Regulation and function of glutamate synthase in Neurospora crassa

In Neurospora crassa two enzymes can provide glutamate: the NADPH dependent GDH and the NADH dependent GOGAT. An elevated GOGAT activity was found in Neurospora wild-type under ammonium limitation in contrast to a 4-fold lower activity on excess of a$\text{m}$ monium. Glutamate and glutamine repress this enzyme. On excess of ammonium the GDH-NADPH deficient mutant am-1 grows poorly with an elevated GOGAT activity. A GOGAT less mutant was found. It presented a lag-phase to grow on ammonium. It is concluded that N. crassa glutamate synthase provides glutamate from low am-monium concentrations. The enzyme was purified to homogeneity and shown to be composed of a single type of monomer with a molecular weight above 200,000.     

NADH-dependent glutamate synthase and nitrogen metabolism in Neurospora crassa

The GDH (NADPH) mutant strain $\text{am-1}$ of $\text{N. crassa}$ has sizable pools of glutamine and glutamate under ammonium-limited conditions for which requires an elevated glutamine synthetase activity. Glutamine in the pre$\text{s}$ ence of 2-oxoglutarate, stimulated nicotinamide nucleotide oxidation by crude and purified extracts of the $\text{am-1}$ strain and led to a reductant dependent formation of two molecules of glutamate. Aminooxyacetate did not have any effect on the reaction, whereas azaserine inhibited it completely. It is concluded that in $\text{N. crassa}$ glutamine synthetase and glutamate synthase are responsible for the assimilation of low ammonium concentrations.     

Induction of single-strand regions in nuclear DNA by adriamycin.

Incubation of adriamycin with isolated nuclei converts nuclear DNA to a form which is susceptible to hydrolysis by $\text{Neurospora}$$\text{crassa}$ nuclease an enzyme highly specific for the cleavage of single-stranded DNA. The effect of adriamycin on nuclear DNA incubated in the presence of the nuclease can be determined by measuring the release of acid-soluble nucleotides or by analyzing the DNA after centrifugation in neutral sucrose gradients. Similar changes in chromatin structure are not observed during incubation of nuclei with adriamycin alone. In addition to adriamycin, daunomycin and ethidium bromide are also active in inducing the formation of DNA structures which are susceptible to the $\text{Neurospora}$$\text{crassa}$ nuclease. The results suggest that certain antitumor agents can induce the formation of single-strand regions in nuclear DNA and that these sites probably occur as a result of a DNA strand separating event.     

5' Cap methylation of homologous poly A(+) RNA by a RNA (guanine-7) methyltransferase from Neurospora crassa.

RNA (guanine-7) methyltransferase, partially purified from $\text{N.}\text{crassa}$ mycelia, catalyzed the transfer of the methyl group from S-adenosylmethionine to the 5′ terminus of both $\text{N.}\text{crassa}$ poly A(+) RNA and reovirus unmethylated mRNA. RNase T₂ digestion of the $\text{in}\text{vitro}$ methylated poly A(+) RNA from $\text{N.}\text{crassa}$ yielded the “cap” structures m ⁷G(5′)pppAp and m ⁷G(5′)pppGp in a ratio of 2:1 respectively. RNase T₂ digestion of the $\text{in}\text{vitro}$ methylated reovirus mRNA yielded m ⁷G(5′)pppGp exclusively. The absence of mRNA 2′-0-methyltransferase activity in the enzyme preparation is consistent with the absence of 2′-0-methylation in $\text{N.}\text{crassa}$ mRNA [Seidel, B. L. and Somberg, E. W. (1978) Arch. Biochem. Biophys. $\text{187}$, 108–112]. This is the first isolation of an eucaryotic, cellular RNA (guanine-7) methyltransferase that has been shown to methylate homologous substrate.     

Ca 2+ -dependent allosteric regulation of nicotinamide nucleotide transhydrogenase from Pseudomonas aeruginosa

Nicotinamide nucleotide transhydrogenase from $\text{Pseudomonas}\text{aeruginosa}$ exhibits allosteric properties and has been shown to be regulated by the prevailing $\text{[NADPH]}\text{[NADP}{}^{\mathrm{+}}\text{]}$ ratio or by 2′-AMP. The present data obtained with membrane fragments from $\text{P.}\text{aerug}$. show that Ca²⁺ strongly influences the concentration of 2′-AMP or NADPH required for half-maximal stimulation. Saturating concentrations of Ca²⁺ cause full activation of the enzyme; Mn²⁺, Mg²⁺ and K⁺ are considerably less efficient and antagonistic to Ca²⁺. Some implications of these findings for the regulatory mechanism and possible physiological function of the enzyme are considered.     

The identification of MYO-inositol:NAD(P)+ oxidoreductase in mammalian brain.

$\text{myo}$-Inositol:NAD(P)⁺ oxidoreductase ($\text{myo}$-inositol oxidoreductase) has been identified in bovine brain. This enzyme elutes from DEAE cellulose with 0.3 M KCl in 50 mM Tris buffer, pH 7.5. Using NADH as cofactor $\text{myo}$-inosose-2 is reduced selectively to $\text{myo}$-inositol. With NADPH the enzyme forms both $\text{myo}$-inositol and $\text{scyllo}$-inositol, however, at a lower rate. The enzyme was chromatographed on G-100 Sephadex and found to have an apparent molecular weight of 74,000. This enzyme differs in DEAE binding, molecular weight and cofactor specificity from the previously described $\text{scyllo}$-inositol oxidoreductase which utilizes NADPH exclusively to produce 3 fold more $\text{scyllo}$-inositol than $\text{myo}$-inositol.     

Magic-angle N-15 NMR study of nitrate metabolism of Neurospora crassa

Magic-angle cross-polarization ¹⁵N nmr spectra of intact lyophilized mycelia from $\text{N.}\text{crassa}$ cultured on media containing [¹⁵N] nitrate have been obtained at 9.12 MHz. The time development of the uptake and distribution of label into protein and amino-acid metabolites can be observed directly. Nitrate metabolism is delayed about one hour if the cells innoculating the culture are grown on nitrate-free medium.     

NADPH-dependent reductase solubilized from microsomes by peroxidation and its activity

Isolated rat liver microsomes were subjected to enzymatic or non-enzymatic lipid peroxidation $\text{in vitro}$. NADPH-dependent cytochrome $\text{c}$ reductase activity was released from the microsomes into the media during peroxidation. This activity could be recovered from the media by DEAE-cellulose chromatography. The recovered enzyme retained high activity for the reduction of cytochrome $\text{c}$ and a lower level of activity for the reduction of cytochrome P-450. The active fractions were capable of enzymatically supporting the peroxidation of isolated mitochondria in the presence of organically complexed Fe⁺³ and NADPH, and in this respect the specific activity was found to be about ten times higher than in microsomes.     

The biosynthesis of lubimin from [1-14C]isopentenyl pyrophosphate by cell-free extracts of potato tuber tissue inoculated with an elicitor preparation from Phytophthora infestans

A cell-free enzyme system, which catalyses the incorporation of radiolabel from [$\text{1}\text{2}$-¹⁴C]isopentenyl pyrophosphate into the sesquiterpenoid phytoalexin lubimin, has been prepared from tuber tissue of Solanum tuberosum inoculated with an elicitor preparation from Phytophthora infestans. Biosynthesis of lubimin is optimum at pH 7.$\text{3}\text{2}$-7.5 and is dependent upon Mg²⁺ and NADPH. Lubimin labelling by cell-free enzyme system prepared from tissue 48 hr after treatment with elicitor rises rapidly to a maximum over the first 30 min of incubation and does not decline for a further 150 min. The biosynthetic capacity for lubimin in cell free extracts can be observed as early as 3 hr after inoculation of tuber tissue, and rises to a maximum at about 48 hr after treatment, declining thereafter. Lubimin labelling is inhibited by iodoacetamide, the effect of which is reversed by 3,3-dimethylallylpyrophosphate. Preliminary observations on the cell-free system show that it will also catalyse the incorporation of [2-¹⁴C]mevalonic acid into lubimin in the presence ofan ATP-generating system.     

Transient kinetic studies of malic enzyme. A conformational change associated with substrate inhibition by malate.

The oxidative decarboxylation of l-malate catalyzed by malic enzyme has been studied by stopped-flow spectrophotometry and by initial rate measurements with large concentrations of NADP⁺, malate, and Mn²⁺. The results show that hybride transfer is fast, $\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{< 0.7}$ ms. The formation of enzyme-bound NADPH in an amount equivalent to about half of the enzyme active center concentration is followed by turnover at a rate which is initially faster than the steady-state rate, under conditions such that substrate inhibition by malate is observed in the steady state. The steady-state rate is reached after about 0.5 s. It is suggested that a conformational change in the abortive complex of enzyme, manganese, NADPH, and malate is responsible for the malate inhibition and for the slow approach to the true steady state. The relief of malate inhibition by increasing Mn²⁺ concentrations is described, and the results are described in relation to other evidence of nonidentical binding sites for, or negatively cooperative binding of, substrate and activator and possible half-of-the-sites reactivity.     

Mandelic acid-4-hydroxylase, a new inducible enzyme from Pseudomonas convexa

A soluble fraction of $\text{Pseudomonas convexa}$ catalyzed the hydroxylation of mandelic acid to $\text{p}$-hydroxymandelic acid. The enzyme had a pH optimum of 5.4 and showed an absolute requirement for Fe²⁺, tetrahydropteridine, NADPH. $\text{p}$-Hydroxymandelate, the product of the enzyme reaction was identified by paper chromatography, thin layer chromatography, UV and IR-spectra.     

Lithosperm inhibition of anterior pituitary hormones.

The enzyme system for the synthesis of the pteridine pigment, sepiapterin, from $\text{2-amino-4-hydroxy-6-(D-}\text{erythro}\text{-1',2',3'-trihydroxyprophyl)}$ triphosphate (dihydroneopterin triphosphate) has been found in extracts of $\text{Drosophila melanogaster}$. NADP⁺ or NADPH and Mg²⁺ are required for this enzymatic transformation. No sepiapterin is produced when dihydroneopterin is supplied as substrate in place of dihydroneopterin triphosphate.     

Unmasking of an essential thiol during function of the membrane-bound enzyme II of the phosphoenolpyruvate β-glucoside phosphotransferase system of Escherichia coli

β-Glucoside transport by phosphoenolpyruvate-hexose phosphotransferase system in Escherichia coli is inactivated in vivo by thiol reagents. This inactivation is strongly enhanced by the presence of transported substrates. In a system reconstituted from soluble and membrane-bound components, only the particulate component, the membrane-bound enzyme II^bgl appeared as the target of N-ethylmaleimide inactivation. The same feature was found in the case of methyl-α-d-glucoside uptake via enzyme II^glc.It is shown that the sensitizing effect of substrates is specific and not generalized, methyl-α-d-glucoside only sensitizes enzyme II^bglc and $\text{p-}\text{nitrophenyl-β-}\text{d}\text{-glucoside}$ only sensitizes enzyme II^bgl towards N-ethylmaleimide inactivation.The inactivation of enzyme II^bgl by thiol reagents is also promoted in vivo by fluoride inhibition of phosphoenolpyruvate synthesis. In toluene-treated bacteria, the presence of phosphoenolpyruvate protects against inactivation by thiol reagents of $\text{p-}\text{nitrophenyl-β-}\text{d}\text{-glucoside}$ phosphorylation. Both results suggest that the inactivator resistent form of enzyme II^bgl is an energized form of the enzyme.     

Dependence of Escherichiacoli pyridine nucleotide transhydrogenase on phospholipids and its sensitivity to N-ethylmaleimide

A partially purified preparation of pyridine nucleotide transhydrogenase (E.C. 1.6.1.1.) (energy-independent) has been obtained from membranes of $\text{Escherichia}\text{coli}$ by means of deoxycholate extraction and DEAE-cellulose chromatography in the presence of Triton X-100. The enzyme was lipid-depleted by treating with cholate and ammonium sulfate. The preparation was reactivated by various phospholipids, in particular, bacterial cardiolipin and phosphatidyl glycerol. Phosphatidyl ethanolamine, the major phospholipid in the outer membrane of $\text{E.}\text{coli}$, was relatively ineffective in stimulating activity. The membrane-bound pyridine nucleotide transhydrogenase is slowly inhibited by N-ethylmaleimide. Protection against inhibition was achieved with NAD⁺ and NADP⁺, but NADPH served to accelerate the rate of inhibition.     

Proteolytic inactivation of a pentafunctional enzyme conjugate: coordinate protection by the first substrate.

The $\text{arom}$ pentafunctional enzyme conjugate of $\text{Neurospora crassa}$ was exposed to trypsin, chymotrypsin, or a protease preparation from $\text{Neurospora}$ in the presence and absence of the first substrate, 3-deoxy-D-$\text{arabino}$-heptulosonate 7-phosphate. It was found that the first substrate coordinately protects all five activities from proteolytic inactivation, which indicates a conformational change induced by this compound. In addition, the data presented are consistent with the “domain” theory of conjugate structure. It is also argued that coordinate protection may be of physiological significance.     

Inhibition of p-nitroanisole O-demethylation in perfused rat liver by potassium cyanide

The effect of potassium cyanide on p-nitroanisole O-demethylation in perfused rat livers has been examined. Cyanide (2 mm), an inhibitor of cytochrome oxidase, diminished p-nitroanisole O-demethylation by 50–75% in perfused livers from normal and phenobarbital-treated rats, but had much less effect on hepatic microsomal p-nitroanisole O-demethylation. The inhibition was also observed in livers where the activity of the pentose phosphate shunt was abolished by pretreatment with 6-aminonicotinamide. Cyanide infusion decreased hepatic $\text{ATP}\text{ADP}$ ratios and cellular concentrations of glutamate, α-ketoglutarate, and isocitrate, but caused an increase in the $\text{NADPV}{}^{\mathrm{+}}\text{NADPH}$ ratio. Rates of NADPH generation via the pentose phosphate shunt were unchanged by cyanide, and hepatic concentrations of glucose 6-phosphate were markedly increased by cyanide. Thus, inhibition of p-nitroanisole metabolism could not be explained solely by a direct interaction of cyanide with mixed-function oxidases or diminished NADPH generation via the pentose cycle. These data indicate that cyanide inhibits mixed-function oxidation in intact cells by diminishing the generation of NADPH from sources other than the pentose cycle. Further, these data are consistent with the hypothesis that some NADPH for mixed-function oxidation arises from cyanidesensitive mitochondrial sources.     

Synthesis of carnitine from ε-N-trimethyllysine in post mitochondrial fractions of Neurospora crassa

An enzyme system in the post mitochondrial fraction of $\text{Neurospora}$$\text{crassa}$ when supplemented with appropriate cofactors formed carnitine from ε-N-trimethyllysine. These findings together with previous studies of ε-N-lysine methylation in this fungi, illustrate that carnitine synthesis in $\text{Neurospora}$ differs markedly in certain features from mammalian systems in that the entire synthesis is carried out employing free intermediates and cytosolic enzymes.     

Detection of two forms of glutamine synthetase in Bacillus brevis (A. G. 4)

A laboratory isolate of $\text{Bacillus}\text{brevis}$ could grow and sporulate on an amino acid, viz., alanine or glutamate or aspartate as single source of carbon and nitrogen. It failed to sporulate if the amino acid was replaced by the corresponding keto acid and ammonium sulphate in the medium, although, normal growth was observed. One of the key enzymes in nitrogen assimilation, the glutamine synthetase, has been purified by DE-52 and affinity column chromatography from both alanine and pyruvate grown cells. The kinetic and other properties of both of these enzymes were studied. The enzyme isolated from alanine grown cells differed significantly from that isolated from pyruvate grown cells (viz.,pH optima, response to Mg⁺⁺ and other effectors). A possible role of glutamine synthetase in the initiation of bacterial sporulation is discussed.     

Cyclic 3',5'-AMP phosphodiesterase of Neurospora crassa

Cyclic 3′,5′-AMP (cAMP) phosphodiesterase activity can be demonstrated in extracts of $\text{Neurospora}\text{crassa}$. The activity is particulate, has a pH optimum of 7.4, and consists of two forms that have different cAMP binding constants. Methylxanthines, inorganic phosphate, and EDTA are inhibitors of the diesterase as are ATP, ADP, and 8-bromo-cAMP. The enzymatic activity is stimulated by histamine and imidazole. These properties suggest that the $\text{Neurospora}$ enzyme is more closely related to the mammalian than to bacterial cAMP phosphodiesterases.