PubMed Health⌕ Search

Biomedical subjects

J L Hoffman

Publications and source records attributed to J L Hoffman.

At least 37 records · Page 2Linked to original sources
PubMed1990

Biosynthesis and urinary excretion of methyl sulfonium derivatives of the sulfur mustard analog, 2-chloroethyl ethyl sulfide, and other thioethers.

Thioether methyltransferase was previously shown to catalyze the S-adenosylmethionine-dependent methylation of dimethyl selenide, dimethyl telluride, and various thioethers to produce the corresponding methyl onium ions. In this paper we show that the following thioethers are also substrates for this enzyme in vitro: 2-hydroxyethyl ethyl sulfide, 2-chloroethyl ethyl sulfide, thiodiglycol, t-butyl sulfide, and isopropyl sulfide. To demonstrate thioether methylation in vivo, mice were injected with [methyl-3H]methionine plus different thioethers, and extracts of lungs, livers, kidneys, and urine were analyzed by high-performance liquid chromatography for the presence of [3H]methyl sulfonium ions. The following thioethers were tested, and all were found to be methylated in vivo: dimethyl sulfide, diethyl sulfide, methyl n-propyl sulfide, tetrahydrothiophene, 2-(methylthio)ethylamine, 2-hydroxyethyl ethyl sulfide, and 2-chloroethyl ethyl sulfide. This supports our hypothesis that the physiological role of thioether methyltransferase is to methylate seleno-, telluro-, and thioethers to more water-soluble onium ions suitable for urinary excretion. Conversion of the mustard gas analog, 2-chloroethyl ethyl sulfide, to the methyl sulfonium derivative represents a newly discovered mechanism for biochemical detoxification of sulfur mustards, as this conversion blocks formation of the reactive episulfonium ion that is the ultimate alkylating agent for this class of compounds.

Animals↗
PubMed1988

S-adenosyl-L-methionine:thioether S-methyltransferase, a new enzyme in sulfur and selenium metabolism.

The final urinary excretion product of selenium detoxification is trimethylselenonium ion. An assay has been developed for the enzyme, S-adenosylmethionine:thioether S-methyltransferase, responsible for this final methylation reaction. This assay employed high pressure liquid chromatography separation and quantitation of the trimethylselenonium ion produced by thioether methyltransferase acting on S-adenosylmethionine and dimethyl selenide. The enzyme was shown to reside primarily in the cytosol of mouse lung (30 pmol/mg protein/min) and liver (7 pmol/mg protein/min). Purification from mouse lung to a preparation that exhibited a single band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis was achieved by DEAE, gel filtration, and chromatofocusing chromatographies. Thioether methyltransferase is monomeric with a molecular weight of 28,000 and has a pI of 5.3. The pH optimum was 6.3, and Km values for dimethyl selenide and S-adenosylmethionine were 0.4 and 1.0 microM, respectively. The enzyme was inhibited 50% by 25 microM sinefungin, an analog of S-adenosylmethionine, or 40 microM S-adenosylhomocysteine, the reaction product. Pure thioether methyltransferase methylated selenium in dimethyl selenide, tellurium in dimethyl telluride, and S in dimethyl sulfide and many other thioethers. These data suggest a general role for this novel enzyme in the synthesis of onium compounds with increased aqueous solubility helpful in their excretion.

Animals↗
PubMed1987

Periodate-oxidized adenosine inhibits the formation of dimethylselenide and trimethylselenonium ion in mice treated with selenite.

The metabolic detoxification of selenite and many other selenium compounds involves a series of S-adenosylmethionine-dependent methylations yielding dimethylselenide (DMSe), which is exhaled, and trimethylselenonium ion (TMSe), which is excreted in the urine. This paper shows that periodate-oxidized adenosine (Adox) inhibits these methylation reactions in vivo and increases the toxicity of selenite. When Adox was injected in mice at 100 mumol/kg 30 min before injection of [75Se]selenite at 0.4 mg Se/kg the appearances of [75Se]DMSe in the breath and [75Se]TMSe in the liver were completely inhibited for 90 min. This was mediated by accumulation of S-adenosylhomocysteine, the methyltransferase inhibitor, in the livers of Adox-treated mice due to inhibition of its hydrolase enzyme. During 24 h, Adox-treated mice excreted no detectable urinary [75Se]TMSe and exhaled only 20% as much [75Se]DMSe as controls. The urine of Adox-treated mice also contained S-adenosylhomocysteine at a level (ca. 4 mM), 200 times that of untreated mice, which provided a convenient index of methylation potential in the intact animal. When three groups of three mice each were injected with 100 mumol Adox/kg, selenite at 4 mg Se/kg, or a combination of the two, the mice receiving the combination were dead within 2 days, while the mice in the other two groups all survived at least 4 days. These results verify the enzymatic nature of selenium methylation in vivo, support its importance in detoxification, and indicate the value of Adox in further studies of selenium metabolism.

Adenosine↗
PubMed1986

Chromatographic analysis of the chiral and covalent instability of S-adenosyl-L-methionine.

The chirality of biologically active S-adenosyl-L-methionine (AdoMet) is S,S, where the designations refer to the sulfur and the alpha-carbon, respectively. This paper describes a cation-exchange high-performance liquid chromatographic (HPLC) method for separating (S,S)-AdoMet from the biologically inactive (R,S)-AdoMet that results from racemization at the sulfur. This method was used to measure the rates of the degradation reactions of (S,S)-AdoMet as a function of pH. These reactions and the first-order rate constants, which were found at 37 degrees C and pH 7.5, are racemization, 1.8 X 10(-6) s-1; cleavage to homoserine lactone and 5'-(methylthio)adenosine, 4.6 X 10(-6) s-1; and hydrolysis to adenine and S-pentosylmethionine, 3 X 10(-6) s-1. Racemization showed no change in rate over the pH range from 7.5 to 1.5. The cleavage reaction persisted until the pH was lowered to 1.5, but hydrolysis ceased at pH 6. Commercial samples of nonradioactive AdoMet contained 20-30% (R,S)-AdoMet, while a sample of [methyl-3H]AdoMet had less than 1% (R,S)-AdoMet. Preparing enzyme substrates by mixing such samples will cause an underestimate of specific activity and an overestimate of the amount of product. The (R,S)-AdoMet/(S,S)-AdoMet ratio in mouse liver was 0.03, much less than the value of 0.19 calculated from the above rate constants. An enzyme extract from mouse liver did not degrade (R,S)-AdoMet, but a more thorough search may find such an activity. In any event, the cleavage and hydrolysis reactions partially balance the racemization of (S,S)-AdoMet in vivo and prevent excessive accumulation of (R,S)-AdoMet.

Animals↗
PubMed1983

Fractionation and kinetic properties of rat liver and kidney methionine adenosyltransferase isozymes.

Three isozymes of methionine adenosyltransferase (EC 2.5.1.6; MAT-I, -II, and -III) exist in normal rat liver and are conveniently purified (MAT-III to homogeneity) by a three-step column chromatography procedure. MAT-I shows Michaelis-Menten kinetics with a Km (L-methionine) of 41 microM and a molecular weight of 208 000 and is slightly inhibited by S-adenosyl-L-methionine (Adomet). MAT-II, which is also the only isozyme found in normal rat kidney, shows negative cooperativity with a Hill coefficient of 0.7. It has a L-methionine concentration required for half-maximal velocity [S0.5(Met)] of 8 microM and a molecular weight of 120 000 and is strongly inhibited by Adomet. MAT-I and -II comprise 15% and 5%, respectively, of total MAT activity in rat liver. The predominant isozyme in rat liver, MAT-III, demonstrates positive cooperativity with a Hill coefficient of 1.8. It has a molecular weight of 97 000 and apparently consists of two subunits of identical molecular weight (47 000). This liver-specific isozyme is strongly activated by both dimethyl sulfoxide and Adomet and has a S0.5(Met) of 215 microM.

Animals↗
PubMed1983

Effect of methotrexate with 5-methyltetrahydrofolate rescue and dietary homocystine on survival of leukemic mice and on concentrations of liver adenosylamino acids.

We have increased significantly the survival time of DBA/2 mice bearing methionine-dependent L1210 or L5178Y leukemia cells by i.p. administration of lethal doses of methotrexate (five daily doses of 25 mg/kg body weight) followed by rescue with 5-methyl tetrahydrofolate (five daily doses of 20 mg/kg body weight). The mice were maintained on a semipurified choline- and cyst(e)ine-free diet containing 0.32% L-methionine. We further increased significantly the survival time of the treated animals bearing L5178Y cells, but not those bearing L1210 cells, by substitution of 0.86% DL-homocystine for the methionine in the diet. We have examined the effects of both diets in mice treated with methotrexate and 5-methyl tetrahydrofolate, singly and in combination, on the concentrations of S-adenosylmethionine and S-adenosylhomocysteine in the liver, a tissue highly active in the metabolism of these amino acids. The substitution of homocystine for methionine in the diet of untreated animals led to a significant increase in S-adenosylhomocysteine and decrease in S-adenosylmethionine in the liver, with a resultant profound decrease in the ratio of S-adenosylmethionine to S-adenosylhomocysteine which was not further altered significantly by administration of methotrexate.

Animals↗
PubMed1980

Decarboxylated S-adenosylmethionine in mammalian cells.

The content of decarboxylated S-adenosylmethionine in rat tissues was found to be of the order of 0.9 to 2.5 nmol/g wet weight, about 2 to 4% of the content of S-adenosylmethionine. Three methods were used for determinations: separation by high pressure liquid chromatography followed by quantitation using UV absorbance, separation by paper electrophoresis after labeling with radioactive methionine to get the ratio of S-adenosylmethionine to decarboxylated S-adenosylmethionine, and separation by electrophoresis followed by elution and assay by an isotope dilution technique using spermidine synthase. The three methods gave comparable results, although the labeling with methionine appeared to slightly overestimate levels in the liver. Hepatic decarboxylated S-adenosylmethionine content was reduced by more than 90% for at least 8 h by treatment with methylglyoxal bis(guanylhydrazone), a potent reversible inhibitor of S-adenosylmethionine decarboxylase, but had returned to control levels by 24 h. The related irreversible inhibitor, 1,1'-[(methylethanediylidene)-dinitrilo]bis(3-aminoguanidine), reduced levels slightly less to about 20% of control, but maintained them at this value for 2 days.

Animals↗
PubMed1980

Flagellar tip activation stimulated by membrane adhesions in Chlamydomonas gametes.

Membrane adhesions between the flagella of mating-type "plus" and "minus" gametes of Chlamydomonas reinhardi are shown to stimulate a rapid change in the ultrastructure of the flagellar tips, designated as flagellar tip activation (FTA). A dense substance, termed fibrous tip material (FTM), accumulates between the flagellar membrane and the nine single A microtubules of the tip. The A microtubules then elongate, growing into the distal region of the tip, increasing tip length by 30%. This study describes FTA kinetics during normal and mutant matings, presents experiments designed to probe its role in the mating reaction, and offers the following conclusions: (a) FTA is elicited by agents that cross-link flagellar membrane components (including natural sexual agglutinins, antiflagellar antisera, and concanavalin A) but not by flagellar adherence to polylysine-coated films. (b) FTA is reversed by flagellar disadhesion. (c) Gametes can undergo repeated cycles of FTA during successive rounds of adhesion/disadhesion. (d) FTA, flagellar tipping, and sexual signaling are simultaneously blocked by colchicine and by vinblastine, suggesting that tubulinlike molecules, perhaps exposed at the membrane surface, are involved in all three responses. (e) FTA is not blocked by short exposure to chymotrypsin, by cytochalasins B and D, nor by concanavalin A, even though all block cell fusion; the response is therefore autonomous and experimentally dissociable from later stages in the mating reaction. (f) Under no experimental conditions is mating-structure activation observed to occur unless FTA also occurs. This study concludes that FTA is a necessary event in the sexual signaling sequence, and presents a testable working model for its mechanism.

Agglutinins↗
PubMed1980

Experimental dissection of flagellar surface motility in Chlamydomonas.

Experiments have explored the possible relationships between the flagellar surface motility of chlamydomonas, visualized as translocation of polystyrene beads by paralyzed (pf) mutants (Bloodgood, 1977, J. Cell Biol. 15:983-989), and the capacity of gametic flagella to participate in the mating reaction. While vegetative and gametic flagella bind beads with equal efficiencies and are capable of transporting them along entire flagellar lengths, beads on vegetative flagella are primarily associated with the proximal half of the flagella whereas those of gametic flagella exhibit no such preference. This difference may relate to the "tipping" response of gametes during sexual flagellar agglutination (Goodenough and Jurivich, 1978, J. Cell Biol. 79:680-693). Colchicine, vinblastine, chymotrypsin, cytochalasins B and D, and anti-beta-tubulin antiserum are all able to inhibit the binding of beads to the flagellar suface. Trysin digestion and an antiserum directed against whole chlamydomonas flagella have no effect on the ability of flagella to bind beads, but the beads remain immobile. These results suggest that at least two flagellar activities participate in surface motility: (a) bead binding, which may involve a tubulin-like component at the flagellar surface; and (b) bead translocation, which may depend on a second component (e.g. an ATPase) of the flagellar surface. Surface motility is shown to be distinct from gametic adhesiveness per se, but it may participate in concentrating dispersed agglutinins, in driving them toward the flagellar tips, and/or in generating a signal-to-fuse from the flagellar tips to the cell body. Directly supporting these concepts is the observation that bound beads remain immobilized at the flagellar tips during the "tip-locking" stage of pf x pf matings, and the observation that bound ligands such as antibody fail to be tipped by trypsinized flagella.

Antigen-Antibody Complex↗
PubMed1980

Membrane-membrane and membrane-ligand interactions in Chlamydomonas mating.

Our investigations of the mating reaction of Chlamydomonas revealed a surprisingly intricate series of interrelated events. Adhering sites are moved to the flagellar tips in a fashion highly reminiscent of the capping of surface ligands over the centriolar regions of lymphocytes (28). Tipping is prevented by the gam-1 mutation and by agents that interact with tubulin; the molecular mechanism(s) for the inhibition effects are currently being sought. Tip locking appears to be accompanied by the accumulation of a dense material beneath the tip membrane, a postulated alteration of axonemal structure, and an immobilization of component(s) involved in surface motility. Two mating signals are then transduced to the locked-in cells who respond by shedding cell walls, activating mating structures, and fusing together. Signal transmission and/or reception is sensitive to such agents as trypsin, chymotrypsin, and cold temperature. Once zygotic cell fusion has occurred, tip unlocking and a reversal of the tip activation response appear to occur in parallel. Since all of these events can occur within 30 sec, the mating reaction serves as an experimental paradigm for studying rapid cellular responses to specific membrane-membrane interactions.

Agglutination↗
PubMed1978

Biosynthesis of S-N6-methyladenosylhomocysteine, an inhibitor of RNA methyltransferases.

This paper demonstrates that N6-methyladenosine (6-methylaminopurine ribonucleoside) will condense in vitro with homocysteine to form S-N6-methyladenosylhomocysteine in a reaction catalyzed by mouse liver S-adenosylhomocysteine hydrolase. Injection of mice with N6-methyladenosine is followed by accumulation of S-N6-methyladenosylhomocysteine in the liver. Studies from other laboratories have shown that S-N6-methyladenosylhomocysteine is nearly as potent an RNA methyltransferase inhibitor as S-adenosylhomocysteine. This indicates that administration of N6-methyladenosine may be a general method for blocking in vivo RNA methylation in studies to determine the role of methylation in RNA processing and translational function.

Animals↗