Non-sterol metabolism of mevalonate in vitro: artifacts and realities.

Commercial [5-¹⁴C]mevalonate is shown to contain several radioactive impurities, which give artifactually high amounts of Hyamine bound, volatile acidic radioactivity when incubated with killed or living rat renal cortex slices, as compared with [5-¹⁴C]mevalonate purified either by liquid-liquid partition chromatography or through the enzymically generated $\text{R}\text{-5-phospho-[5-}{}^{14}\text{C]mevalonate}$ by ion-exchange chromatography. The artifactual ${}^{14}\text{CO}{}_2$ results were not diluted by incubation with increasing amounts of unlabelled mevalonate, whereas the ${}^{14}\text{CO}{}_2$ and [${}^{14}\text{C}$]cholesterol produced by rat renal cortex slices incubated with purified [5-¹⁴C]mevalonate were both diluted to the same extent by unlabelled mevalonate. It is concluded that $\text{R}\text{[5-}{}^{14}\text{C]mevalonate}$ is genuinely oxidized to ${}^{14}\text{CO}{}_2\text{in}\text{vitro}$, and that purification of substrate before its use is necessary. Production of ${}^{14}\text{CO}{}_2$ and various [${}^{14}\text{C}$]lipids from purified [5-¹⁴C]mevalonate, as a function of time and substrate concentration, by renal cortex and liver slices, is described.     

Structure and action of heteronemertine polypeptide toxins Binding of cerebratulus lacteus toxin B-IV to axon membrane vesicles

The binding of the crustacean selective protein neurotoxin, toxin B-IV, from the nemertine Cerebratulus lacteus to lobster axonal vesicles has been studied. A highly radioactive, pharmacologically active derivative of toxin B-IV has been prepared by reaction with Bolton-Hunter reagent. Saturation binding and competition of ¹²⁵I-labeled toxin B-IV by native toxin B-IV have shown specific binding of ¹²⁵I-labeled toxin B-IV to a single class of binding sites with a dissociation constant of 5–20 nM and a binding site capacity, corrected for vesicle sidedness, of 6–9 pmol per mg membrane protein. This compares to a value of 3.8 pmol [³H]saxitoxin bound per mg in the same tissue. Analysis of the kinetics of toxin B-IV association ($\text{k}{}_{\mathrm{+1}}\text{=7.3·10}{}^5\text{M}{}^{\mathrm{?1}}\text{·s}{}^{\mathrm{?1}}$) and dissociation ($\text{k}{}_{\mathrm{?\; 1}}\text{=2·10}{}^{\mathrm{?3}}\text{s}{}^{\mathrm{?1}}$) shows a nearly identical $\text{K}{}_{\mathrm{<mtext>d</mtext>}}$ of about 3 nM. There is no competition of toxin B-IV binding by purified toxin from Leiurus quinquestriatus venom while Centruroides sculpturatus Ewing toxin I appears to cause a small enhancement of toxin B-IV binding.     

Ligand-dependent tryptic inactivation of the ouabain sensitivity of ADP-ATP exchange catalyzed by canine renal Na+, K+-ATPase.

Phosphate transporter of bovine heart mitochondria was purified by solubilization of submitochondrial particles with octylglucoside and fractionation of the extract with ammonium sulfate. After reconstitution into liposomes the purified protein catalyzed phosphate transport which was sensitive to mersalyl and other SH reagents. Transport measured either as $\text{P}{}_{\mathrm{i}}\text{OH}$ or $\text{P}{}_{\mathrm{i}}\text{P}{}_{\mathrm{i}}$ exchange was proportional to protein concentration and time. The $\text{P}{}_{\mathrm{i}}\text{OH}$ but not the $\text{P}{}_{\mathrm{i}}\text{P}{}_{\mathrm{i}}$ exchange was stimulated several fold by valinomycin plus nigericin in the presence of K⁺. The reconstituted system provides a suitable assay during purification of the mitochondrial phosphate transporter.     

Studies on (K+ + H+)-ATPase IV. Effects of phospholipase C treatment

(1) The total phospholipid content of a gradient purified ($\text{K}{}^{\mathrm{+}}\text{+ H}{}^{\mathrm{+}}$)-ATPase preparation from pig gastric mucosa is 105 μmol per 100 mg protein, and consists of 29% sphingomyelin, 29% phosphatidylcholine, 28% phosphatidylethanolamine, 10% phosphatidylserine and 4% phosphatidylinositol. The cholesterol content corresponds to 50 μmol per 100 mg protein. (2) Treatment with phospholipase C (from Clostridium welchii and Bacillus cereus) results in an immediate decrease of the phosphate content. Up to 50% of the phospholipids are hydrolyzed by each phospholipase C preparation alone, without further hydrolysis by increased phospholipase concentration or prolonged incubation time. Combined treatment with the two phospholipase C preparations, sequentially or simultaneously, hydrolyzes up to 65% of the phospholipids. (3) The ($\text{K}{}^{\mathrm{+}}\text{+ H}{}^{\mathrm{+}}$)-ATPase and K⁺ stimulated $\text{p-}\text{nitrophenylphosphatase}$ activities are decreased proportionally with the total phospholipid content, indicating that these enzyme activities are dependent on phospholipids. (4) Phospholipase C treatment does not change optimal pH, $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ value for ATP and temperature dependence of the gastric ($\text{K}{}^{\mathrm{+}}\text{+ H}{}^{\mathrm{+}}$)-ATPase, but slightly decreases the $\text{K}{}_{\mathrm{<mtext>a</mtext>}}$ value for K⁺. (5) Phospholipase C treatment lowers the $\text{Ado}\text{PP[}\text{NH}\text{]P}$ binding and phosphorylation capacities, suggesting that inactivation occurs primarily on the substrate binding level. (6) Most of the results can be understood by assuming that hydrolysis of the phospholipids by phospholipase C leads to aggregation of the membrane protein molecules and complete inactivation of the aggregated ATPase molecules.     

Effects of 5-hydroxylysine on acetylene reduction and NH4+-assimilation in the cyanobacterium Anabaena cylindrica.

5-hydroxylysine, an analogue of glutamate and lysine, causes $\text{NH}{}_4{}^{\mathrm{+}}$ production by N₂-fixing A. cylindrica; it also reversibly inhibits GS activity in vitro but has no effect on alanine dehydrogenase or GOGAT. On adding 5-hydroxylysine intracellular pools of glutamine, glutamate and aspartate decrease; those of alanine and serine increase. 5-hydroxylysine alleviates the inhibitory effect of $\text{NH}{}_4{}^{\mathrm{+}}$ on heterocyst production and C₂H₂ reduction and in $\text{NH}{}_4{}^{\mathrm{+}}\text{-grown}$ cultures results in heterocyst synthesis and in C₂H₂ reduction. The data suggest that the GS-GOGAT pathway is the sole route of importance in primary $\text{NH}{}_4{}^{\mathrm{+}}$ assimilation in A. cylindrica, that $\text{NH}{}_4{}^{\mathrm{+}}$ alone does not inhibit nitrogenase and heterocyst production, and that GS and/or a product is involved in regulating the production of both.     

Binding of native, glucosamine-labeled, luteinizing hormone to Leydig cells.

A method for obtaining pituitary glycoprotein hormones labeled either in the protein backbone or in the carbohydrate moiety, by incubation of pituitary slices in the presence of radioactive leucine or glucosamine, has been developed. This report describes the subsequent purification of the “native” labeled luteinizing hormone from the incubation medium and its use in binding studies with testicular tissue. The purified radioactive luteinizing hormone specifically bound to Leydig cells could be displaced with excess human chorionic gonadotropin, but was unaffected by excess follicle stimulating hormone. Binding data indicated that $\text{K}{}_{\mathrm{d}}\text{= 5.2 × 10}{}^{\mathrm{?9}}\text{m}$ and approximately 6 × 10⁴ binding sites per cell.     

Saturation transfer electron paramagnetic resonance on membrane bound proteins. II-Absence of rotational diffusion of the cholinergic receptor protein in Torpedo marmorata membrane fragments.

We have applied the technique of saturation transfer electron paramagnetic resonance to study the rotational diffusion of spin labeled membrane bound cholinergic receptors from Torpedo marmorata. Two different spin labels were used: a spin labeled maleimide derivative which binds covalently to proteins and a long chain spin labeled acylcholine which binds reversibly with a high affinity to the receptor protein. The maleimide spin label has a motion whose rotational correlation time is τ₂ > 10^?3 sec. The long chain spin labeled acylcholine indicates slightly more motion ($\text{τ}{}_2\text{? 10}{}^{\mathrm{?4}}\text{sec}$), but the nitroxide in this latter case is probably more loosely bound.     

Mitochondrial phosphate transport and the N-ethylmaleimide binding proteins of the inner membrane

Mitochondria have been prepared from the flight muscles of mature blowflies (Sarcophaga bullata). Phosphate transport by these mitochondria, determined by rates of passive swelling in ammonium phosphate, is sensitive to inhibition by N-ethylmaleimide. 20 nmol of N-ethylmaleimide/nmol cytochrome a inhibit the swelling by 90%. When the mitochondria are inhibited by $\text{N-[}{}^3\text{H}\text{]}\text{ethylmaleimide}$, then solubilized in dodecyl sulfate/mercaptoethanol at 100°C and then electrophoresed on dodecyl sulfate-polyacrylamide gels, many labeled protein bands can be detected, including a large labeled peak that has the same mobility as the tracking dye, bromophenol blue. Sonic submitochondrial particles that are prepared from the $\text{N-[}{}^3\text{H}\text{]}\text{ethylmaleimidelabeled}$ mitochondria, solubilized, and electrophoresed on dodecyl sulfatepolyacrylamide gels, possess only seven major labeled protein bands with no radioactive peak at the tracking dye. These labeled proteins have molecular weights of 71, 68, 64, 45, 32, 30, and approx. 10 · 10³. The nmol $\text{N-[}{}^3\text{H}\text{]-}\text{ethylmaleimide}$ bound to each of these proteins per nmol cytochrome a are 0.15, 0.19, 0.35, 0.45, 0.87, 0.10, and 0.17, respectively, when the mitochondria are inhibited with 21.5 mol $\text{N-[}{}^3\text{H}\text{]}\text{ethylmaleimide/mol}$ cytochrome a at 10 μM cytochrome a. Coty and Pedersen ((1975) J. Biol. Chem. 250, 3515–3521) sensitized rat liver mitochondria to $\text{N-[}{}^3\text{H}\text{]}\text{ethylmaleimide}$ and identified five labeled proteins. Only the labeled 32 · 10³ dalton and the 45 · 10³ dalton proteins are common to both systems     

Interaction of lymphocytes and macrophage cell line cells (M1 cells). I. Functional maturation and appearance of Fc receptors im M1 cells

M₁ cells, which are cell line cells established from myeloid leukemia cells of the SL strain mouse, can differentiate from blast cells ($\text{M}{}_1{}^{\mathrm{?}}$) to mature macrophages ($\text{M}{}_1{}^{\mathrm{+}}$) within 48 hr, when they are cultured with conditioned medium (CM) obtained from murine embryonic fibroblasts. While $\text{M}{}_1{}^{\mathrm{?}}$ cells have no phagocytic activity nor Fc receptor (FcR), $\text{M}{}_1{}^{\mathrm{+}}$ cells possess both characteristics. The appearance of FcR is temperature-dependent and inhibited by a metabolic inhibitor, cycloheximide. FcR on $\text{M}{}_1{}^{\mathrm{+}}$ cells is resistant to trypsin and pronase. $\text{M}{}_1{}^{\mathrm{+}}$ cells improve the viability of macrophage-depleted SL splenic lymphocytes and restore the in vitro secondary plaque forming cell response of macrophage-depleted spleen cells to particulate and soluble antigens. $\text{M}{}_1{}^{\mathrm{?}}$ cells lack this macrophage-substituting capacity. Mm₁ cells, mutant cells from M₁ cells, having FcR and higher phagocytic activity than $\text{M}{}_1{}^{\mathrm{+}}$ cells, are also devoid of this capacity.     

Respiration-driven proton translocation with nitrite and nitrous oxide in Paracoccus denitrificans

(1) $\text{H}$⁺/electron acceptor ratios have been determined with the oxidant pulse method for cells of denitrifying Paracoccus denitrificans oxidizing endogenous substrates during reduction of O₂, NO^?₂ or N₂O. Under optimal H⁺-translocation conditions, the ratios $\text{H}{}^{\mathrm{+}}\text{O}$, $\text{H}{}^{\mathrm{+}}\text{N}{}_2\text{O}$, $\text{H}{}^{\mathrm{+}}\text{NO}{}^{\mathrm{?}}{}_2$ for reduction to N₂ and $\text{H}{}^{\mathrm{+}}\text{NO}{}^{\mathrm{?}}{}_2$ for reduction to N₂O were 6.0–6.3, 4.02, 5.79 and 3.37, respectively. (2) With ascorbate/N,N,N′,N′-tetramethyl-p-phenylenediamine as exogenous substrate, addition of NO^?₂ or N₂O to an anaerobic cell suspension resulted in rapid alkalinization of the outer bulk medium. $\text{H}{}^{\mathrm{+}}\text{N}{}_2\text{O}$, $\text{H}{}^{\mathrm{+}}\text{NO}{}^{\mathrm{?}}{}_2$ for reduction to N₂ and $\text{H}{}^{\mathrm{+}}\text{NO}{}^{\mathrm{?}}{}_2$ for reduction to N₂O were ?0.84, ?2.33 and ?1.90, respectively. (3) The $\text{H}{}^{\mathrm{+}}\text{oxidant}$ ratios, mentioned in item 2, were not altered in the presence of $\text{valinomycin}\text{K}{}^{\mathrm{+}}$ and the triphenylmethylphosphonium cation. (4) A simplified scheme of electron transport to O₂, NO^?₂ and N₂O is presented which shows a periplasmic orientation of the nitrite reductase as well as the nitrous oxide reductase. Electrons destined for NO^?₂, N₂O or O₂ pass two H⁺-translocating sites. The $\text{H}{}^{\mathrm{+}}\text{electron}$ acceptor ratios predicted by this scheme are in good agreement with the experimental values.     

Heterozygote frequencies in small subpopulations.

The mean fixation index within subpopulations (F_IS) has been defined as $\text{F}\text{̄}{}_{\mathrm{IS}}\text{= ∑w}{}_{\mathrm{i}}\text{F}{}_{\mathrm{ISi}}\text{or as}\text{F}\text{̂}{}_{\mathrm{IS}}\text{=}\text{∑w}{}_{\mathrm{i}}\text{p}{}_{\mathrm{i}}\text{q}{}_{\mathrm{i}}\text{F}{}_{\mathrm{ISi}}\text{∑w}{}_{\mathrm{i}}\text{p}{}_{\mathrm{i}}\text{q}{}_{\mathrm{i}}$. The latter definition is preferred because it can be obtained from the two other fixation indices, F_ST and F_IT and because it is unaffected by the mean gene frequency. The expected frequency of heterozygotes in small subpopulations of dioecious organisms will exceed Hardy-Weinberg expectations and this can be measured by $\text{F}\text{̂}{}_{\mathrm{IS}}$. In an isolated subpopulation of constant variance effective size $\text{N,}\text{F}\text{̂}{}_{\mathrm{IS}}$ rapidly tends to $\text{1 − 4N}{}^2\text{(N − 1 + [N}{}^2\text{+ 1]}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{)}{}^2$. In the Island model of population structure, $\text{F}\text{̂}{}_{\mathrm{IS}}$ is approximately $\text{−(1 − m)}\text{N}\text{where}\text{m}$ is the immigration rate.When a sample is drawn from a natural population, the observed F_IS will depend upon the genetic structure of the population. The values of F_IS expected in three different types of population structure are discussed.     

Effects of L-methionine-DL-sulfoximine and beta-N-oxalyl-L-alpha, beta-diaminopropionic acid on nitrogenase biosynthesis and activity in Rhodopseudomonas capsulata.

Inhibitors of glutamine synthetase cause derepression of nitrogenase biosynthesis in the presence of $\text{NH}{}_4{}^{\mathrm{+}}$ in the photosynthetic bacterium Rhodopseudomonas capsulata. A new derepressor of nitrogenase biosynthesis, β-N-oxalyl-L-α,β-diaminopropionic acid (ODAP), is here compared with the widely used L-methionine-DL-sulfoximine (MSX). With both compounds, a quantitative correlation has been observed between inhibition of glutamine synthetase and derepression of nitrogenase biosynthesis. We also find that both MSX and ODAP inhibit nitrogenase activity in vivo in R. capsulata. The latter effect seems to be indirect and related to the previously reported reversible inhibition of nitrogenase activity in vivo by $\text{NH}{}_4{}^{\mathrm{+}}$. As a control it was observed that neither $\text{NH}{}_4{}^{\mathrm{+}}$ nor MSX nor ODAP inhibit nitrogenase activity in vivo in Clostridium pasteurianum.     

The effect of formate on cytochrome aa3 and on electron transport in the intact respiratory chain

1. Formate inhibits cytochrome $\text{c}$ oxidase activity both in intact mitochondria and submitochondrial particles, and in isolated cytochrome $\text{aa}{}_3$. The inhibition increases with decreasing pH, indicating that HCOOH may be the inhibitory species.2. Formate induces a blue shift in the absorption spectrum of oxidized cytochrome $\text{aa}{}_3\text{(a}{}^{\mathrm{3+}}\text{a}{}_3{}^{\mathrm{3+}}$) and in the half-reduced species ($\text{a}{}^{\mathrm{2+}}\text{a}{}_3{}^{\mathrm{3+}}$). Comparison with cyanide-induced spectral shifts, towards the red, indicates that formate and cyanide have opposite effects on the $\text{aa}{}_3$ spectrum, both in the fully oxidized and the half-reduced states. The formate spectra provide a new method of obtaining the difference spectrum of $\text{a}{}_3{}^{\mathrm{2+}}$ minus $\text{a}{}_3{}^{\mathrm{3+}}$, free of the difficulties with cyanide (which induces marked high → low spin spectral shifts in cytochrome $\text{a}{}_3{}^{\mathrm{3+}}$) and azide (which induces peak shifts of cytochrome $\text{a}{}^{\mathrm{2+}}$ towards the blue in both α- and Soret regions).3. The rate of formate dissociation from cytochrome $\text{a}{}^{\mathrm{2+}}\text{a}{}_3{}^{\mathrm{3+}}\text{-}\text{HCOOH}$ is faster than its rate of dissociation from $\text{a}{}^{\mathrm{3+}}\text{a}{}_3{}^{\mathrm{3+}}\text{-}\text{HCOOH}$, especially in the presence of cytochrome $\text{c}$. The $\text{K}{}_{\mathrm{<mtext>i</mtext>}}$ for formate inhibition of respiration is a function of the reduction state of the system, varying from 30 mM (100% reduction) to 1 mM (100% oxidation) at pH 7.4, 30 °C.4. Succinate-cytochrome $\text{c}$ reductase activity is also inhibited by formate, in a reaction competitive with succinate and dependent on [formate]².5. Formate inhibition of ascorbate plus $\text{N,N,N′,N′-}\text{tetramethyl}\text{-p-}\text{phenyl-enediamine}$ oxidation by intact rat liver mitochondria is partially released by uncoupler addition. Formate is permeable through the inner mitochondrial membrane and no differences in ‘on’ or ‘off’ inhibition rates were observed when intact mitochondria were compared with submitochondrial particles.6. NADH-cytochrome $\text{c}$ reductase activity is unaffected by formate in submitochondrial particles, but mitochondrial oxidation of glutamate plus malate is subject both to terminal inhibition at the cytochrome $\text{aa}{}_3$ level and to a slow extra inhibition by formate following uncoupler addition, indicating a third site of formate action in the intact mitochondrion.     

Kinetics of phospholipid exchange between bilayer membranes

In an accompanying publication by Duckwitz-Peterlein, Eilenberger and Overath ((1977) Biochim. Biophys. Acta 469, 311–325) it is shown that the exchange of lipid molecules between negatively charged vesicles consisting of total phospholipid extracts from Escherichia coli occurs by the transfer of single lipid monomers or small micelles through the water. Here a kinetic interpretation is presented in terms of a rate constant, $\text{k}{}_{\mathrm{?}}$, for the escape of lipid molecules from the vesicle bilayer into the water. The evaluated rate constants are $\text{k}{}_{\mathrm{?}}{}^{\mathrm{<mtext>P</mtext>}}\text{= (0.86 ± 0.05) · 10}{}^{\mathrm{?5}}\text{s}{}^{\mathrm{?1}}$ and $\text{k}{}_{\mathrm{?}}{}^{\mathrm{<mtext>E</mtext>}}\text{= (1.09 ± 0.13) · 10}{}^{\mathrm{?6}}\text{s}{}^{\mathrm{?1}}$ for phospholipid molecules with $\text{trans-}\text{Δ}{}^9\text{-hexadecenoate}$ and $\text{trans-}\text{Δ}{}^9\text{-octadecenoate}$, respectively, as the predominant acyl chain component. The rate constants are discussed in terms of the acyl chain and polar head group composition of the lipids.     

Influence of ionic strength upon the proton inventory for the water-catalyzed hydrolysis of N-acetylbenzotriazole

Proton inventory investigations of the hydrolysis N-acetylbenzotriazole at pH 3.0 (or the equivalent point on the pD rate profile) have been conducted at two different temperatures and at ionic strengths ranging from 0 to 3.0 M. The solvent deuterium isotope effects and proton inventories are remarkably similar over this wide range of conditions. The proton inventories suggest a cyclic transition state involving four protons contributing to the solvent deuterium isotope effect for the water-catalyzed hydrolysis. The hydrolysis data are described by the equation $\text{k}{}_{\mathrm{n}}\text{= k}{}_{\mathrm{<mtext>o</mtext>}}\text{(1 ? n + nπ}{}_{\mathrm{<mtext>a</mtext>}}{}^1\text{)}{}^4$ with $\text{π}{}_{\mathrm{<mtext>a</mtext>}}{}^1\text{～ 0.74}$, where k_o is the observed first-order rate constant in protium oxide, n is the atom fraction of deuterium in the solvent, k_n is the rate constant in a protium oxide-deuterium oxide mixture, and $\text{π}{}_{\mathrm{<mtext>a</mtext>}}{}^1$ is the isotopic fractionation factor.     

Photophosphorylation in bacterial chromatophores entrapped in alginate gel: Improvement of the physical and biochemical properties of gel beads with barium as gel-inducing agent

The immobilization of Rhodopseudomonas capsulata chromatophores by entrapment in an alginate gel is described. Alginate beads were prepared with Ba²⁺, Sr²⁺ and Ca²⁺ as gel-forming agents and compared for their mechanical strength, chemical resistance against disruption by phosphate-induced swelling, and yield of photophosphorylation activity. Barium alginate beads proved to have better physico-chemical properties than the more commonly used calcium alginate beads. After embedding in barium alginate gel, R. capsulata chromatophores retained a high yield (up to 70%) of their photophosphorylation capacity. Alginate entrapment did not cause a large increase in the Michaelis constant for ADP and phosphate, the substrates of adenosinetriphosphatase (ATPase). These constants were $\text{K}{}^{\mathrm{ADP}}{}_{\mathrm{<mtext>m</mtext>}}\text{= 1.4 × 10}{}^{\mathrm{?5}}\text{m}$ and $\text{K}{}^{\mathrm{P}}{}_{\mathrm{i}}{}_{\mathrm{<mtext>m</mtext>}}\text{= 2.2 × 10}{}^{\mathrm{?4}}\text{m}$ for free chromatophores and $\text{K}{}^{\mathrm{ADP}}{}_{\mathrm{<mtext>m</mtext>}}\text{= 2.3 × 10}{}^{\mathrm{?4}}\text{m}$ and $\text{K}{}^{\mathrm{P}}{}_{\mathrm{i}}{}_{\mathrm{<mtext>m</mtext>}}\text{= 5.6 × 10}{}^{\mathrm{?4}}\text{m}$ for chromatophores entrapped in barium alginate gel. However, embedding gave no additional protection against rapid inactivation of chromatophores upon storage at 3°C. Preliminary results with a batch reactor for continuous ATP regeneration are presented. The barium alginate method has two features which are not generally encountered at the same time, extremely mild conditions for entrapment and excellent physical properties of the gels beads, which make this method a suitable tool for the construction of bioreactors with immobilized cells or organelles.     

Preferential solvation of methylmercurated calf thymus deoxyribonucleic acid.

The changes in polymer-solvent interactions that occur when native calf thymus DNA is dialyzed against Na₂SO₄ solutions of a given ionic strength and buffer concentration but of varying concentrations in methylmercuric hydroxide have been investigated with the help of solution density measurements at 25 °C and pH 6.8–7.0. From measurements executed under equilibrium dialysis conditions at the three salt levels 5 mm, 0.05 m, and 0.5 m Na₂SO₄ (m refers to molality) and in the presence of 5 mm cacodylic acid buffer, the density increments $\text{(}\text{??}\text{?c}{}_2\text{)}{}_{\mathrm{\mu }}{}^0$ for native calf thymus DNA were determined as a function of CH₃HgOH concentration. $\text{(}\text{??}\text{?c}{}_2\text{)}{}_{\mathrm{\mu }}{}^0$ was found not to vary with organomercurial concentration, irrespective of the concentration of supporting electrolyte, until a certain CH₃HgOH concentration level has been reached, viz., , beyond which $\text{(}\text{??}\text{?c}{}_2\text{)}{}_{\mathrm{\mu }}{}^0$ increases strongly with increasing concentration of CH₃HgOH. As is shown by optical melting, $\text{(}\text{??}\text{?c}{}_2\text{)}{}_{\mathrm{\mu }}{}^0$ becomes a function of organomercurial concentration the moment DNA undergoes denaturation brought about by the complexing of CH₃HgOH with the various N-binding sites of the base residues in the DNA double helix.Polymer-solvent interactions, expressed in terms of preferential water interactions (“net hydration”) and preferential salt interactions (“salt solvation”), were derived from the $\text{(}\text{??}\text{?c}{}_2\text{)}{}_{\mathrm{\mu }}{}^0$ data in combination with data obtained on the preferential interaction of CH₃HgOH with denatured DNA and data on the partial specific volumes of all major solution components, gathered from density measurements on solutions with fixed concentrations of diffusible components. Evidence is presented which shows that denaturation in general decreases the net hydration while salt becomes preferentially associated with the polyelectrolyte. This process is further amplified by the interaction of CH₃HgOH with denatured DNA: Methylmercurated DNA alters the redistribution of diffusible components at dialysis equilibrium to such an extent that in a formal sense large amounts of water are rejected from the immediate vicinity of the polymer. The molecular implications of these findings are explored. The results are further discussed in the light of previous findings where the methylmercury-induced denaturation of DNA had been studied with the help of buoyant density measurements in a Cs₂SO₄ density gradient and by velocity-sedimentation in a variety of sulfate media.     

Nitrogen fixation in cultured cowpea Rhizobia: inhibition and regulation of nitrogenase activity.

Nitrogenase activity in agar cultures of cowpea rhizobia, strain 32H1, was rapidly inhibited by $\text{NH}{}_4{}^{\mathrm{+}}$ but this was relieved by increased O₂ tension. Inhibition was more rapid than that caused by inhibitors of protein synthesis and was not relieved by methionine sulfoximine or methionine sulfone. Under conditions were nitrogenase activity was inhibited by $\text{NH}{}_4{}^{\mathrm{+}}$, glutamine synthetase and glutamate synthase were substantially unaffected. Glutamate dehydrogenase was undetected in either nitrogenase active or $\text{NH}{}_4{}^{\mathrm{+}}$ inhibited cultures. These results indicate that $\text{NH}{}_4{}^{\mathrm{+}}$ inhibition of nitrogenase activity in strain 32H1 is not effected through glutamine synthetase regulation of nitrogenase synthesis.