Origin of guanine nucleotides in isolated heart mitochondria

Presence of guanine nucleotide within the matrix of mitochondria is uncontested; the mechanism by which GTP takes up residence in the matrix is unknown. In this report, we demonstrate for the first time that direct transport of guanine nucleotide across the inner membrane of heart mitochondria is possible. Transport of guanine nucleotides from the medium to the matrix was suggested by inhibition of translation in isolated rat heart mitochondria when GTP-gamma-S was added to the medium. This result suggested that GTP was one source of matrix GTP. Other sources were investigated by measuring matrix uptake and conversion to GTP of several purines, purine nucleosides, and purine nucleotides. Results demonstrated that [14C]-guanine and [3H]-guanosine were not taken up by isolated mitochondria and were not converted to any other compound. While [14C]-ATP and [3H]-AMP were taken up readily into the matrix, radioactivity was never associated with a guanine compound. [3H]-IMP was not taken up into the matrix and was never converted to another compound. Our data showed that label added as [3H]-GTP, [3H]-GDP, or [3H]-GMP was readily taken up and concentrated in the matrix of isolated mitochondria.     

Changes in the activity of enzymes involved in purine "salvage" and nucleic acid degradation during the growth of Catharanthus roseus cells in suspension culture

Changes during growth in the activity of several enzymes involved in purine "salvage", adenine phosphoribosyltransferase (EC 2.4.2.7), guanine phosphoribosyl-transferase (EC 2.4.2.8), hypoxanthine phosphoribosyltransferase (EC 2.4.2.8) and adenosine kinase (EC 2.7.1.20), the enzymes which catalyze the conversion of nucleoside monophosphate to triphosphate, nucleoside monophosphate kinase (EC 2.7.4.4) and nucleoside diphosphate kinase (EC 2.7.4.6), and several degradation enzymes, deoxyribonucleae(s), ribonuclease(s). phosphatase(s), nucleosidase (EC 3.2.2.1), 3'-nucleotidase (EC 3.1.3.6) and 5'-nucleotidase (EC 3.1.3.5) were examined in cells of Catharanthus roseus (L.) G. Don cultured in suspension. In addition, the incorporation of [8-¹⁴C] adenine, [8-¹⁴C] adenine, [8-¹⁴C]hypoxanthine. [8-¹⁴C] adenosine and [8-¹⁴C]inosine into nucleotides and nucleic acids was also determined using intact cells.
The activities of all purine "salvage" enzymes examined and those of nucleoside monophosphate and diphosphate kinases increased rapidly during the lag phase and decreased during the following cell division and cell expansion phases. The rate of incorporation of adenine, guanine, hypoxanthine, and adenosine into nucleotides and nucleic acids was higher in the lag phase cells than during the following three phases. The highest rate of [8-¹⁴C]inosine incorporation was observed in the stationary phase cells. The activity of all degradation enzymes examined decreased when the stationary phase cells were transferred to a new medium.
These results indicated that the increased activity of purine "salvage" enzymes observed in the lag phase cells may contribute to an active purine "salvage" which is required to initiate a subsequent cell division.     

Profiles of purine biosynthesis, salvage and degradation in disks of potato (Solanum tuberosum L.) tubers

To find general metabolic profiles of purine ribo- and deoxyribonucleotides in potato (Solanum tuberosum L.) plants, we looked at the in situ metabolic fate of various ¹⁴C-labelled precursors in disks from growing potato tubers. The activities of key enzymes in potato tuber extracts were also studied. Of the precursors for the intermediates in de novo purine biosynthesis, [¹⁴C]formate, [2-¹⁴C]glycine and [2-¹⁴C]5-aminoimidazole-4-carboxyamide ribonucleoside were metabolised to purine nucleotides and were incorporated into nucleic acids. The rates of uptake of purine ribo- and deoxyribonucleosides by the disks were in the following order: deoxyadenosine > adenosine > adenine > guanine > guanosine > deoxyguanosine > inosine > hypoxanthine > xanthine > xanthosine. The purine ribonucleosides, adenosine and guanosine, were salvaged exclusively to nucleotides, by adenosine kinase (EC 2.7.1.20) and inosine/guanosine kinase (EC 2.7.1.73) and non-specific nucleoside phosphotransferase (EC 2.7.1.77). Inosine was also salvaged by inosine/guanosine kinase, but to a lesser extent. In contrast, no xanthosine was salvaged. Deoxyadenosine and deoxyguanosine, was efficiently salvaged by deoxyadenosine kinase (EC 2.7.1.76) and deoxyguanosine kinase (EC 2.7.1.113) and/or non-specific nucleoside phosphotransferase (EC 2.7.1.77). Of the purine bases, adenine, guanine and hypoxanthine but not xanthine were salvaged for nucleotide synthesis. Since purine nucleoside phosphorylase (EC 2.4.2.1) activity was not detected, adenine phosphoribosyltransferase (EC 2.4.2.7) and hypoxanthine/guanine phosphoribosyltransferase (EC 2.4.2.8) seem to play the major role in salvage of adenine, guanine and hypoxanthine. Xanthine was catabolised by the oxidative purine degradation pathway via allantoin. Activity of the purine-metabolising enzymes observed in other organisms, such as purine nucleoside phosphorylase (EC 2.4.2.1), xanthine phosphoribosyltransferase (EC 2.4.2.22), adenine deaminase (EC 3.5.4.2), adenosine deaminase (EC 3.5.4.4) and guanine deaminase (EC 3.5.4.3), were not detected in potato tuber extracts. These results suggest that the major catabolic pathways of adenine and guanine nucleotides are AMP → IMP → inosine → hypoxanthine → xanthine and GMP → guanosine → xanthosine → xanthine pathways, respectively. Catabolites before xanthosine and xanthine can be utilised in salvage pathways for nucleotide biosynthesis.     

Guanine uptake and metabolism in Neurospora crassa

Guanine is transported into germinated conidia of Neurospora crassa by the general purine base transport system. Guanine uptake is inhibited by adenine and hypoxanthine but not xanthine. Guanine phosphoribosyltransferase (GPRTase) activity was demonstrated in cell extracts of wild-type germinated conidia. The Km for guanine ranged from 29 to 69 micro M in GPRTase assays; the Ki for hypoxanthine was between 50 and 75 micro M. The kinetics of guanine transport differ considerably from the kinetics of GPRTase, strongly suggesting that the rate-limiting step in guanine accumulation in conidia is not that catalyzed by GPRTase. Efflux of guanine or its metabolites appears to have little importance in the regulation of pools of guanine or guanine nucleotides since very small amounts of 14C label were excreted from wild-type conidia preloaded with [8-14C]guanine. In contrast, excretion of purine bases, hypoxanthine, xanthine, and uric acid appears to be a mechanism for regulation of adenine nucleotide pools (Sabina et al., Mol. Gen. Genet. 173:31-38, 1979). No label from exogenous [8-14C]guanine was ever found in any adenine nucleotides, nucleosides, or the base, adenine, upon high-performance liquid chromatography analysis of acid extracts from germinated conidia of wild-type of xdh-l strains. The 14C label from exogenous [8-14C]guanine was found in GMP, GDP, GTP, and the GDP sugars as well as in XMP. Xanthine and uric acid were also labeled in wild-type extracts. Similar results were obtained with xdh-l extracts except that uric acid was not present. The labeled xanthine and XMP strongly suggest the presence of guanase and xanthine phosphoribosyltransferase in germinated conidia.     

Purine nucleoside cycles in chicken tissues

The activity of enzymes catalyzing the reactions of successive degradation to the IMP and GMP bases as well as reactions of the reutilization and degradation of the hypoxanthine and guanine bases in the chicken liver and spleen is determined. The passage rate of [8-14C]hypoxanthine label through IMP and [8-3H]guanine label through GMP is studied together with the metabolism intensity of adenine-hypoxanthine-, xanthine- and guanine-containing components and labelled acid of the acid-soluble fraction of the test tissues in experiments in vivo. The results obtained evidence for functioning of conjugated ways of hypoxanthine- and guanine-derivatives in the so-called nucleoside cycles in the chicken tissues, the activity of the guanosine cycle (GMP----guanosine----guanine----CMP) in the liver being higher than that in inosine one (IMP----inosine----hypoxanthine----IMP), whereas in the spleen, vice versa, the activity of the metabolism of hypoxanthine derivatives is higher than that of guanine derivatives.     

Transfer of purine metabolites between cells through the medium and via cell contacts in cocultures of HGPRT+ and HGPRT- cells

Cells with and without hypoxanthine-guanine phosphoribosyltransferase (HGPRT) activity were used to examine the transfer of purine metabolites through the medium and via cell contacts. HGPRT- Chinese hamster and human fibroblasts were able to incorporate 3H-labeled purine metabolite(s) from medium in which mouse HGPRT+ B82 cells had been grown for 24 h with [3H]hypoxanthine, but mouse A9 fibroblasts that were deficient in HGPRT, adenine phosphoribosyltransferase (APRT), and methylthioadenosine phosphorylase (MTAP) were unable to incorporate these metabolites. This suggests that in recipient cells incorporation is due to [3H]MTA, which has been shown previously to be the major 3H-labeled purine metabolite to accumulate in B82 medium, being cleaved by MTAP to [3H]adenine, which is phosphoribosylated by APRT to [3H]AMP. Incorporation by recipient cells of metabolites from the medium is referred to as contact-independent metabolite transfer (CIMT). In autoradiograms of B82/A9 cocultures that were labeled with [3H]hypoxanthine, grains were found over A9 that were not in contact with B82, although A9 did not act as recipients of CIMT. This is termed proximity-dependent metabolite transfer (PDMT). Both CIMT and PDMT interfered with the assessment of nucleotide exchange between HGPRT+ and HGPRT- cells through cell contacts, which is referred to as contact-dependent metabolite transfer (CDMT). These problems were unique to HGPRT+ mouse L cells. However, HGPRT- mouse L cells, A9, could be used as potential recipients. A9 were positive recipients of CDMT with only one of five cell lines tested, which suggested that these cells were selective communicators. CDMT could not be studied with [3H]guanine because the nuclei of HGPRT- cells became labeled.     

Synthesis and antiviral evaluation of 9-(S)-[3-alkoxy-2-(phosphonomethoxy)propyl]nucleoside alkoxyalkyl esters: inhibitors of hepatitis C virus and HIV-1 replication

We reported previously that octadecyloxyethyl 9-(S)-[3-hydroxy-2-(phosphonomethoxy)-propyl]adenine (ODE-(S)-HPMPA) was active against genotype 1b and 2a hepatitis C virus (HCV) replicons. This is surprising because acyclic nucleoside phosphonates have been regarded as having antiviral activity only against double stranded DNA viruses, HIV and HBV. We synthesized octadecyloxyethyl 9-(S)-[3-methoxy-2-(phosphonomethoxy)propyl]-adenine and found it to be active in genotype 1b and 2a HCV replicons with EC?? values of 1-2 μM and a CC?? of > 150 μM. Analogs with substitutions at the 3'-hydroxyl larger than methyl or ethyl, or with other purine bases were less active but most compounds had significant antiviral activity against HIV-1 in vitro. The most active anti-HIV compound was octadecyloxyethyl 9-(R)-[3-methoxy-2-(phosphonomethoxy)propyl]guanine with an EC?? < 0.01 nanomolar and a selectivity index of > 4.4 million.     

The transfer of 5'-methylthioadenosine from B82 cells through the medium to CHW-1102 cells

The Chinese hamster cell line. CHW-1102, which is deficient in hypoxanthine guanine phosphoribosyl transferase (HGPRT+), incorporated a [3H]purine metabolite(s) from medium in which B82 cells, but not V79, A9 and BHK cells, had been grown for 24 h with [3H]hypoxanthine. A thin-layer chromatographic comparison of the medium revealed a large radioactive peak that was unique to the B82 medium and co-chromatographed with methylthioadenosine (MTA), but not with most other common purine bases and nucleosides. The addition of either MTA, adenine, or adenosine to B82 medium reduced the amount of radioactive material incorporated by CHW-1102 cells. Methylglyoxal bis(guanylhydrazone) inhibited the production of the [3H]metabolite(s) that were incorporated from B82 medium by CHW-1102 cells. Little MTA phosphorylase activity was detected in the mouse L cell lines, L929, B82, and A9, but activity was present in CHW-1102 cells. These results suggest that one of the metabolites in B82 medium is [3H]MTA, and this is taken up and cleaved by CHW-1102 cells to yield [3H]adenine, which is incorporated into nucleic acids. This accounts for the majority of contact-independent metabolite transfer (CIMT). In cocultures some interactions between B82 and CHW-1102 cells were positive for contact-dependent metabolite transfer (CDMT) or metabolic cooperation.     

Incorporation of label from root-applied N6[8-14C]furfiuryIadenine into the guanine nucleotide fraction of pea bud ribonucleic acid

The pattern of incorporation of label into the nucleotides of axillary bud ribonucleic acid was investigated in Pisum sativum L. cv. Meteor following the application of N ⁶[8-^I4C]furfuryladenine or of [8-¹⁴C]adenine to the root system of decapitated plants and to cultured excised buds. When N ⁶[8-¹⁴C]furifaryladenine was applied to the root system label was confined to the guanine nucleotide moiety of the axillary bud ribonucleic acid; label from [8-¹⁴C]adenine was incorporated preferentially into adenine nucleotide in the molar ratio adenine nucleotide/guanine nucleotide = 3.23. When isolated buds were incubated in media containing [8-¹⁴C]adenine or N ⁶[8-¹⁴C]furfuryladenine, label was incorporated into both purine moieties of the ribonucleic acid. However, the relative incorporation into the guanine nucleotide fraction was considerably greater for N ⁶[8-^I4C]furfuryladenine (adenine nucleotide/guanine nucleotide = 2.23) than for [8-¹⁴C]adenine (ratio = 4.67).
It was concluded that the pattern of metabolism of adenine to guanine and its incorporation into the guanine nucleotide moiety of pea axillary bud ribonucleic acid, is influenced by the presence of a substitution in the N ⁶ position of the adenine base.     

Guanine for DNA synthesis. A compulsory route through ribonucleotide reductase.

Two alternative pathways for the synthesis of dGTP and its incorporation into DNA were studied: guanine (Gua)----GMP----GDP----dGDP----dGTP----DNA and dG----dGMP----dGDP----dGTP----DNA. To determine the contribution of each pathway to DNA synthesis independently of each other, [14C]Gua and [3H]dG tracer experiments were performed in a double-mutant S-49 mouse T-lymphoma cell line, dGuo-L, with purine nucleoside phosphorylase (EC 2.4.2.1)-deficiency and dGTP-feedback-resistant ribonucleotide reductase (RR, EC 1.17.4.1). In this cell line, dGTP pools can be selectively elevated by exogenous dG without affect RR and DNA synthesis. Although [3H]dG, but not [14C]Gua (up to 200 microM), readily expanded the cellular dGTP pool in a dose-dependent fashion in asynchronous cells, only a small fraction of the Gua flux into DNA was derived from [3H]dG, with the major fraction coming from [14C]Gua. H.p.l.c. analysis of G1- and partially enriched S-phase cells revealed that [3H]dGTP only accumulates in G1- but not in S-phase cells because of a rapid turnover of the dGTP pool during DNA synthesis. These results fail to provide evidence for cellular dGTP compartmentation and suggest that the pathway dG----dGMP----dGDP----dGTP alone has insufficient capacity to maintain DNA synthesis.     

Leishmania mexicana: Purine metabolism in promastigotes,axenic amastigotes,and amastigotes derived from vero cells

Leishmania mexicana mexicana promastigotes, axenic amastigotes, and amastigotes derived from Vero cells were examined for de novo purine synthesis and mechanisms of purine salvage. Both promastigotes and axenic amastigotes were incapable of de novo purine synthesis, as shown by the lack of [¹⁴C]formate and [¹⁴C]glycine incorporation into purine nucleotide pools. However, the ready incorporation of [¹⁴C]hypoxanthine, [¹⁴C]adenine, and [¹⁴C]guanine suggested that purine salvage pathways were operating. In addition, a significant percentage (?60%) of the total label from these purine precursors was associated with adenylate nucleotides. Nucleotide pool levels of axenic amastigotes were consistently greater but the specific activities were less than those of promastigotes, suggesting a slower rate of purine metabolism in the axenic amastigote form. Similar results were obtained from amastigotes isolated from infected Vero cells.     

Guanine nucleotides stimulate production of inositol trisphosphate in rat cortical membranes.

The guanine nucleotides guanosine 5'[beta, gamma-imido]triphosphate (Gpp[NH]p), guanosine 5'-[gamma-thio]-triphosphate (GTP gamma S), GMP, GDP and GTP stimulated the hydrolysis of inositol phospholipids by a phosphodiesterase in rat cerebral cortical membranes. Addition of 100 microM-Gpp[NH]p to prelabelled membranes caused a rapid accumulation of [3H )inositol phosphates (less than 30 s) for up to 2 min. GTP gamma S and Gpp [NH]p caused a concentration-dependent stimulation of phosphoinositide phosphodiesterase with a maximal stimulation of 2.5-3-fold over control at concentrations of 100 microM. GMP was as effective as the nonhydrolysable analogues, but much less potent (EC50 380 microM). GTP and GDP caused a 50% stimulation of the phospholipase C at 100 microM and at higher concentrations were inhibitory. The adenine nucleotides App[NH]p and ATP also caused small stimulatory effects (64% and 29%). The guanine nucleotide stimulation of inositide hydrolysis in cortical membranes was selective for inositol phospholipids over choline-containing phospholipids. Gpp[NH]p stimulated the production of inositol trisphosphate and inositol bisphosphate as well as inositol monophosphate, indicating that phosphoinositides are substrates for the phosphodiesterase. EGTA (33 microM) did not prevent the guanine nucleotide stimulation of inositide hydrolysis. Calcium addition by itself caused inositide phosphodiesterase activation from 3 to 100 microM which was additive with the Gpp[NH]p stimulation. These data suggest that guanine nucleotides may play a regulatory role in the modulation of the activity of phosphoinositide phosphodiesterase in rat cortical membranes.     

Muscarinic Cholinergic Stimulation of Exogenous Phosphatidylinositol Hydrolysis Is Regulated by Guanine Nucleotides in Rabbit Brain Cortical Membranes

Rabbit brain cortical membranes, which have been extracted with 2 M KCl, hydrolyze exogenously added [3H]phosphatidylinositol [( 3H]PI) in a guanine nucleotide- and carbachol-dependent manner. Both oxotremorine-M and carbachol are full agonists with EC50 values of 8 and 73 microM, respectively. Pirenzepine and atropine inhibit carbachol-stimulated [3H]PI hydrolysis. The hydrolysis-resistant guanine nucleotide analog guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) is the most potent in supporting carbachol-stimulated hydrolysis of PI. There is no effect of carbachol in the absence of guanine nucleotides or in the presence of 100 microM adenosine 5'-O-(3-thiotriphosphate), adenosine-5'-(beta, gamma-imido)triphosphate, or sodium pyrophosphate. Guanylyl-5'-(beta,gamma-imido)triphosphate [Gpp(NH)p] in the presence of carbachol also stimulates PI hydrolysis although much less than that seen with GTP gamma S. GDP and Gpp(NH)p are potent antagonists of the GTP gamma S-dependent carbachol response. Optimal stimulation by carbachol and GTP gamma S was observed at 0.3-1 microM free Ca2+ and 6 mM MgCl2. Limited trypsinization resulted in loss of receptor-regulated PI breakdown and a slight decrease in basal activity. These results demonstrate that phospholipase C hydrolysis of exogenous PI by rabbit cortical membranes may be stimulated by carbachol in a guanine nucleotide-dependent manner.     

Studies on extracellular ribonucleases of Ustilago sphaerogena. Characterization of substrate specificity with special reference to purine-specific ribonucleases

1. Ribonuclease U(1) splits only the phosphodiester bonds of guanosine 3'-phosphates in RNA. It may be regarded as a guanyloribonuclease [ribonucleate (guanine nucleotide)-2'-transferase (cyclizing), EC 2.7.7.26] similar to ribonuclease T(1) (Egami, Takahashi & Uchida, 1964). It seems to be identical with the extracellular ribonuclease described by Glitz & Dekker (1963, 1964a,b). 2. Ribonucleases U(2) and U(3) are novel enzymes with a strict specificity. They split the internucleotide bonds between purine 3'-nucleotides and 5'-hydroxy groups of adjacent nucleotides in RNA with the intermediary formation of purine nucleoside 2',3'-(cyclic)-phosphates, which are slowly hydrolysed to purine 3'-nucleotides. So they may be classified as ;puryloribonucleases [ribonucleate (purine nucleotide)-2'-transferase (cyclizing)]'. Double-stranded RNA is scarcely split by ribonucleases U(2) and U(3). 3. Ribonuclease U(4) has no absolute base specificity, and produces the mononucleotides 3'-adenylate, 3'-guanylate, 3'-cytidylate and 3'-uridylate from RNA.     

Differential effect of N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) on [3H]SCH23390 and [3H]forskolin binding in rat striatum

The binding of [3H]forskolin to a homogeneous population of binding sites in rat striatum was enhanced by NaF, guanine nucleotides and MgCl2. These effects of NaF and guanylylimidodiphosphate (Gpp(NH)p) were synergistic with MgCl2, but NaF and Gpp(NH)p together elicited no greater enhancement of [3H]forskolin binding. These data suggest that [3H]forskolin may label a site which is modulated by the guanine nucleotide regulatory subunit which mediates the stimulation of adenylate cyclase (NS). The D1 dopamine receptor is known to stimulate adenylate cyclase via NS. In rat striatum, the Bmax of [3H]forskolin binding sites in the presence of MgCl2 and NaF was approximately two fold greater than the Bmax of [3H]SCH23390-labeled D1 dopamine receptors. Incubation of striatal homogenates with the protein modifying reagent EEDQ elicited a concentration-dependent decrease in the binding of both [3H]SCH23390 and [3H]forskolin, although EEDQ was approximately 14 fold more potent at inactivating the D1 dopamine receptor. Following in vivo administration of EEDQ there was no significant effect on [3H]forskolin binding sites using a dose of EEDQ that irreversibly inactivated greater than 90% of D1 dopamine receptors. These data suggest that EEDQ is a suitable tool for investigating changes in the stoichiometry of receptors and their second messenger systems.     

Stereochemistry of the hydrogen transfer to NAD catalyzed by (S)alanine dehydrogenase from Bacillus subtilis.

The stereochemistry of the hydrogen transfer to NAD catalyzed by (S)alanine dehydrogenase [ (S)alanine: NAD oxidoreductase (EC 1.4.1.1) ] from B. subtilis was investigated. The label at C-2 of (S) [2,3--3H] alanine was enzymatically transferred to NAD, and the [4--3H]NADH produced isolated and the stereochemistry at C-4 investigated. It was found that the label was exclusively located at the (R) position which indicates that (S)alanine dehydrogenase is an A-type enzyme. This result was confirmed in an alternate way by reducing enzymatically [4--3H]NAD with non labeled (S)alanine and (S)alanine dehydrogenase and investigating the stereochemistry of the ]4--3H]NADH produced. As expected, the label was now exclusively located at the (S) position. This proves that (S)alanine dehydrogenase isolated from B. subtilis should be classified as an A-enzyme with regard to the stereochemistry of the hydrogen transfer to NAD.     

Complete Conversion of Brain D2 Dopamine Receptors from the High- to the Low-Affinity State for Dopamine Agonists, Using Sodium Ions and Guanine Nucleotide

Since previous work had shown that brain D2 3,4-dihydroxyphenylethylamine (dopamine) receptors were only partly converted from their high-affinity state to their low-affinity state, we here tested whether it was possible to obtain a complete 100% conversion of these receptors into their low-affinity state. It was first essential to resolve the components of [3H]spiperone binding to dopaminergic sites and nondopaminergic sites in rat striatal homogenates. In the presence of 50 microM S-sulpiride (to occlude the dopaminergic sites), therefore, we first determined that the residual binding of [3H]spiperone (approximately 20%) was inhibited by serotonergic agonists much more effectively than dopamine or noradrenaline, thus identifying the serotonergic component of [3H]spiperone binding. Thus, dopamine (or ADTN) inhibited the binding of [3H]spiperone at a high-affinity site (with dissociation constant of 10 nM dopamine), at a low-affinity site (with dissociation constant of 2,000 nM dopamine), and at the serotonergic site (with dissociation constant of 50,000 nM dopamine). In the absence of sodium ions, the high-affinity site was about 50% occupied by [3H]spiperone, and guanine nucleotide had no effect on this proportion. In the presence of 120 mM NaCl, however, the high-affinity site was reduced to 15% and guanine nucleotide completely eliminated this high-affinity site, 100% of the sites having been completely converted to their low-affinity state. Using [3H]N-propyl-norapomorphine to label the high-affinity state of the dopamine receptor, 50% conversion into the low-affinity state occurred at 45 mM LiCl, 69 mM NaCl, and 202 mM KCl. We conclude that it is possible to convert brain D2 dopamine receptors completely into their low-affinity state, in the presence of NaCl and a guanine nucleotide, providing that appropriate allowance is made for the serotonergic component of [3H]spiperone binding.     

Biosynthesis of riboflavine by a purine-requiring mutant strain of Escherichia coli

1. Riboflavine biosynthesis occurs in non-proliferating cultures of a purine-requiring strain of Escherichia coli (ATCC no. 13863). 2. No significant incorporation of radioactivity from [1-¹⁴C]glycine into either C-4a and C-9a of riboflavine or into nucleic acid purines is detected under the above conditions; appreciable incorporation of label into 5-aminoimidazole-4-carboxamide occurs. However, the label of [6-¹⁴C]guanine is incorporated significantly into C-4 of riboflavine and into nucleic acid adenine and guanine; the specific radioactivity of the riboflavine is approximately twice that of either adenine or guanine of nucleic acid. 3. These results show that a purine derivative is an obligatory intermediate in riboflavine biogenesis.     

Investigation of human lymphocyte plasma membrane associated nucleic acid.

Plasma membranes were prepared from the human lymphocyte cell line WIL23A by hypotonic swelling, Dounce homogenization, differential and equilibrium centrifugation. The resulting vesiculated membrane fragments were found to have densities of 1.10 and 1.17 g/ml, and were defined by lactoperoxidase mediated whole cell iodination, L-[3H] fucose incorporation, 5'-nucleotidase activity (EC 3.1.3.5) and electron micrographic visualization. Recovery of plasma membrane from whole cell homogenates was estimated to be approximately 30-35% as judged by the recovery of 125I-labeled cell surface protein. When plasma membranes were prepared from cells which had been incubated for 18 h in the presence of 0.5 muCi/ml [3H] thymidine such that greater than 10(9) acid insoluble counts could be demonstrated in the whole cell homogenates, no [3H] thymidine label and presumably, therefore, no DNA, could be shown to be coincident with either the 1.10 or 1.17 density. Similar experiments with [3H] uridine suggested that 90% of the plasma membranes did not contain RNA, while 10% remained questionable.     

Binding of [3H]forskolin to human platelet membranes. Regulation by guanyl-5'-yl imidodiphosphate, NaF, and prostaglandins E1 and D2

[3H]Forskolin binds to human platelet membranes in the presence of 5 mM MgCl2 with a Bmax of 125 fmol/mg of protein and a Kd of 20 nM. The Bmax for [3H]forskolin binding is increased to 455 and 425 fmol/mg of protein in the presence of 100 microM guanyl-5'-yl imidodiphosphate (Gpp(NH)p) and 10 mM NaF, respectively. The increase in the Bmax for [3H]forskolin in the presence of Gpp(NH)p or NaF is not observed in the absence of MgCl2. The EC50 values for the increase in the number of binding sites for [3H]forskolin by Gpp(NH)p and NaF are 600 nM and 4 mM, respectively. The EC50 value for Gpp(NH)p to increase the number of [3H]forskolin binding sites is reduced to 35 mM and 150 nM in the presence of 50 microM PGE1 or PGD2, respectively. The increase in the number of [3H]forskolin binding sites observed in the presence of NaF is unaffected by prostaglandins. The binding of [3H]forskolin to membranes that are preincubated with Gpp(NH)p for 120 min or assayed in the presence of PGE1 reaches equilibrium within 15 min. In contrast, a slow linear increase in [3H]forskolin binding is observed over a period of 60 min when Gpp(NH)p and [3H]forskolin are added simultaneously to membranes. A slow linear increase in adenylate cyclase activity is also observed as a result of preincubating membranes with Gpp(NH)p. In human platelet membranes, agents that activate adenylate cyclase via the guanine nucleotide stimulatory protein (Ns) increase the number of binding sites for [3H]forskolin in a magnesium-dependent manner. This is consistent with the high affinity binding sites for [3H]forskolin being associated with the formation of an activated complex of the Ns protein and adenylate cyclase. This state of the adenylate cyclase may be representative of that formed by a synergistic combination of hormones and forskolin.