Inhibitory Effect Of High Concentrations Research Articles

Angiotensin II receptors and mechanisms of action in adrenal glomerulosa cells

The plasma-membrane receptors, coupling mechanisms, and effector enzymes that mediate target-cell activation by angiotensin II (All) have been characterized in rat and bovine adrenal glomerulosa cells. The All holoreceptor is a glycoprotein of Mr∼125,000 under non-denaturing conditions. Photoaffinity labeling of All receptors with azido-AII derivatives has shown size heterogeneity among the All binding sites between species and target tissues, with Mr values of 55,000 to 79,000. Such variations in molecular size probably reflect differences in carbohydrate content of the individual receptor sites. The adrenal All receptor, like that in other tissues, is coupled to the inhibitory guanine nucleotide inhibitory protein (N i). However, studies with pertussis toxin have shown that stimulation of aldosterone production by All is not mediated by N i but by a pertussis-insensitive nucleotide regulatory protein of unidentified nature. Although N i is not involved in the stimulatory action of All on steroidogenesis, it does mediate the inhibitory effects of high concentrations of All upon aldosterone production. The actions of All on adrenal cortical function are thus regulated by at least two guanine nucleotide regulatory proteins that are selectively activated by increasing All concentrations. The principal effector enzyme in All action is phospholipase C, which is rapidly stimulated in rat and bovine glomerulosa after All receptor activation. All-induced breakdown of phosphatidylinositol bisphosphate (PIP 2) and phosphatidylinositol phosphate (PIP) leads to formation of inositol 1,4,5-trisphosphate IP 3) and inositol 1,4-bisphosphate (IP 2). These are metabolized predominantly to inositol-4-monophosphate, which serves as a marker of polyphosphoinositide breakdown, whereas inositol-1-phosphate is largely derived from phosphatidylinositol hydrolysis. The All-stimulated glomerulosa cell also produces inositol 1,3,4-trisphosphate, a biologically inactive IP 3 isomer formed from Ins-l,4,5-trisphosphate via inositol tetrakisphosphate (IP 4) during ligand activation in several calcium-dependent target cells. The Ins-l,4,5-P3 formed during All action binds with high affinity to specific intracellular reeceptors that have been characterized in the bovine adrenal gland and other All target tissues, and may represent the sites through which (IP 3) causes calcium mobilization during the initiation of cellular responses.

The role of hydrogen peroxide in basophil histamine release and the effect of selected flavonoids

Studies on hydrogen peroxide (H2O2)-induced histamine release from human basophils indicate that H2O2 is a weak stimulus of histamine release, that the release process is Ca2+ and energy-dependent, and that histamine release is not influenced by theophylline (in keeping with previous observations with rat mast cells). Low concentrations of H2O2 appeared to augment and high concentrations to inhibit histamine release induced by anti-IgE. However, the inhibitory effect of high concentrations of H2O2 were completely abrogated by catalase, which destroys H2O2, and thus indicates that basophils retain immunologic responsivity and are not irreversibly effected by high concentrations of H2O2. Leukocyte suspensions relatively enriched in monocytes, lymphocytes, basophils, neutrophils, and neutrophils plus eosinophils were prepared by Percoll-gradient centrifugation. Anti-IgE stimulated H2O2 formation only in the fraction richest in basophils. Opsonized zymosan, on the other hand, stimulated H2O2 generation in both the basophil and monocyte fractions, indicating activation of both monocytes and basophils by this stimulus. Mixtures of basophil-containing leukocyte suspensions plus purified neutrophils and opsonized zymosan stimulated histamine release in proportion to concomitant generation of H2O2. Addition of catalase reduced histamine release under these conditions, whereas scavengers of other toxic oxygen derivatives (superoxide dismutase, alpha-tocopherol, D-mannitol) had little or no effect on histamine release. These findings suggest that neutrophil-derived H2O2 can cause basophil histamine release in mixed populations of activated leukocytes. Three naturally occurring flavonoids, quercetin, apigenin, and taxifolin (dihydroquercetin) were examined for their effect on anti-IgE-induced histamine release and H2O2 generation in basophil-containing leukocyte suspensions.(ABSTRACT TRUNCATED AT 250 WORDS)

Combined Effect of Some Growth Regulators on Growth and Gametangial Formation in the LiverwortRiccia gangeticaAhmad

The present work deals with the effect of IAA-kinetin, IAA-GA3 and GA3-ABA interactions on growth and gametangial formation in Riccia gangetica in vitro. Inhibitory effect of high concentration of IAA on vegetative growth is overcome by the co-addition of kinetin. The best response in terms of fresh and dry weight yields of thalli is obtained by a combination of 10−5 mol dm−3 kinetin+ 10−7 mol dm−3 IAA. Interaction of IAA and kinetin has an additive effect on archegonial formation. Co-addition of IAA and GA3 decreases production of archegonia and antheridia as compared to those produced in response to IAA and GA3 alone, respectively. Combination of GA3 and ABA reduces vegetative growth, as well as the number of anthendia and archegonia.

Opposing α- and β-adrenergic mechanisms mediate dose-dependent actions of noradrenaline on supraoptic vasopressin neurones in vivo

The effects of pressure-applied noradrenaline (NA) on the activity of neurosecretory cells of the supraoptic nucleus (SON) were examined in anaesthetized male rats. Spontaneously active, antidromically identified neurosecretory cells were classified as vasopressin (VP)-secreting on the basis of activity patterns and responsiveness to baroreceptor activation. The probability of encountering VP units was enhanced by confining electrode penetrations to the caudal aspect of the SON. Application of low concentrations of NA (50–150 μM) excited 75% of putative VP neurones tested (n= 45), while very high concentrations (1–100 mM) were inhibitory (79%, n tested= 14). The excitatory effects of NA were blocked by the α 1 antagonist prazosin (0.1–5 μM, n= 9) and mimicked by application of the α 1 agonist methoxamine (300 μM–1mM, n= 29). The α 2 agonist clonidine (800 μM–1 mM) also frequently elicited mild excitations (92%,n tested= 13); however, this was commonly followed by an extended period of quiescence. Neither the α 2 antagonist yohimbine(5 μM, n = 4) nor the β-adrenoreceptor antagonist timolol (5–20 μM, n= 6) blocked NA-induced excitations. The inhibitory effects of high concentrations of NA, however, were blocked by the application of timolol (5–20 μM, n= 5). It is suggested that the excitatory effect of low concentrations of NA on VP neurones reflects the actions of this substance when endogenously secreted at normal sites of release within the SON.

Effects of protease inhibitiors on adenylate cyclase activation and aldosterone production in rat adrenal zona glomerulosa cells

It has been shown that serine proteases are involved in aldosterone and 18-hydroxycorticosterone production by the rat adrenal zona glomerulosa in response to a variety of stimulants. From evidence presented for various tissues, including the rat adrenal cortex, the observation that adenylate cyclase can be activated by proteolytic enzymes and inhibited by protease inhibitors has led to the suggestion that serine proteases may also be involved in the hormonal stimulation of adenylate cyclase. In studies designed to test this hypothesis using protease inhibitors, only high concentrations (greater than 10(-4) M) of TAME (p-tosyl-L-arginine methyl ester) inhibited ACTH stimulated steroid and cAMP production in rat adrenal glomerulosa cells. TPCK (tosyl-L-phenylalanine chloromethylketone) and TLCK (tosyl-L-lysine chloromethylketone) were found to have a similar effect at very high concentrations (10(-2) M) but had no effect at the serine protease inhibitory concentration of 5 X 10(-6) M. Other protease inhibitors tested had no effect on ACTH-stimulated cAMP but the inhibitory effect of high concentrations of protease inhibitors on ACTH-stimulated adenylate cyclase was duplicated by the polyanion dextran sulphate. The results suggest that the inhibitors act through non-specific membrane effects and that proteases are not involved in the activation of zona glomerulosa adenylate cyclase by ACTH. In view of these findings it is concluded that a more rigorous approach should be applied to the use of protease inhibitors in whole cell systems, and that the concept of hormonal activation of adenylate cyclase via proteolytic events, which is based on studies with such inhibitors, should be reconsidered.

Growth stimulatory precipitates of Ca2+ and pyrophosphate.

Inorganic pyrophosphate (PPi) forms an insoluble precipitate with calcium in growth medium when its concentration exceeds about 0.1 mM. This PPi precipitate can reproduce the effects of 10% calf serum on all cell processes examined in Balb/c 3T3 cells, including hexose uptake and metabolism to lactate, 3H-uridine, and 3H-choline uptake, and the incorporation of 3H-leucine and 3H-thymidine into trichloroacetic acid (TCA)-insoluble material. Concentrations of PPi insufficient to form a precipitate are without effect on cell metabolism. The precipitates are most effective when prepared with concentrations of PPi just sufficient to result in precipitate formation and become considerably less effective as the PPi concentration increases, even though the quantity of precipitate formed continues to increase with PPi concentration up to 1 mM PPi. Precipitates formed at low PPi concentrations consist largely of Ca2+ (81% of cations), PPi (77% of anions), and Pi (23% of anions). Precipitates formed with higher concentrations of PPi contain proportionately less Ca2+ and Pi and more monovalent cations and PPi. We have distinguished cell surface-bound PPi from intracellular PPi by differential extraction. The quantity of surface-bound PPi increases sharply when the PPi concentration reaches the point of precipitate formation. If the precipitate is prevented from binding to the cell surface by inverting monolayer cultures in precipitate-containing medium, the cells are not stimulated. These findings suggest that the binding of PPi precipitate to the cell surface is involved in the stimulation of cell metabolism by PPi. PPi precipitates do not absorb serum mitogens or inhibitors from the culture medium, nor do they affect the binding of 125I-platelet-derived growth factor to its specific cell-surface receptor, suggesting that PPi precipitates do not act directly through either of these mitogen-receptor systems. In analogy to cell stimulation by epidermal growth factor and by antigens, we suggest that PPi may be active only in the form of a precipitate because multivalent binding of receptors with formation of clusters is required for stimulation. The inhibitory effects of high concentrations of PPi may be due to interference by free PPi with formation of active receptor clusters.

Impaired pentose phosphate shunt function in sickle cell disease: a potential mechanism for increased Heinz body formation and membrane lipid peroxidation.

The red cells' antioxidant defense mechanisms were compared between individuals with sickle cell disease and those with hemolytic anemia and reticulocytosis. In sickle cell disease, there was a significant increase in incubated Heinz body formation (p less than .001), a decrease in reduced glutathione concentration (p less than .01), an increase in glucose-6-phosphate dehydrogenase activity (p less than .01), and a decrease in glutathione reductase activity (p less than .005). The patients with sickle cell disease hd an absolute increase in the activity of the pentose shunt in the intact red cell after methylene blue stimulation (p less than .05) and in red cell hemolysates (p less than .0250. Heinz body formation (r = .75) and pentose shunt activity in red cell hemolysates (r = .83) were strongly related to the degree of reticulocytosis. Although there was a correlation between the pentose shunt activity in the stimulated red cell and in red cell hemolysates for the patients with hemolytic anemia (r = .58), stimulated shunt activity did not increase as the hemolysate shunt activity increased for the patients with sickle cell disease. There were very strong relationships between the ATP concentration and the reticulocyte count (r = .80) and the hemolysate pentose shunt activity (r = .77) in sickle cel disease. These data suggest that in spite of an absolute increase in stimulated pentose shunt activity, there Is a relative suppression of stimulated shunt activity in the youngest sickle erythrocytes. This may be related, in part, to the inhibitory effects of high concentrations of ATP on the activity of glucose-6-phosphate dehydrogenase.

Effects of high concentrations of sodium chloride and polyethylene glycol on the growth and ion absorption in plants

To provide a better understanding of plant responses to salt and water stresses, the effects of high concentrations of sodium chloride (NaCl) and polyethylene glycol (PEG) on the growth and ion absorption in plants were examined at similar osmotic potential conditions, using the water culture method. The inhibitory effect on plant growth appeared to be only slightly larger in PEG treatment than in NaCl treatment. However, the depressive effects on K, Ca and Mg contents of plants were more severe with NaCl treatment than with PEG treatment. No depressive effects of NaCl or PEG were observed for P content. From the short term absorption experiment using a tracer technique, it was also evident that the inhibition of K absorption was more drastic in NaCl treatment than in PEG treatment. On the other hand, the inhibitory effects of high concentrations of NaCl and PEG were very small on P absorption.

Modulation of transmission in rat sympathetic ganglia by activation of presynaptic alpha- and beta-adrenoceptors.

1 Conditions under which transmission in rat isolated superior cervical ganglia may be affected by activation of presynaptic alpha- and beta-adrenoceptors have been investigated by means of an extracellular recording method. 2 Clonidine caused a small hyperpolarization of the ganglia (mean EC50 approximately 2 nM) in unstimulated preparations; with continuous preganglionic stimulation at 0.2 Hz, clonidine markedly decreased the height of the compound action potential (mean EC50 approximately 18 nM). 3 Phentolamine (0.1-3 microM) per se increased the height of the compound action potential by up to 15%, and antagonized the inhibitory effects of adrenaline and clonidine. 4 Using a higher frequency of stimulation (0.5 Hz), the effect of phentolamine (1 microM) was unchanged, whereas the inhibitory effectiveness of adrenaline on the height of the compound action potential was reduced. 5 (+/-)-Propranolol (0.1 microM) did not affect the height of the compound action potential, whereas the inhibitory effects of high concentrations of adrenaline were enhanced. 6 During an infusion of clonidine (1 microM), adrenaline (1-100 microM) and, less effectively, noradrenaline (10-100 microM) increased the height of the compound action potential by up to 14%; these effects were antagonized by propranolol (0.1 microM). 7 In the presence of noradrenaline (10 and 30 microM) adrenaline (100 microM) caused a small (up to 5%) enhancement of the height of the compound action potential. 8 The results obtained are consistent with the existence of presynaptic alpha- and beta-adrenoceptors on preganglionic terminals. The alpha-adrenoceptor may be part of a trans-synaptic inhibitory feedback. mechanism; however the functional role of the facilitatory beta-adrenoceptor is not clear.

In vitro immunization to KLH. II. Limiting dilution analysis of antigen-reactive cells in primary and secondary culture.

Limiting dilution analysis was used to estimate the frequency of human peripheral blood T lymphocytes that proliferate in response to in vitro immunization with keyhole limpet hemocyanin (KLH). Antigen-reactive cells (ARC) were estimated 9 days after primary immunization with KLH. The ARC frequency of lymphocytes from 12 subjects ranged from 1:23,800 to 1:52,631. Lymphocytes from five of these subjects were also primed for 12 days with KLH, rechallenged in secondary culture with fresh adherent cells and KLH, and assayed 4 days later. The ARC frequency increased to 1:1,123 to 1:7,247, indicating that T cell clones responsive to KLH had expanded during primary culture. In addition, we observed that the proliferative response of lymphocytes from 5 of the 12 subjects were inhibited at high cell concentrations. Depletion of OKT8+ T cells before culturing with KLH however did not alter the inhibitory effect of high concentrations of T cells.

Acute Inhibition of Corticosteroidogenesis by Inhibitors of Calmodulin Action*

To identify the possible role of calmodulin in ACTH function, we tested the ability of chlorpromazine (CP) and other calmodulin antagonists to inhibit steroidogenesis of isolated adrenocortical cells of the rat. CP reversibly inhibited maximal ACTH-induced corticosterone (B) production. The presence of the drug did not alter the ED50 of ACTH stimulation (3.2 X 10(3) pg/ml), suggesting that it inhibited ACTH-induced steroidogenesis in a noncompetitive manner. The CP concentration required for half-maximal inhibition was 8.2 microM, a value close to the dissociation constant of the CP-calmodulin complex (5.3 microM). Concentrations greater than 40 microM resulted in complete inhibition. Similar concentrations of CP inhibited ACTH-induced cAMP accumulation in a dose-dependent manner, indicating an effect of the drug on early events in ACTH action. In addition, CP also apparently acted at a site distal to the point of cAMP formation, as shown by the finding that it inhibited cAMP-induced B production. CP inhibition of ACTH-induced B production was independent of the Ca2+ concentration, suggesting that the drug did not compete with Ca2+ directly. Concentrations of CP greater than 20 microM inhibited protein synthesis as measured by leucine incorporation into cellular proteins. Thus, although the inhibitory effect of high concentrations of CP on steroidogenesis might be explained by an effect on protein synthesis, the inhibition seen at 10 microM appeared to be independent of protein synthesis. Other antagonists of calmodulin action inhibited maximal ACTH-induced B production with the following relative potencies: trifluoperazine greater than CP greater than haloperidol greater than chlordiazepoxide. This order is similar to that reported for inhibition of calmodulin-activated phosphodiesterase and for binding to calmodulin. These findings suggest that calmodulin may modulate the effect of ACTH on steroidogenesis at multiple sites.

Combined effects of ATP and phosphate on rubidium exchange mediated by Na-K-ATPase reconstituted into phospholipid vesicles.

1. Phospholipid vesicles reconstituted with Na-K-ATPase show an (ATP+phosphate)-stimulated Rb-Rb exchange, with properties similar to the K-K exchange of human red cells. This includes a rate 15-20% of the rate of active ATP-dependent Na-K exchange.2. We have studied activation of this Rb-Rb exchange by ATP at fixed phosphate concentrations and by phosphate at fixed ATP concentrations. It is found for both ATP and phosphate that with low concentrations of the fixed ligand an increase in concentration of the complementary ligand produces first stimulation and then inhibition of Rb-Rb exchange. At high concentrations of the fixed ligand the complementary ligand shows only saturation behaviour.3. The pattern of activation and of inhibition by ATP and by phosphate is affected by the Rb(0) concentration in the exterior medium, in that higher concentrations of Rb(0) counteract inhibitory effects of high concentrations of ATP and phosphate.4. (ATP+phosphate)-stimulated Rb-Rb exchange is activated by Rb(0) in the exterior medium along a sigmoid curve. An increase of Rb(i) within the vesicles, which raises the maximal velocity of Rb-Rb exchange, is accompanied by a smaller increase in the Rb(0) concentration required for half-maximal stimulation of the Rb-Rb exchange.5. The data are interpreted in terms of a model similar to those proposed by Karlish & Stein (1982a,b), but extended to include simultaneous effects of ATP and phosphate. Inhibitions by high concentration of ATP or phosphate arise as a result of stabilization of E(1) ATP or E(2)-P forms respectively, in the presence of low concentrations of the complementary ligand. With high concentrations of the fixed ligand, saturation behaviour of the varying ligand is observed because the occluded Rb forms become the dominant transport intermediates. The occluded Rb forms bind both ATP and phosphate weakly and independently. The effects of ATP together with phosphate are accounted for by a simple combination of their separate effects on the Rb-Rb exchange.6. We suggest that the functional role of the occluded Rb form E(2) (Rb)(occ) in active transport is to minimize passive cation leaks through the system and allow control of the direction of cation movements by binding of physiological ligands such as ATP or phosphate.

A pharmacological analysis of chloride transport across the amphibian cornea.

1. Active Cl, but not Na, transport across the toad cornea was inhibited by mepacrine, which is a phospholipase A2 inhibitor; trifluoperazine, which blocks the action of calmodulin; and meclofenamic acid, which inhibits synthesis of prostaglandins. Bumetanide and DIDS (4,4'-diisothiocyano-2,2'-stilbene disulphonic acid) have previously been shown to inhibit this Cl transport. The interactions of these antagonists with several agonists that increase Cl transport were studied. 2. The effects of adrenaline, prostaglandin E2, dibutyryl cyclic AMP (DBcAMP) and the Ca ionophore A23187 were inhibited by mepacrine, trifluoperazine and bumetanide. 3. The inhibitory effects of high concentrations of DIDS, however, could be overcome by all of the agonists except DBcAMP. 4. Meclofenamic acid only blocked the effects of A23187. 5. A model is proposed to account for the observed actions and interactions of the various antagonists and agonists on Cl transport. This involves possible roles for Ca, calmodulin and phospholipase A2.

Interactions of zinc with prostaglandins E 1 and E 2 in vascular smooth muscle

In the isolated mesenteric vascular bed of the rat, zinc had actions similar to those of PGE 1 and unlike those of PGE 2. In the preparations whose vascular reactivity to norepinephrine was first blocked by indomethacin and then restored by PGE 2, zinc at all concentrations had no significant effect on vascular responses. Zinc (100 pg/ml) blocked the inhibitory effect of high concentrations of PGE 1. PGE 1 (10 ng/ml and 100 pg/ml, respectively) pushed the Zn dose-response curve to the right and blocked the inhibitory effect of high concentrations of zinc. PGE 2 (10 ng/ml) has only marginal effects on the zinc dose-response curve. These results suggest that in this preparation, zinc effects depend upon endogenous PG synthesis and that zinc preferentially affects the actions of PGE 1 over those of PGE 2.

MEMBRANE LIPID OXIDATION IN A MUCROSOMAL FRACTION OF RED HAKE MUSCLE

A microsomal fraction which catalyzes the oxidation of the membrane lipid in the presence of NADH and Fe has been isolated from the muscle of red hake. Unlike other microsomal membrane systems previously reported which prefer, and in some cases require, NADPH, very little lipid oxidation is observed with NADPH in the fish muscle system. The enzymic nature of the reaction is indicated by the preference of NADH versus NADPH, the inhibitory effect of high concentrations of NADH, and the loss of the activity of the system upon heating. The effect of temperature on production of TBA-reactive substances and the loss of NADH from the reaction medium was followed. There is a sharp decline in production of TBA-reactive substances between 6°C and 0°C. Phosphate buffer was shown to inhibit the oxidation compared to histidine buffer at low concentrations of ADP. High concentrations of ADP were inhibitory to the lipid peroxidation system when histidine was used as buffer.

Stimulation of valyl- and isoleucyl-tRNA synthetase reactions by polyamines

The aminoacylation of tRNA catalysed by valyl-tRNA synthetase (EC 6.1.1.9) and isoleucyl-tRNA synthetase (EC 6.1.1.5) fromMycobacterium smegmatis is dependent on the presence of divalent metal ions. Polyamines alone, in the absence of metal ions, do not bring about aminoacylation. In the presence of suboptimal concentrations of Mg2+, polyamines significantly stimulate the reaction. Of the cations tested, only Mn2+, Co2+ and Ca2+ can partially substitute for Mg2+ in aminoacylation, and spermine stimulates aminoacylation in the presence of these cations also. At neutral pH, spermine deacylates nonenzymatically aminoacyl tRNA. AMP and pyrophosphate-dependent enzymatic deacylation of aminoacyl-tRNA (reverse reaction) is also stimulated by spermine. The inhibitory effect of high concentration of KC1 on aminoacylation is counteracted, by spermine. The low level of activity between pH 8.5–9.0 at 1.2 mM Mg2+ is restored to normal level on the addition of spermine. The inhibitory effect of high pH on aminoacylation in the presence of low concentration of Mg2+ is also prevntedvby spemine.

Effects of heavy metals and other trace elements on the fermentative activity of the rumen microflora and growth of functionally important rumen bacteria.

The inhibitory effects of high concentrations of essential and non-essential trace elements were tested on the rumen microflora using the rate of fermentation in vitro as the assay. The elements (and the concentration causing 50% inhibition) in decreasing order of toxicity were Hg2+ (20 microgram/ml), Cu2+ (21 microgram/ml), Cr6+ (70 microgram/ml), Se4+ (73 microgram/ml), Ni2+ (160 microgram/ml), Cd2+ (175 microgram/ml), As3+ (304 microgram/ml) and As5+ (1610 microgram/ml). The elements tested that were either weak or noninhibitory at concentrations greater than 400 microgram/ml included Zn2+, Cr2+, Fe2+, Mn2+, Pb2+, and Co2+. Methylmercury was as inhibitory as mercuric chloride to the fermentation. When the inhibitory effect of Cd2+ was tested on separated bacterial and protozoal fractions, it was more inhibitory to the bacteria. The inhibitory effects of trace elements were also determined for a number of axenic cultures of rumen bacteria. The bacteria which most frequently exhibited the greatest sensitivity were Bacteroides succinogenses, Ruminococcus albus, Bacteroides amylophilus, and Eubacterium ruminantium. Those often exhibiting intermediate sensitivities included Butyrivibrio fibrisolvens, Selenomonas ruminantium, and Megasphera elsdenii, while Streptococcus bovis was very refractory to all elements tested. Rumen fluid provided a modest protective effect for the bacteria.

Inhibition of Human Erythrocyte Lactate Dehydrogenase by High Concentrations of Pyruvate. Evidence for the Competitive Substrate Inhibition

The mechanism of the inhibitory effect of high concentrations of pyruvate on human erythrocyte lactate dehydrogenase has been studied by the use of a new parameter, delta, defined as the difference between the reciprocals of initial reaction rates obtained from experimental measurements and hypothetical linear Lineweaver-Burk plots. This parameter served as a method for differentiating between the competitive and umcompetitive substrate inhibition. Results of this study indicate that pyruvate is a competitive substrate inhibitor. It is suggested that the inhibitory effect of pyruvate is due to its competition with NADH for binding to the free enzyme and formation of an inactive enzyme-pyruvate binary complex. The competitive nature of pyruvate inhibition is further supported by the results of a kinetic study with NADH as the variable substrate. The dissociation constnat of the inactive enzyme-pyruvate binary complex was determined to be 101 micrometer. The physiological significance of the inhibitory effect could be to preserve a level of NADH concentration necessary for other vital enzymic reactions of living cells despite the presence of a high concentration of pyruvate.

Effect of simazine, lindane and ceresan on soil respiration and nitrification rates

The effect of three pesticidesviz, simazine, lindane and ceresan upto 100-fold of field application was studied on organic matter mineralization as well as on nitrification rates in Delhi alluvial soil. The normal rate of simazine (2 ppm) did not influence CO2 evolution but higher concentrations suppressed it. Lindane and ceresan inhibited CO2 production from soil by normal concentrations but the same CO2 production was enhanced in berseem roots treated soil upto 10 ppm of ceresan. The nitrification of ammonium sulphate in soil due to these pesticides was impaired significantly for varying periods. The inhibitory effect of higher concentrations was marked only upto 3 weeks, subsequently the rate of nitrification was restored. Among the nitrifiers, Nitrobacter were more susceptible as compared to Nitrosomonas.

Characteristics of acetaldehyde oxidation in rat liver mitochondria.

Rat liver mitochondria oxidized acetaldehyde (180 muM) at the rate of approximately 12 nmol/min/mg of protein at 37 degrees. This was stimulated by 88% with the addition of ADP. The ADP/O ratio (2.6) was similar to that with glutamate as substrate. 2,4-Dinitrophenol and phenazine methosulfate also stimulated the rate of acetaldehyde oxidation in the mitochondria. By contrast, acetaldehyde metabolism was virtually abolished by rotenone and antimycin A. These results indicate that acetaldehyde oxidation is linked to the mitochondrial respiratory chain and coupled with mitochondrial oxidative phosphorylation. Indeed, little acetaldehyde was metabolized when mitochondrial membranes were disrupted by sodium deoxycholate. In the disrupted mitochondria, however, acetaldehyde oxidation was fully recovered by addition of NAD+, suggesting that the ability of mitochondria to supply NAD+ controls the rate of acetaldehyde oxidation in intact mitochondria. The stimulatory effect of ADP on mitochondrial acetaldehyde oxidation was diminished by increasing the acetaldehyde concentration. Concomitantly, the ADP/O ratio decreased, suggesting an inhibitory effect of high concentrations of acetaldehyde on mitochondrial respiration. Chronic feeding of ethanol significantly reduced the capacity of intact liver mitochondria to oxidize acetaldehyde. This was associated with a significant reduction of the mitochondrial respiration. By contrast, the activity of aldehyde dehydrogenase in disrupted mitochondria remained unchanged.