The role of environmental antigens in the spontaneous development of autoimmunity in MRL-lpr mice

It has been proposed that the "normal" stimulation of the immune system that occurs from interactions with environmental stimuli, whether infectious or dietary, is necessary for the initiation and/or continuation of autoimmunity. We tested this hypothesis by deriving a group of MRL-lpr mice into a germfree (GF) environment. At 5 mo of age, no differences between GF and conventional MRL-lpr mice were noted in lymphoproliferation, flow cytometric analysis of lymph node cells (LN), or histologic analysis of the kidneys. Autoantibody levels were comparably elevated in both groups. A second experiment tested the role of residual environmental stimuli by contrasting GF mice fed either a low m.w., ultrafiltered Ag-free (GF-AF) diet or an autoclaved natural ingredient diet (GF-NI). At 4 mo of age, both groups showed extensive lymphoproliferation and aberrant T cell formation, although the GF-AF mice had approximately 50% smaller LNs compared with sex-matched GF-NI controls. Autoantibody formation was present in both groups. Histologic analysis of the kidneys revealed that GF-AF mice had much lower levels of nephritis, while immunofluorescence analysis demonstrated no difference in Ig deposits but did reveal a paucity of C3 deposition in the kidneys of GF-AF mice. These data do not support a role for infectious agents in the induction of lymphoproliferation and B cell autoimmunity in MRL-lpr mice. Furthermore, they suggest that autoantibodies do not originate from B cells that were initially committed to exogenous Ags. They do suggest a possible contributory role for dietary exposure in the extent of lymphoproliferation and development of nephritis in this strain.

Effects of a diet deficient in tyrosine and queuine on germfree mice

A chemically-defined diet consisting of amino acids (including tyrosine), vitamins, trace elements, glucose, etc., known to support growth and reproduction through many generations when fed to germfree mice has been in use for many years in our laboratory. Classical nutritional studies showed that tyrosine was not a dietary requirement for higher mammals if an adequate amount of phenylalanine was present. Therefore, it was unexpected that when tyrosine was removed from this diet, the germfree mice developed ocular, neurological and other abnormalities which resulted in 100% fatalities usually within two weeks. Adding tyrosine back to the diet prevented the abnormalities from occurring. Conventional mice with a normal intestinal flora showed none of these symptoms when fed the same tyrosine-deficient diet. We added queuine to the tyrosine-deficient diet at a concentration of 0.1 microM. The germfree mice that were fed the diet supplemented with queuine were asymptomatic and remained alive until the termination of the experiments.

Cloning and characterization of cDNA encoding the rabbit tRNA-guanine transglycosylase 60-kilodalton subunit

Eukaryotes synthesize queuosine (nucleoside Q) by the irreversible base-for-base exchange of queuine (Q base) for guanine at tRNA position 34, a reaction catalyzed by tRNA-guanine transglycosylase (TGT). The physiological role of Q remains unknown but the tRNA of tumor cells often is undermodified with respect to Q. Toward an understanding of the function of Q in normal and neoplastic cells we have isolated and characterized the cDNA for rabbit TGT. Rabbit erythrocyte TGT was reported previously to be a dimer of 60- and 43-kDa subunits (N. K. Howes and W. R. Farkas, 1978, J. Biol. Chem. 253, 9082-9078). Here we present the cDNA sequence for the apparent 60-kDa subunit; it contains an open reading frame encoding a 493-residue protein. The rabbit TGT 60-kDa subunit shares significant sequence similarity with the deubiquitinating enzyme family (F. R. Papa and M. Hochstrasser, 1993, nature 366, 313-319), especially with sequence elements that include conserved Cys and His residues.

Absence of plasma amine oxidase in some frequently used animal models

1. Plasma amine oxidase (PAO), an enzyme that oxidizes primary amines is generally assumed to be present in the sera of all mammals. 2. We studied plasma from eleven different mammals to determine if they could oxidize a group of amines of biomedical interests. 3. Our results indicate that PAO is not present in all mammals and also that substrate specificity of the enzyme varies within the species tested. Rodents which are frequently used as animal models in pharmacology and toxicology do not contain PAO. 4. The common polyamines, spermine, spermidine and putrecine were oxidized only by bovine plasma.

The lysine analog L-oxalysine is an inhibitor of RNA synthesis

1. The lysine analog L-4-oxalysine was found to be a potent inhibitor of RNA synthesis in Candida albicans. 2. The compound was a weak inhibitor of protein synthesis and DNA synthesis was not affected. 3. The inhibition of RNA synthesis was reversed by L-lysine but not D-lysine. 4. The decrease in the level of newly synthesized RNA in cells treated with L-oxalysine was due to inhibition of de novo synthesis rather than to degradation of RNA.

Queuine metabolism and cadmium toxicity in Drosophila melanogaster

Queuine can replace guanine in the anticodon of certain tRNAs and is a hypermodified guanine derivative that can be synthesized by bacteria but not by mice. The study demonstrates that Drosophila can incorporate dietary queuine into tRNA but cannot synthesize it de novo for this purpose. Since an earlier study had shown that dietary CdCl2 caused Drosophila to increase greatly the proportion of queuine-containing tRNA over non-queuine tRNA the ability of dietary queuine to counteract cadmium toxicity was evaluated. When queuine was present in the cadmium-containing medium more pupae matured into adults than when queuine was absent. Other studies had demonstrated that the transglycosylase enzyme, that catalyzes the replacement of guanine in the anticodon of tRNA by queuine, is present in Drosophila larvae but the tRNA is virtually devoid of queuine. This study shows that in the presence of dietary queuine the larval tRNA contains abundant amounts of queuine. Therefore, we postulate a significant role for bacteria in supplying queuine to Drosophila for its incorporation into tRNA and that the control of this process by Drosophila is passive, i.e. is not an essential feature in differentiation.

Queuine is incorporated into brain transfer RNA

Pig brain tRNA was assayed for the presence of queuosine in the first position of the anticodon for each of the Q-family of tRNAs (aspartyl, asparaginyl, histidyl and tyrosyl). The brain tRNA was aminoacylated with each of the four amino acids and the aminoacylated tRNA's analyzed by RPC-5 chromatography. The results of this study show that for all four tRNAs of the family, queuine is substituted for guanine in virtually 100% of the anticodons. Therefore, it can be concluded that queuine is able to cross the blood-brain barrier and that brain contains guanine-queuine tRNA transglycosylase, the enzyme responsible for the excision of guanine from the original transcripts of these tRNAs and insertion of queuine. The determination of whether the tRNA contained queuine was made from the elution profile of the RPC-5 chromatograms and the results confirmed by a change in the RPC-5 elution profile when the tRNAs were reacted with BrCN or NaIO4.

The absence of the diet-derived 7-deazapurine, queuine in artificial liquid diets

Queuine is a derivative of guanine found in the transfer RNAs of most organisms including man. Higher mammals cannot synthesize queuine and must obtain it either from their diets or intestinal microflora. Tumor cells often contain much less queuine in their transfer RNAs than do normal cells. Cancer patients are frequently fed artificial liquid diets or are nourished by chemically defined intravenously administered liquids. In this report we present the results of our examination of five common artificial nutrition preparations obtained from a hospital pharmacy with respect to their content of queuine.

Possible involvement of queuine in oxidative metabolism

The possibility that the base, queuine, or the queuine family of tRNAs may play a role in oxidative metabolism has been investigated. (i) The enzymatic insertion of queuine into tRNA requires oxygen. This is true for both the mammalian and bacterial enzyme. (ii) (q-) LM cells (murine fibroblast line) grown in culture had 53% less of the manganese-containing superoxide dismutase than (q+) cells. (iii) There was less thiobarbituric-acid-reactive material in queuine-deficient mouse liver and kidney than in (q+) liver and kidney.

Interaction of lysinoalanine with the protein synthesizing apparatus

Lysinoalanine [N epsilon-(DL-2-amino-2-carboxyethyl)-L-lysine; LAL], a nephrotoxic lysine analog, inhibits the lysyl-tRNA-synthetase (EC 6.1.1.6) of prokaryotic and eukaryotic cells competitively at micromolar concentrations. Incorporation of [14C]lysine into protein by a cell-free eukaryotic protein-synthesizing system was inhibited by LAL. Inhibition was 69.7% and 18.4% at LAL concentrations of 1.0 mM and 0.1 mM, respectively. LAL was incorporated into protein as well as being an inhibitor as indicated by the incorporation of [14C]LAL into protein by the cell-free eukaryote protein-synthesizing system. The proteins labeled with [14C]LAL co-electrophoresed with those labeled with [14C]lysine. These results indicate that LAL is an inhibitor of both prokaryote and eukaryote lysyl-tRNA-synthetase. Furthermore, it is incorporated into protein. Both of these actions can be factors in the nephrotoxicity of this common food contaminant. Possible mechanisms for the toxicity of lysinoalanine are discussed.

Elevated urinary excretion of beta-aminoisobutyric acid and exposure to inorganic lead

beta-Aminoisobutyric acid (beta-AIB), a normal degradation product of thymine, a constituent of DNA and, to a lesser extent, of transfer RNA, is excreted in low levels in human urine. We found that a group of iron workers occupationally exposed to inorganic lead excreted high levels of urinary beta-AIB. Elevated urinary excretion of beta-AIB was also observed in marmosets, Callithrix jacchus, that received lead acetate in drinking water. Our results suggest that increased urinary excretion of beta-AIB could stem from damage to DNA on exposure to lead.

Elevation of blood levels of zinc protoporphyrin by radiomimetic drugs and Friend leukemia virus

Sublethal doses of whole-body irradiation induced the elevation of zinc protoporphyrin (ZPP). Experiments were conducted to determine if recovery from radiomimetic drugs also resulted in elevation of ZPP. Daily injections with hydroxyurea and other cytotoxic drugs for 10 days caused ZPP elevation and a dose of radiation too low to cause ZPP elevation by itself caused ZPP elevation when hydroxyurea was administered prior to irradiation. Friend leukemia virus also brought about an elevation of ZPP. However, not all factors that increased erythropoiesis brought about ZPP elevation. The elevated erythropoiesis in response to hypoxia and the enhanced erythropoiesis that followed administration of folic acid to folic acid-deficient mice was not accompanied by ZPP elevation.

Secretion of ceruloplasmin by a human clear cell carcinoma maintained in nude mice

Ceruloplasmin is the best known but least understood copper protein. Studies preliminary to investigating the control of ceruloplasmin synthesis have utilized a human renal cell carcinoma maintained in nude mice for 73 passages over a 5-year period. In vitro cultures of these cells were accomplished and the mRNAs were extracted prior to microinjection into Xenopus oocytes. The media examined by SE-HPLC and immunological techniques demonstrated that (1) after in vitro culture, ceruloplasmin was secreted as an uncleaved polypeptide chain with a MW of 135,000; (2) the translational product of ceruloplasmin mRNA injected into Xenopus oocytes was cleaved into fragments with MWs of 110,000, 67,000, and 50,000. The results indicate that mRNA for human ceruloplasmin can be obtained to serve as a template for the synthesis of a cDNA probe to investigate the control of human ceruloplasmin's synthesis.

Translation of messenger RNA from a renal tumor into a product with the biological properties of erythropoietin

The renal tumor RCC-3-JCK, when transplanted into immunodeficient mice, caused an erythrocytic polycythemia. When grown in culture, the tumor cells secreted a substance into the culture medium that chromatographed by size-exclusion high-performance liquid chromatography similarly to purified human erythropoietin (Ep) and was positive when assayed for Ep by its ability to stimulate erythropoiesis in fetal mouse liver cells (the FMLC assay). The poly(A) + RNA was extracted from the tumor cells and injected into Xenopus oocytes, inducing the appearance of Ep(FMLC) in the oocyte culture medium. Both the tumor cells and oocyte culture media were fractionated by size-exclusion high-performance liquid chromatography, and two fractions with Ep(FMLC) activity were found in the tumor-cell culture medium. Three active fractions were found in the medium from the mRNA-injected oocytes. The largest component from both culture media had the same elution time as a human standard (Ep). The poly(A) + RNA was fractionated by sucrose density-gradient centrifugation and the 8S and 10S fractions were found to induce Ep(FMLC) synthesis when they were injected into the oocytes. We conclude that poly(A) + RNA isolated from the Ep-producing tumor RCC-3-JCK included mRNA for Ep and that the Ep was a translational product of Xenopus oocytes injected with this mRNA.

The elevation of blood levels of zinc protoporphyrin in mice following whole body irradiation

Elevation of zinc protoporphyrin (ZPP) levels in the blood has served as an indicator of lead poisoning and iron deficiency anemia for many years. We have discovered that sublethal doses of whole body irradiation with x-rays also elevates ZPP 2-3-fold over normal levels. The ZPP level does not begin to increase until days 12-14 postirradiation and peaks between days 18 and 20 before returning to normal levels between days 28 and 35. Increasing the radiation dose delays the onset of the rise in ZPP, but does not affect the magnitude of the elevation. At lethal doses, ZPP elevation is not observed. Neither of the two previously described mechanisms that cause elevations of ZPP, namely iron deficiency and inhibition of ferrochelatase, are responsible for the radiation-induced elevation of ZPP. The elevation of ZPP appears to be correlated with the recovery of the hematopoietic system from radiation injury.

Substrate and inhibitor specificity of tRNA-guanine ribosyltransferase

We have tested as inhibitors or substrates of tRNA-guanine ribosyltransferase (EC 2.4.2.29) a number of compounds, including derivatives of 7-deazaguanine, pteridines, purines, pyrimidines and antimalarials. Virtually all purines and pteridines that are inhibitors or substrates of the rabbit reticulocyte enzyme have an amino nitrogen at the 2 position. In addition the 9 position and the oxygen at the 6 position may be important for recognition by the enzyme. Saturation of the double bond in the cyclopentenediol moiety of queuine reduces substrate activity and queuine analogs that lack the cyclopentenediol moiety, such as 7-deazaguanine and 7-aminomethyl-7-deazaguanine, are relatively poor substrates for the enzyme. While adenosine is not an inhibitor, neplanocin A (an adenosine analog in which a cyclopentenediol replaces the ribose moiety) is a poor inhibitor. The incorporation of 7-aminomethyl-7-deazaguanine into the tRNA of L-M cells results in a novel chromatographic form of tRNAAsp, indicating that L-M cells cannot modify this Q precursor (in Escherichia coli) to queuosine. The specific incorporation of 7-deazaguanine and 8-azaguanine into tRNA by L-M cells also results in novel chromatographic forms of tRNAAsp. With intact L-M cells, the enzyme-catalyzed insertion into tRNA of queuine, dihydroqueuine, 7-aminomethyl-7-deazaguanine, or 7-deazaguanine is irreversible, while guanine or 8-azaguanine incorporation is reversible; suggesting that it is the substitution of C-7 for N-7 which prevents the reversible incorporation of queuine into tRNA.

Preparation of purified erythropoietin by high performance liquid chromatography

Highly purified erythropoietin was not available in quantities needed to carry out planned investigations and, therefore, the use of high performance liquid chromatography was explored. This technique permits the separation of proteins with high efficiency and resolution. Three types of chromatography were used. Size exclusion or gel permeation, reversed phase, and ion exchange columns were utilized with different solvent systems. The chromatographic fractions were assayed either by an exhypoxic polycythemic mouse assay or by the fetal liver cell assay. In addition, selected fractions were tested for their capability to stimulate CFU-E and BFU-E colony formation in methyl cellulose. The results of the techniques of size exclusion and ion exchange chromatography were found to be rapid and reproducible. Although reversed phase chromatography gave excellent resolution, the results were somewhat variable. Using different chromatographic combinations, erythropoietin with a specific bioactivity in the range of 50,000 u/mg protein was isolated. Although the erythropoietin gene has now been cloned and the hormone purified by utilizing monoclonal antibodies, high performance liquid chromatography may be useful in the removal of unwanted contaminants, as a tool in chemical characterization, and as a possible method for the hormone's identification and measurement in clinical laboratories.

Partial purification and characterization of dihydrodipicolinic Acid reductase from maize

Dihydrodipicolinic acid reductase, an enzyme which catalyzes the pyridine nucleotide-linked reduction of dihydrodipicolinic acid to tetrahydrodipicolinic acid in the biosynthetic pathway leading to l-lysine, has been partially purified from maize (Zea mays cv Pioneer 3145) kernels. The crude maize extract and the partially purified enzyme were assayed for dihydrodipicolinic acid reductase by their ability to restore the capability of crude extracts of a mutant Escherichia coli (CGSC 4549; defective in dihydrodipicolinic acid reductase) to synthesize diaminopimelic acid from aspartic acid and pyruvic acid.In a study of its properties, the Michaelis constant obtained for dihydrodipicolinic acid was 4.3 x 10(-4) and for NADPH the K(m) was 4.6 x 10(-5). The enzyme had a pH optimum close to 7 and was much more temperature labile than the bacterial enzyme. Its molecular weight was 8.0 x 10(4).Several compounds, viz., alpha-picolinic acid, l-pipecolic acid, isophthalic acid, and isocinchomeronic acid, with structures similar to dihydrodipicolinic acid inhibited the reductase reaction. Dipicolinic acid, the most potent of these inhibitors, acted as a competitive inhibitor with a K(i) of 9.0 x 10(-4). The competitive inhibition of the reductase reaction by dipicolinic acid (oxidized dihydrodipicolinic acid) suggests that the substrate for this enzyme was in the ring rather than the open chain form. Oxidized pyridine nucleotide inhibited the activity slightly.

Isolation of queuine from bovine amniotic fluid

A facile method for isolation of large quantities of queuine from bovine amniotic fluid is described. Queuine was sequentially purified by cation-exchange chromatography, adsorption chromatography on Sephadex G-25, and size-exclusion chromatography on Sephadex G-10. The queuine isolate was identified by its participation in the queuine-guanine tRNA transglycosylase reaction and comparisons with authentic queuine.

Purification and properties of guanine, queuine-tRNA transglycosylase from wheat germ

Guanine, queuine-tRNA transglycosylase has been purified from wheat germ to homogeneity. The specific activity is 2,000 pmol h-1 mg-1 of protein. The enzyme has an apparent Mr = 140,000. It migrates as a single band with Mr = 68,000 on electrophoresis in sodium dodecyl sulfate gels, indicating two Mr = 68,000 subunits. Both guanine (Km = 6.0 X 10(-8) M) and queuine (KI = 9.5 X 10(-8) M) are substrates but 7-(aminomethyl)-7-deazaguanine is not. The enzyme requires a monovalent or a divalent cation; Na+ and Mg2+ are more effective activators than other cations. The optimum pH is 7.6. Six tRNA isoacceptors found in wheat germ are substrates for the enzyme.

Queuine-containing isoacceptor of tyrosine tRNA in Drosophila melanogaster. Alteration of levels by divalent cations

Dietary cadmium causes the queuine-containing, Q(+), isoacceptors to increase relative to the guanine-containing, Q(-), ones of tRNATyr, tRNAHis and tRNAAsp of Drosophila melanogaster. Of the other divalent cations examined, Sr2+, Ni2+, Cu2+, Zn2+ and Hg2+, only Hg2+ failed to cause an increase in Q(+)tRNATyr. For these results, all pre-adult stages of the organism were spent on media containing the divalent ions. Adult flies that had developed on a normal diet also responded to divalent ions; Hg2+ as well as Cd2+, Sr2+ and Zn2+ caused an increase in Q(+)tRNATyr in 4 days. Using adult flies, the rate of the response was measured; when placed on a Cd2+-containing diet, they formed significantly more Q(+)tRNATyr within 24 h as compared to adults on a normal diet. Whether the queuine is derived from the diet or from de novo synthesis is yet to be determined. Since the metal ions represent a range of values in the 'hard-soft' classification, different sites of reaction are expected, yet for Drosophila a common result is an alteration in the ratio of Q(+) and Q(-) isoacceptors of these tRNAs. The transition to Q(+)tRNA may be an early indication of the metabolic imbalances resulting from the presence of the divalent cation.

Administration of exogenous queuine is essential for the biosynthesis of the queuosine-containing transfer RNAs in the mouse

1. Normal mouse liver contains predominantly tRNA that contains queuosine in the first position of the anticodon ((Q+)tRNA). 2. Germ-free mice fed a chemically defined diet devoid of queuine for 1 year have no queuine in all four of the tRNAs that respond to the NAUC codons. 3. The synthesis of (Q+)tRNAs can be induced by injecting queuine, feeding free queuine, or by feeding (Q+)tRNA. 4. When mice that have no (Q+)tRNA are titrated with exogenous queuine, tRNAAsp is modified to the (Q+) state before tRNAHis.

Presence of queuine in Drosophila melanogaster: correlation of free pool with queuosine content of tRNA and effect of mutations in pteridine metabolism

Queuine, a modified form of 7-deazaguanine present in certain transfer RNAs, is shown to occur in Drosophila melanogaster adults in a free form and its concentration varies as a function of age, nutrition and genotype. In several, but not all mutant strains, the concentrations of queuine and the Q(+) (queuine-containing) form of tRNATyr are correlated. The bioassay employs L-M cells which respond to the presence of queuine by an increase in their Q(+)tRNAAsp that is accompanied by a decrease in the Q(-)tRNAAsp isoacceptors. The increase in Q(+)tRNATyr in Drosophila that occurs on a yeast diet is accompanied by an increase in queuine. Similarly the increase of Q(+)tRNAs with age also is accompanied by an increase in free queuine. In two mutants, brown and sepia, these correlations were either diminished or failed to occur. Indeed, the extract of both mutants inhibited the response of the L-M cells to authentic queuine. When the pteridines that occur at abnormally high levels in sepia were used at 1 x 10(-6)M, the inhibition of the L-M cell assay occurred in the order biopterin greater than pterin greater than sepiapterin. These pteridines were also inhibitory for the purified guanine:tRNA transglycosylase from rabbit but the relative effectiveness then was pterin greater than biopterin greater than sepiapterin. Pterin was competitive with guanine in the enzyme reaction with Ki = 0.9 x 10(-7)M. Also when an extract of sepia was chromatographed on Sephadex G-50, the pteridine-containing fractions only were inhibitory toward the L-M cell assay or the enzyme assay. These results indicate that free queuine occurs in Drosophila but also that certain pteridines may interfere with the incorporation of queuine into RNA.

Specific changes in Q-ribonucleoside containing transfer RNA species during Friend leukemia cell erythroid differentiation

Changes in specific tRNA isoacceptors during Friend leukemia cell (F.L.C.) erythroid differentiation have been found to be concomitant with differences in the extent of the Q-base modification in certain species of tRNA. Transfer RNA was isolated from F.L.C. cultures after 0, 36, 48, 72, and 96 hr of DMSO induced differentiation. Changes in 17 isoacceptors of tRNAasn, tRNAasp, tRNAhis and tRNAtyr were compared by RPC-5 chromatography. Isoacceptors of these tRNA changed in relative amounts, following consistent trends throughout cell differentiation. The amount and distribution of Q-base containing tRNA isoacceptors was assayed by measuring the quanine-tRNA transferase catalyzed incorporation of [3H]-labeled guanine into tRNA species undermodified in Q-base followed by RPC-5 chormatography of the tRNA. The amount of Q-base containing tRNA species decreased in the first 48 hr after the induction, then increased again, indicating the level of Q-modification is correlated to the process of differentiation. Isoacceptors that lacked the Q-base were eluted late from RPC-5.

Effect of diet on the queuosine family of tRNAs of germ-free mice

The transfer RNAs for aspartic acid, asparagine, histidine, and tyrosine respond to codons in the third column of the genetic code and contain a hypermodified nucleoside known as queuosine (Q) in the first position of the anticodon of the major isoacceptor tRNA. Nothing is known about the physiological or biochemical function of Q. Germ-free mice were maintained for a period of nine tRNA half-lives on a chemically defined diet known to contain all essential constituents of the rodent diet but no Q or its base, queuine. The tRNAs for histidine and asparagine contained only 15% of the Q-containing isoacceptor tRNA. On the other hand, the Q-containing isoacceptor comprised 88% of the tRNAHis and 85% of the tRNAAsn in conventional mice and germ-free mice fed commercial mouse chow. Transfer RNAAsp and tRNATyr were completely modified with respect to Q in germ-free mice maintained on the chemically defined diet as well as on normal mouse chow. Germ-free mice fed the chemically defined diet contained normal amounts of the hypermodified base wye in tRNAPhe.

Effects of plumbous ion on guanine metabolism

The enzyme guanine aminohydrolase (guanase) is inhibited by low levels of Pb2+. The inhibition is noncompetitive and the Ki is 3.0 X 10(-6) M. The only other heavy metals that are inhibitory at low concentrations are Ag+, which is 36% more, and Hg2+, which is about 50% less inhibitory than Pb2+. The inhibition of guanase by Pb2+ and Hg2+ is synergistic and the inhibition of the enzyme was readily reversed by EDTA. The relationship of these studies with guanase and to the etiology and treatment of saturnine gout, which appears in humans suffering from lead poisoning, is discussed.

A factor in serum and amniotic fluid is a substrate for the tRNA-modifying enzyme tRNA-guanine transferase

Q factor, a substance found in animal serum that enables cultured mammalian cells (L-M) to produce tRNA containing queuine (the base of "nucleoside Q", queuosine), has been purified to homogeneity from bovine amniotic fluid. Q factor causes the appearance of Q-containing tRNAAsp in the L-M cells cultivated in serum-free medium, and this was used as an assay to monitor the purification of Q factor. Q factor is a competitive inhibitor of guanine for rabbit reticulocyte tRNA-guanine trnsferase, with a K1 of 4.5 x 10(-8) M. Q factor is inactivated in both the L-M cell and tRNA-guanine transferase assays by treatment with periodate or cyanogen bromide, both of which react with queuine. In L-M cells, nearly complete conversion of Q-free to Q-containing tRNAAsp is observed within 24 hr after addition of pure Q factor to the medium; actinomycin D, cycloheximide, and cycloleucine, inhibitors of RNA synthesis, protein synthesis, and nucleic acid methylation, respectively, do not inhibit this conversion. The product of the reaction, catalyzed by pure rabbit reticulocyte tRNA-guanine transferase, between Q factor and rabbit reticulocyte Q-free tRNAHis is chromatographyically indistinguishable from Q-containing tRNAHis.

Studies with a homogeneous enzyme from rabbit erythrocytes catalyzing the insertion of guanine into tRNA

An enzyme that catalyzes a post-transcriptional modification of tRNA, resulting in replacement of a base from tRNA by guanine, has been purified 2600-fold from rabbit erythrocyte cytosol. The purest preparation migrates as a single protein band on polycrylamide gel electrophoresis and the enzymatic activity co-electrophoreses with this protein. The native enzyme has a molecular weight of 104,000 and is dissociated into two subunits of Mr= 60,000 and 43,000. The Km for guanine is 1.5 x 10(-7) M and for a pure guanine-accepting tRNA is 3.3 x 10(-9) M. The amino acid composition of the pure enzyme has been determined. To our knowledge this is the first study in which the molecular characteristics of a pure enzyme capable of modifying an internal position in tRNA has been reported.

Saturnine gout: lead-induced formation of guanine crystals

Intravenous injection of a sublethal dose of lead acetate into a domestic pig resulted in a 4.5-fold increase of guanine in the urine, indicating an impairment in the conversion of guanine to xanthine. This impairment is probably due to the inhibition of guanine aminohydrolase (guanase), since the activity of this enzyme is inhibited by Pb2+ (the inhibition constant being 3.0 X 10(-6)M). Postmortem histological examination revealed concretions of crystalline material in the epiphyseal plate of the femoral head. Extraction of the section containing the concretions showed that they were guanine. The relation of these findings to saturnine gout is discussed.

Identification of the minor guanylated tRNA of rabbit reticulocytes

Two of the tRNA's found in rabbit reticulocytes are substrates for a post-transcriptional modification leading to the incorporation of guanine into the polynucleotide chain. The major guanylated tRNA was previously identified as tRNA (His). In the present report we show that the minor guanylated tRNA is tRNA (Asn), and that just as in the case of tRNA (His), the guanine is located in an internal position. There are only two tRNA (Asn) in reticulocytes. We further show that one of these, the one that is not labeled with guanine, contains the hypermodified base known as Q. tRNA (Asn) does not contain Q.

Partial purification and properties of the reticulocyte guanylating enzyme

The guanylating enzyme which catalyzes the insertion of a guanine residue into one of the isoacceping tRNAHis of rabbit reticulocytes has been purified approximately one-hundred fold. It is free of nuclease activity. The enzyme does not catalyze the replacement of inserted radioactive guanine by unlabeled guanine, indicating that the reaction is irreversible. We have separated the histidyl-tRNA of reticulocytes into three isoacceptors. Previous work showed that the last histidyl-tRNA to elute from RPC-5 columns was the product of the guanylation reaction. This reports shows that the same late-eluting peak also contains the substrate for the guanylating enzyme, indicating that the guanine insertion reaction is chromatographically silent. The isoaccepting tRNAHis that is the substrate for the guanylating enzyme does not contain the hypermodified base known as Q. It is the other major reticulocyte tRNAHis that coantains Q, showing that at least in the reticulocyte the role of the guanylating enzyme is not the conversion of the Q form of tRNA to the homogeneic G form. The purified enzyme does not insert any base other than guanine into tRNA.

Effect of plumbous ion on messenger RNA

The effects of Pb2+, a potent catalyst for the depolymerization of RNA have been studied on brome mosaic virus (BMV) RNA, rabbit globin m-RNA and polyuridylic acid. After exposure of these natural and synthetic messengers to a sufficiently high concentration of Pb2+, they all lost their ability to stimulate amino acid incorporation in cell-free protein-synthesizing systems. There were differences in the susceptibilities of the messengers; gloing the m-RNA for 40 min revealed that there was a threshold Pb2+ concentration below which no loss of m-RNA activity was observed. The threshold concentration was considerably greater than the Pb2+ concentration at which protein synthesis is inhibited in reticulocytes and overt symptoms of plumbism are observed. However, when m-RNA were incubated for an extended period (24 h), even with sub-threshold concentrations of Pb2+, there was destruction of messenger function and globin m-RNA was more susceptible than BMV-RNA. Also the susceptibility of m-RNA to Pb2+ is temperature-dependent, which would indicate that m-RNA, like t-RNA, exists as a population of molecules in different conformational states that are not readily interconvertible.

Guanine aminohydrolase in rat and mouse red cells: a potent inhibitor of guanylation of tRNA

1. The red blood cells of mice and rats contained guanine aminohydrolase (EC 3.5.4.3). This enzyme was not present in rabbit, sheep or human red blood cells. 2. The enzyme from rat blood cells was separated into two activities by column chromatography on DEAE-cellulose. Both isozymes were labile but it was possible to show that the more abundant enzyme followed Michaelis-Menten kinetics, had an apparent Km of 4.0-10-6 M and was not activated by GTP nor inhibited by allantoin. 3. We believe, therefore, that guanine aminohydrolase was the protein in rat and mouse red blood cells that inhibited the enzyme (in rabbit reticulocytes) responsible for guanylation of tRNA.

The effects of plumbous ion on protein biosynthesis in reticulocytes

Plumbous ion a potent inhibitor of hemoglobin synthesis by reticulocytes in vivo, is not as effective in inhibiting in vitro globin synthesizing systems prepared from these cells. The reticulocyte is considerably more sensitive to Pb2+ than are leukemic leukocytes, HeLa cells or bacteria. In fact, protein synthesis in leukemic leukocytes is actually stimulated by Pb2+. The synthesis of non-heme protein as well as hemoglobin is inhibited by Pb2+ in reticulocytes and the synthesis of alpha chains is inhibited to a greater degree tcids into reticulocytes. All of these observations are consistent with the biosynthesis of heme rather than any of the steps in protein biosynthesis being the locus for inhibition of hemoglobin synthesis by low levels of plumbous ion. Also, there is marked biological variations in the susceptibilities of reticulocytes from different rabbits to Pb2+.