase in the oxidation of [1-14C]glucose but not [6-14C]glucose by OCC, supporting a proposed role for the pentose phosphate pathway in meiotic induction.     

Conversion of dolichyl pyrophosphate N,N'-diacetylchitobiose to lipid-tri- to heptasaccharides by liver microsomes from hibernating ground squirrels (Citellus citellus L.)

Incubation of liver microsomes from hibernating ground squirrel with GDP-[14C]mannose and exogenous dolichyl phosphate resulted in the synthesis of dolichyl phosphate [14C]mannose. The mannosyltransferase activity was about 3-fold higher in microsomes from hibernating ground squirrels than in those from active animals. Incubation for 30 min of liver microsomes from hibernating animals with dolichyl pyrophosphate N,N'-diacetyl-[14C]chitobiose and GDP-[14C]mannose led to the synthesis of lipid-[14C]trisaccharide. When liver microsomes were incubated with lipid-[14C]trisaccharide and unlabelled GDP-mannose, lipid-tetra- to heptasaccharides were discovered in the chloroform-methanol (2:1) extract. Since, under the experimental conditions, negligible synthesis of dolichyl phosphate mannose was observed, it was assumed that GDP-mannose was a donor of mannose in the conversion of lipid-trisaccharide into lipid-oligosaccharides containing 2-5 mannose residues.     

Anomeric configuration of the dolichyl D-mannosyl phosphate formed in calf pancreas microsomes.

Dolichyl D-[14C]mannosyl phosphate formed in calf pancreas microsomes was compared to dolichyl alpha-D-[14C]mannopyranosyl phosphate, a chemical synthesis of which is described. Jack bean alpha-mannosidase, which converted citronellyl alpha-D-mannopyranosyl phosphate, but not its beta anomer, to citronellyl phosphate and D-mannose, was effective in releasing D-[14C]mannose from dolichyl alpha-D-[14C]manopyranosyl phosphate in the presence of detergent. In contrast, alpha-mannosidase did not cause any significant release from the pancreatic dolichyl D-[14C]mannosyl phosphate. Alkali treatment (0.1 M NaOH in propanol at 65 and 90 degrees) degraded both dolichyl D-[14C]mannosyl phosphates with the formation of water-soluble 14C-labeled products. The pattern of 14C-labeled breakdown products formed from the synthetic dolichyl alpha-D-[14C]mannopyranosyl phosphate differed from that obtained from the pancreatic dolichyl D-[14C]mannosyl phosphate. Dolichyl alpha-D-[14C]mannopyranosyl phosphate yielded several 14C-labeled products, including a trace of D-[14C]mannosyl phosphate, and an acidic fraction which appeared to result from degradation of D-[14C]mannose. The pancreatic dolichyl D-[14C]mannosyl phosphate gave various products, depending on the temperature of the reaction: at 90 degrees, 20 to 30% of the radioactivity was found in D-[14C]mannosyl phosphate and the rest in acidic breakdown products; at 65 degrees, about two-thirds of the radioactivity was recovered in a compound which behaved as D-MANNOSE 2-PHOSPHATE, A Product characteristic of a beta-linked D-mannosyl residue. It is concluded that the pancreatic compound is dolichyl beta-D-[14C]mannosyl phosphate.     

Effect of dexamethasone on the synthesis of dolichol-linked saccharides and glycoproteins in hepatocytes prepared from control and inflamed rats.

Hepatocytes were prepared from control and inflamed rats. The incorporation of [14C]mannose into protein was increased in inflamed compared with control hepatocytes. The incorporation of [14C]mannose into protein was also increased when the hepatocytes were cultured in presence of dexamethasone (1 microM), either from control or inflamed rats. At the same time the incorporation of [14C]mannose into dolichol phosphate mannose and dolichol-linked oligosaccharide was increased due to inflammation. The presence of dexamethasone in the hepatocyte culture caused an increased formation of these two products; in particular its effect on oligosaccharide lipid formation was very pronounced. The ratios of activities of formation of [14C]mannose-labelled oligosaccharide lipid in inflamed over control hepatocytes gradually decrease when increasing amounts of exogenous dolichol phosphate was added in cell homogenate assay mixture. These results suggest that the increase of oligosaccharide lipid formation in inflammation could be due to a higher concentration of endogenous dolichol phosphate, as was shown for dolichol phosphate mannose formation in inflammation [Sarkar & Mookerjea (1984) Biochem. J. 219, 429-436]. In contrast, the ratio of activities of [14C]mannose-labelled oligosaccharide lipid between dexamethasone-treated and untreated hepatocytes shows only a slight increase when increasing concentrations of exogenous dolichol phosphate were added to the assays. This suggests that the stimulation of dolichol pyrophosphate oligosaccharide synthesis observed in dexamethasone treatment is probably due to the higher level of enzymes involved in oligosaccharide synthesis rather than higher level of endogenous dolichol phosphate in these cells.     

Steroid hormone biosynthesis by a sesterterpene pathway in the rat and rabbit testis

Rat and rabbit testis preparations were incubated with [4-14C]cholesterol and 23,24-dinor-[7 alpha-3H]5-cholen-3 beta-ol, the latter being a proposed intermediate in the sesterterpene pathway for steroid biosynthesis. Steroids were isolated, purified by thin-layer chromatography and crystallised to constant specific activity. It was found that rat and rabbit testis can utilise 23,24-dinor-5-cholen-3 beta-ol to produce testosterone. The tritium/carbon-14 ratios in the testosterone and androstenedione isolated indicated that these tissues differentiated between the two substrates. This finding is supported by the observation that, on stimulation with HCG, the tritium/carbon-14 ratios in the testosterone isolated were increased compared to the controls. The results of further experiments implied that, while the biosynthesis of testosterone from cholesterol occurred in the rat testis mitochondrial fraction, its biosynthesis from 23,24-dinor-5-cholen-3 beta-ol occurred in the microsomal fraction.     

3-Deazaguanosine is metabolized to the triphosphate derivative in Chinese hamster cells deficient in hypoxanthine-guanine phosphoribosyltransferase

3-Deazaguanosine containing a 14C label in the ribose moiety was prepared using [U-14C]inosine as the [14C] ribose donor and commercial purine-nucleoside phosphorylase (EC 2.4.2.1) both to degrade the inosine, in the presence of phosphate, and to synthesize [14C-ribosyl]3-deazaguanosine in reduced phosphate and an excess of 3-deazaguanine. Purification was by high-pressure liquid chromatography (HPLC). [14C-ribosyl]3-Deazaguanosine was metabolized by Chinese hamster ovary cells to two metabolites, one major and one minor, eluting in the triphosphate region after HPLC analysis, and appeared to be incorporated into perchloric acid-insoluble material. Cell line TGR-3, deficient in hypoxanthine-guanine phosphoribosyltransferase (EC 2.4.2.8) and resistant to 3-deazaguanine, also formed both metabolites. Line TGR-1/DGRR-9, deficient in hypoxanthine-guanine phosphoribosyltransferase and resistant to both 3-deazaguanine and 3-deazaguanosine, formed greatly reduced levels of the major metabolite. 3-Deazaguanosine 5'-triphosphate, prepared enzymically from authentic 3-deazaguanosine 5'-monophosphate, co-eluted with the major metabolite peak during HPLC analysis. Treatment of a metabolite-containing extract with bacterial alkaline phosphatase (EC 3.1.3.1) resulted in the formation of 3-deazaguanosine. These observations indicate that 3-deazaguanosine can be metabolized, in Chinese hamster ovary cells, to the triphosphate derivative in lieu of the action of hypoxanthine-guanine phosphoribosyltransferase.