Effect of silicon on manganese tolerance of bean plants (Phaseolus vulgaris L.)

Summary The effect of silicon on manganese tolerance of bean plants (Phaseolus vulgaris L. var. ‘Red Kidney’) grown in water culture was studied at different levels of manganese supply. Without silicon, growth depression and toxicity symptoms occurred already at 5 × 10⁻⁴ mM Mn in the nutrient solution. After addition of Aerosil (0.75 ppm Si), the plants tolerated 5 × 10⁻³ mM Mn and, at a higher silicon supply of 40 ppm, as much as 10⁻² mM Mn in the nutrient solution without any growth depression. This increase in manganese tolerance was not caused by a depressing effect of silicon on uptake or translocation of manganese but rather by an increase in the manganese tolerance of the leaf tissue. In absence of silicon, 100 ppm Mn was already toxic for the leaf tissue, whereas with a supply of 40 ppm Si, this ‘critical level’ in the leaves was increased to more than 1000 ppm Mn. At lower manganese levels in the leaf tissue, a molar ratio Si/Mn of 6 within the tissue was sufficient to prevent manganese toxicity. Above 1000 ppm Mn, however, even a much wider Si/Mn ratio (> 20) could not prevent growth depression by manganese toxicity. With⁵⁴Mn and autoradiographic studies, it could be demonstrated that, in absence of silicon, even at optimal manganese supply (10⁻⁴ mM), the distribution of manganese within the leaf blades was inhomogeneous and characterized by spot-like accumulations. In presence of silicon, however, the manganese distribution was homogeneous in the lower concentration range of manganese and still fairly homogeneous in the high concentration range. This effect of silicon on manganese distribution on the tissue level was also reflected on the cellular level. In the presence of silicon, a higher proportion of the leaf manganese could be found in the press sap,i.e., had been transported into the vacuoles, than in the absence of silicon. The increase in manganese tolerance of bean leaves by silicon therefore seems to be primarily caused by the prevention of local manganese accumulation within the leaf tissue which leads to local disorders of the metabolism and, correspondingly, growth depression.     

Regulation of Mn-SOD activity in the mouse heart: glucose effect

Intraperitoneal injection of glucose was found to cause a dose and time dependent suppression of superoxide dismutase activity in mouse heart. Manganese superoxide dismutase was more sensitive to glucose suppression than Cu-Zn superoxide dismutase. While glucose suppressed the Mn form of the enzyme at the concentration of 1.5 mg/kg, it did not have a significant effect on Cu-Zn superoxide dismutase activity at this concentration. The maximum suppression for both forms of superoxide dismutase activity occurred at 4.5 mg/kg. Glucose also suppressed manganese superoxide dismutase activity in mouse heart for a longer period of time compared to Cu-Zn superoxide dismutase. Glucose suppression also occurred in mouse brain. The glucose suppression effect on manganese superoxide dismutase activity in the heart was partially alleviated by X-irradiation.     

The effect of manganese on the production of Phanerochaete flavido-alba ligninolytic peroxidases in nitrogen limited cultures

Manganese supplementation of culture medium affected Phanerochaete flavido-alba FPL 106507 growth, glucose consumption and extracellular protein accumulation. Both the titre and time of detection of lignin peroxidase (LiP) were affected by manganese concentration in the medium, whereas with manganese peroxidase (MnP) only the titre was affected. In high Mn(II) containing cultures highest manganese peroxidase levels and a decrease in extracellular veratryl alcohol accumulation were observed. After FPLC a number of haemprotein peaks showing manganese peroxidase activity were detected in Mn(II) supplemented cultures. On the contrary, only haemprotein peaks of lignin peroxidase were detected in culture medium not supplemented with Mn(II).     

Tissue antioxidant status in streptozotocin-induced diabetes in rats

Interactions between manganese (Mn) deficiency and streptozotocin (STZ)-diabetes with respect to tissue antioxidant status were investigated in male, Sprague-Dawley rats. All rats were fed either a Mn-deficient (1 ppm) or a Mn-sufficient (45 ppm) diet for 8 wk. Diabetes was then induced by tail-vein injection of STZ (60 mg/kg body weight), after which the rats were kept for an additional 4 or 8 wk. The control groups comprised rats not injected with STZ and fed either Mn-deficient or Mn-sufficient diets for a total of 12 wk. The Mn-deficient diet decreased the activities of manganese superoxide dismutase (MnSOD) in kidney and heart, and of copperzinc superoxide dismutase (CuZnSOD) in kidney, in the non-diabetic animals. In the diabetic rats, the Mn-deficient diet induced more pronounced decreases in activities of these same enzymes, and also increased liver MnSOD activity. Plasma and hepatic vitamin E levels increased progressively with the duration of diabetes, independent of dietary Mn intake. Lipid peroxidation, as measured by H₂O₂-induced production of thiobarbituric acid reactive substances in erythrocytes, also increased, concomitant with decreased liver and kidney glutathione (GSH) levels. These findings demonstrate for the first time an interactive effective between Mn deficiency and STZ-diabetes, resulting in amplification of tissue antioxidant changes seen with either Mn deficiency or STZ-diabetes alone. This effect of Mn deprivation in experimental diabetes suggests a physiological role for Mn as an antioxidant nutrient.     

Effect of P and Mn on growth response and uptake of Fe,Mn and P by sorghum

Summary The effects of P and Mn on growth response and uptake of Fe, Mn and P by grain sorghum were investigated using nutrient culture. High P and Mn concentrations in solution (greater than 40 and 1 mg/l for P and Mn, respectively) markedly reduced plant height and shoot and root dry weight of 4-week-old sorghum plants. High Mn concentrations in solution increased the concentrations of Mn and P in shoot tissue and uptake of Mn, but depressed the uptake of P. High levels of P enhanced Mn uptake by sorghum and accentuated Mn toxicity at low Mn levels. The tissue Fe and total uptake of Fe were both reduced markedly by the high levels of P and Mn concentrations in solution. The increases of P, Mn and Fe concentrations in root tissue with a concomitant decrease of Fe in shoots suggested that the translocation of Fe from roots to shoots was hindered under high P and Mn conditions. Since coating occurred on root surfaces and intensified with increasing Mn concentrations in the substrate, part of the reduction of Fe in shoots could be attributed to the formation of high valent manganese oxides on the root surfaces which may retain Fe and reduce its absorption by sorghum.Contribution from the Department of Agronomy and Range Sci., University of California, Davis, CA.     

A manganese porphyrin suppresses oxidative stress and extends the life span of streptozotocin-diabetic rats

Enhanced oxidative stress due to hyperglycemia has been implicated in diabetic complications and is considered a major cause of cell and tissue damage. The aim of the present study was to investigate whether synthetic manganese porphyrin, Mn(III) 5,10,15,20-tetrakis(N-methylpyridinium-2-yl)porphyrin (MnTM-2-PyP⁵⁺) can ameliorate diabetes-induced oxidative stress and affect life span of diabetic rats.

Diabetes was induced by a single (60 mg/kg) intraperitoneal injection of streptozotocin in male Wistar rats. Oxidative stress was monitored by measuring malondialdehyde levels (MDA) in blood plasma and erythrocytes using HPLC. The antioxidant status was assessed by measuring the total radical-trapping potential (TRAP) of blood plasma. Life span of the animals was used as an indication of the overall effect of MnTM-2-PyP⁵⁺. MnTM-2-PyP⁵⁺ was administered subcutaneously at 1 mg/kg for the duration of the experiment, five times/week followed by one week of rest.

Diabetes increased plasma and erythrocyte levels of MDA and decreased TRAP. MnTM-2-PyP⁵⁺ had no effect on blood glucose and glycosylated hemoglobin, but significantly increased TRAP and lowered MDA. This Mn porphyrin decreased mortality and markedly extended the life span of the diabetic animals.

MnTM-2-PyP⁵⁺ suppressed diabetes-induced oxidative stress, which presumably accounts for its beneficial effect on the life span of the diabetic rats. The results indicate that Mn(III) N-alkylpyridylporphyrins can be used as potent therapeutic agents in diabetes.     

Bacteriology of Manganese Nodules: I. Bacterial Action on Manganese in Nodule Enrichments

Bacteria, found in manganese nodules from the Atlantic Ocean, enhance the adsorption of Mn from sea water by crushed manganese nodules in the presence of peptone. When bacterial outgrowth from crushed manganese nodules was experimentally delayed, peptone did not enhance Mn adsorption by nodular substance, but hindered it in some cases. A mechanism to explain the role of bacteria in enhancing Mn adsorption by manganese nodules is presented. Oyster shells were shown to adsorb Mn in the absence of bacteria. Peptone did not enhance the rate of Mn adsorption. Adsorbed Mn was not visibly oxidized during experimental observation. These results suggest one way whereby nodule formation may be initiated in the oceans. Some bacteria in the nodules were found to release manganese from them in the presence of glucose and peptone. Bacteria may, therefore, play a role not only in nodule buildup but also in nodule breakdown.     

Manganese accumulation in pancreatic beta-cells and its stimulation by glucose.

Electrothermal atomic-absorption spectroscopy was employed for measuring manganese in beta-cell-rich pancreatic islets microdissected from ob/ob mice. The islet content of endogenous manganese was 80 mumol/kg dry wt., which is about half as much as found in the exocrine pancreas. The initial uptake was characterized by two components, with approximate Km values of 35 microM and 3.7 microM respectively. After 60 min of incubation with 0.25 mM-Mn2+, the intracellular concentration of manganese corresponded to an almost 25-fold accumulation compared with that of the extracellular medium. When exposed to 20 mM-D-glucose, the islets retained more manganese, owing to suppression of its mobilization. The glucose inhibition of efflux was prompt and reversible, as indicated from direct recordings of manganese in a perifusion medium. D-Glucose was an equally potent inhibitor of efflux in the presence of 15 microM- and 1.28 mM-Ca2+. The inhibitory action disappeared when metabolism was suppressed by adding 0.1 mM-N-ethylmaleimide or by lowering the temperature from 37 degrees C to 2 degrees C. At a concentration of 0.25 mM, Mn2+ abolished the insulin-releasing action of D-glucose, exerting only moderate suppression of its metabolism. The addition of Mn2+ resulted in inhibition of basal insulin release in the presence of 1.28 mM-Ca2+, but not in a Ca2+-deficient medium. The studies indicate that the previously observed phenomenon of glucose inhibition of 45Ca efflux has a counterpart in the suppression of manganese mobilization from the pancreatic islets. With the demonstration of a pronounced glucose inhibition of manganese efflux, it is evident that Mn2+ may represent a useful tool for exploring the mechanism of glucose-induced retention of calcium in the pancreatic beta-cells.     

Effect of dietary selenium, zinc and allopurinol supplements on plasma and tissue manganese levels in rats with thiocetamide-induced liver cirrhosis

The effect of thiocetamide-induced liver cirrhosis on plasma and tissuemanganese levels and the protective role of selenium, zinc and allopurinolsupplements was investigated in rats. Control plasma and liver manganese(Mn) levels were found to be (mean ± SD): 8.4 ± 2.4 mg/L and5.7 ± 1.5 mg/g wet weight respectively. Plasma manganese levels weresignificantly increased (p < 0.001) whereas liver manganese levels weresignificantly reduced (p < 0.05) in the cirrhotic rats. Treatment withselenium, zinc and allopurinol reversed this trend and restored themanganese levels close to the normal values. Lung, spleen, and kidneymanganese levels under control conditions were considerably lower than thatof the liver tissue. However, these levels registered a significant increase(p < 0.05) in cirrhotic rats and this change was normalized after selenium,zinc and allopurinol treatment. There were no significant differences in thecomparative efficacy of each of these protective agents. Zinc supplementconsiderably increased the plasma zinc levels and plasma Zn/Mn ratio had agood correlation with plasma zinc concentration. This ratio wassignificantly reduced in cirrhotic rats, but returned to the control levelafter zinc, selenium and allopurinol treatment. The results of this studyindicate that the trace element, manganese, plays an important role instabilizing cell structure and that this effect is mediated possibly bypreserving the antioxidant activity of the tissues.     

Effect of manganese toxicity on the indoleacetic Acid oxidase system of cotton

The effect of substrate manganese on tissue manganese levels and activity of the indoleacetic acid (IAA)-oxidase system of cotton (Gossypium hirsutum, L.) was investigated. A sand culture technique was used with 1, 3, 9, 27 and 81 mg manganese (MnSO₄) per liter nutrient solution applied in various experiments.

The following relationships held for both long-term (126 days) and short-term (12-14 days) exposures to manganese treatment: A) There was a direct relationship between substrate and tissue manganese. B) Only the 81 mg/liter Mn plants exhibited severe manganese toxicity symptoms. C) At the toxic level of manganese an increased IAA-oxidase activity and decreased IAA-oxidase inhibitor activity was observed. There was a direct relationship between degree of enzyme response and severity of visible symptoms. D) With the manganese toxicity plants, but none of the other treatments, extracts of the young leaves contained as much IAA-oxidase activity as extracts of much older leaves. E) Crude extracts from the plants grown with 81 mg manganese per liter solution, in contrast to those of other treatments, destroyed IAA without addition of MnCl₂ to the assay medium.

A hypothesis is advanced stating that manganese toxicity symptoms in cotton are expressions of auxin deficiency caused by IAA-oxidase activity increased by the abnormal tissue levels of manganese.

     

Roles of manganese and organic acid chelators in regulating lignin degradation and biosynthesis of peroxidases by Phanerochaete chrysosporium.

We studied the effect of manganese and various organic chelators on the distribution, depolymerization, and mineralization of synthetic 14C-labeled lignins (DHP) in cultures of Phanerochaete chrysosporium. In the presence of high levels of manganese [Mn(II) or Mn(III)], along with a suitable chelator, lignin peroxidase (LiP) production was repressed and manganese peroxidase (MnP) production was stimulated. Even though partial lignin depolymerization was observed under these conditions, further depolymerization of the polymer to smaller compounds was more efficient when low levels of manganese were present. LiPs were prevalent under these latter conditions, but MnPs were also present. Mineralization was more efficient with low manganese. These studies indicate that MnP performs the initial steps of DHP depolymerization but that LiP is necessary for further degradation of the polymer to lower-molecular-weight products and mineralization. We also conclude that a soluble Mn(II)-Mn(III) organic acid complex is necessary to repress LiP.     

Roles of manganese and organic acid chelators in regulating lignin degradation and biosynthesis of peroxidases by Phanerochaete chrysosporium.

We studied the effect of manganese and various organic chelators on the distribution, depolymerization, and mineralization of synthetic 14C-labeled lignins (DHP) in cultures of Phanerochaete chrysosporium. In the presence of high levels of manganese [Mn(II) or Mn(III)], along with a suitable chelator, lignin peroxidase (LiP) production was repressed and manganese peroxidase (MnP) production was stimulated. Even though partial lignin depolymerization was observed under these conditions, further depolymerization of the polymer to smaller compounds was more efficient when low levels of manganese were present. LiPs were prevalent under these latter conditions, but MnPs were also present. Mineralization was more efficient with low manganese. These studies indicate that MnP performs the initial steps of DHP depolymerization but that LiP is necessary for further degradation of the polymer to lower-molecular-weight products and mineralization. We also conclude that a soluble Mn(II)-Mn(III) organic acid complex is necessary to repress LiP.     

Manganese and iron oxidation by fungi isolated from building stone

Acid and nonacid generating fungal strains isolated from weathered sandstone, limestone, and granite of Spanish cathedrals were assayed for their ability to oxidize iron and manganese. In general, the concentration of the different cations present in the mineral salt media directly affected Mn(IV) oxide formation, although in some cases, the addition of glucose and nitrate to the culture media was necessary. Mn(II) oxidation in acidogenic strains was greater in a medium containing the highest concentrations of glucose, nitrate, and manganese. High concentrations of Fe(II), glucose, and mineral salts were optimal for iron oxidation. Mn(IV) precipitated as oxides or hydroxides adhered to the mycelium. Most of the Fe(III) remained in solution by chelation with organic acids excreted by acidogenic strains. Other metabolites acted as Fe(III) chelators in nonacidogenic strains, although Fe(III) deposits around the mycelium were also detected. Both iron and manganese oxidation were shown to involve extracellular, hydrosoluble enzymes, with maximum specific activities during exponential growth. Strains able to oxidize manganese were also able to oxidize iron. It is concluded that iron and manganese oxidation reported in this work were biologically induced by filamentous fungi mainly by direct (enzymatic) mechanisms.Correspondence to: G. Gomez-Alarcon.     

Clodronate liposomes improve metabolic profile and reduce visceral adipose macrophage content in diet-induced obese mice

Background

Obesity-related adipose inflammation has been thought to be a causal factor for the development of insulin resistance and type 2 diabetes. Infiltrated macrophages in adipose tissue of obese animals and humans are an important source for inflammatory cytokines. Clodronate liposomes can ablate macrophages by inducing apoptosis. In this study, we aim to determine whether peritoneal injection of clodronate liposomes has any beneficial effect on systemic glucose homeostasis/insulin sensitivity and whether macrophage content in visceral adipose tissue will be reduced in diet-induced obese (DIO) mice.

Methodology/Principal Findings

Clodronate liposomes were used to deplete macrophages in lean and DIO mice. Macrophage content in visceral adipose tissue, metabolic parameters, glucose and insulin tolerance, adipose and liver histology, adipokine and cytokine production were examined. Hyperinsulinemic-euglycemic clamp study was also performed to assess systemic insulin sensitivity. Peritoneal injection of clodronate liposomes significantly reduced blood glucose and insulin levels in DIO mice. Systemic glucose tolerance and insulin sensitivity were mildly improved in both lean and DIO mice treated with clodronate liposomes by intraperitoneal (ip) injection. Hepatosteatosis was dramatically alleviated and suppression of hepatic glucose output was markedly increased in DIO mice treated with clodronate liposomes. Macrophage content in visceral adipose tissue of DIO mice was effectively decreased without affecting subcutaneous adipose tissue. Interestingly, levels of insulin sensitizing hormone adiponectin, including the high molecular weight form, were significantly elevated in circulation.

Conclusions/Significance

Intraperitoneal injection of clodronate liposomes reduces visceral adipose tissue macrophages, improves systemic glucose homeostasis and insulin sensitivity in DIO mice, which can be partially attributable to increased adiponectin levels.     

The interaction between manganese and calcium fluxes in pancreatic beta-cells.

Electrothermal atomic-absorption spectroscopy was employed for measuring manganese in beta-cell-rich pancreatic islets isolated from ob/ob mice. The efflux from preloaded islets was estimated from the amounts remaining after 30 min of subsequent test incubations in the absence of Mn2+. An increase in the extracellular Mg2+ concentration promoted the Mn2+ efflux and removal of Na+ from a Ca2+-deficient medium had the opposite effect. Addition of 25 mM-K+ failed to affect Mn2+ outflow as did 3-isobutyl-1-methylxanthine and dibutyryl cyclic AMP. Whereas tolbutamide caused retention of manganese, the ionophore Br-X537A promoted an efflux. D-Glucose was equally potent in retaining the islet manganese when the external Ca2+ concentration ranged from 15 microM to 6.30 mM. Subcellular-fractionation experiments indicated a glucose-stimulated incorporation of manganese into all fractions except the microsomes. The effect was most pronounced in the mitochondrial fraction, being as high as 164%. The glucose-induced uptake of intracellular 45Ca was abolished in the presence of 0.25 mM-Mn2+. When added to medium containing 2.5 mM-Mn2+, glucose even tended to decrease 45Ca2+ uptake. The inhibitory effect of Mn2+ was apparent also from a diminished uptake of 45Ca into all subcellular fractions. The efflux of 45Ca2+ was markedly influenced by Mn2+ as manifested in a prominent stimulation followed by inhibition. In addition to demonstrating marked interactions between fluxes of Mn2+ and Ca2+, the present studies support the view that the glucose inhibition of the efflux of bivalent cations from pancreatic beta-cells is accounted for by their accumulation in the mitochondria.     

Effects of hypophysectomy and acute growth hormone treatment upon glucose metabolism in adipose tissues and isolated adipocytes of rats.

Glucose oxidation and incorporation into lipid were measured in epididymal adipose tissues and isolated adipose cells of normal and hypophysectomized rats in an effort to determine whether the acute hypoglycemic effect of a systemic growth hormone (GH) injection was related to alterations in the glucose metabolism of adipose tissue. The rats were fed rat chow or a high sucrose diet and received 100 mug GH intraperitoneally 30 minutes or three and one-half hours before sacrifice. Hypophysectomized rats showed a lower plasma glucose as compared with normal rats on both diets. Thirty minutes after a GH injection there was a further decrease of the plasma glucose which, however, was not present in those rats receiving GH three and one-half hours before sacrifice. Adipose tissues from hypophysectomized rats fed the high sucrose diet showed a blunted insulin sensitivity as compared with normal rats on a similar diet. The insulin sensitivity of these tissues was further decreased 30 minutes after a GH injection. Basal glucose metabolism of isolated adipocytes from hypophysectomized rats, as compared with normal rats, was depressed if they were fed rat chow, was at normal levels if they were fed the high sucrose diet and was increased if they were fed the sucrose diet and received triiodothyronine and cortisone supplements. No manipulations of diet or hormonal treatments made the isolated adipocyte from hypophysectomized rats sensitive to insulin either 30 minutes or three and one-half hours after a GH injection. Since basal glucose utilization is not enhanced by GH injection and both the blunted insulin sensitivity of adipose tissue and the absent insulin sensitivity of adipopocytes would be expected to produce hyperglycemia rather than hypoglycemia, it is concluded that immediate systemic effects of a GH injection on carbohydrate metabolism are not related to changes in glucose metabolism of the peripheral adipose tissues.     

Relative Bioavailability of Manganese Proteinate for Broilers Fed a Conventional Corn–Soybean Meal Diet

An experiment was conducted to investigate the bioavailability of organic manganese proteinate (Mn) relative to inorganic Mn sulfate for broilers fed a conventional corn–soybean meal basal diet. A total of 448-day-old Arbor Acres commercial male chicks were fed the Mn-unsupplemented basal diet (control) or basal diet supplemented with 60, 120, or 180 mg Mn/kg from each Mn source. At 21 days of age, heart tissue was excised for testing DM, Mn concentration, manganese superoxide dismutase (MnSOD) activity, and MnSOD mRNA level. The Mn concentration, MnSOD activity, and MnSOD mRNA level in heart tissue increased (P < 0.01) linearly as dietary manganese concentration increased. Based on slope ratios from multiple linear regressions of the above three indices on added Mn level, there was no significant difference (P > 0.21) in bioavailability between Mn proteinate and Mn sulfate for broilers in this experiment.     

Determination of the oxidation states of manganese in brain, liver, and heart mitochondria

Excess brain manganese can produce toxicity with symptoms that resemble those of Parkinsonism and causes that remain elusive. Manganese accumulates in mitochondria, a major source of superoxide, which can oxidize Mn2+ to the powerful oxidizing agent Mn3+. Oxidation of important cell components by Mn3+ has been suggested as a cause of the toxic effects of manganese. Determining the oxidation states of intramitochondrial manganese could help to identify the dominant mechanism of manganese toxicity. Using X-ray absorbance near edge structure (XANES) spectroscopy, we have characterized the oxidation state of manganese in mitochondria isolated from brain, liver, and heart over concentrations ranging from physiological to pathological. Results showed that (i) spectra from different model manganese complexes of the same oxidation state were similar to each other and different from those of other oxidation states and that the position of the absorption edge increases with oxidation state; (ii) spectra from intramitochondrial manganese in isolated brain, heart and liver mitochondria were virtually identical; and (iii) under these conditions intramitochondrial manganese exists primarily as a combination of Mn2+ complexes. No evidence for Mn3+ was detected in samples containing more than endogenous manganese levels, even after incubation under conditions promoting reactive oxygen species (ROS) production. While the presence of Mn3+ complexes cannot be proven in the spectrum of endogenous mitochondrial manganese, the shape of this spectrum could suggest the presence of Mn3+ near the limit of detection, probably as MnSOD.     

54Mn absorption and excretion in rats fed soy protein and casein diets

Rats were fed diets containing either soy protein or casein and different levels of manganese, methionine, phytic acid, or arginine for 7 days and then fed test meals labeled with 2 microCi of 54Mn after an overnight fast. Retention of 54Mn in each rat was measured every other day for 21 days using a whole-body counter. Liver manganese was higher (P less than 0.0001) in soy protein-fed rats (8.8 micrograms/g) than in casein-fed rats (5.2 micrograms/g); manganese superoxide dismutase activity also was higher in soy protein-fed rats than in casein-fed rats (P less than 0.01). There was a significant interaction between manganese and protein which affected manganese absorption and biologic half-life of 54Mn. In a second experiment, rats fed soy protein-test meals retained more 54Mn (P less than 0.001) than casein-fed rats. Liver manganese (8.3 micrograms/g) in the soy protein group was also higher than that (5.7 micrograms/g) in the casein group (P less than 0.0001), but manganese superoxide dismutase activity was unaffected by protein. Supplementation with methionine increased 54Mn retention from both soy and casein diets (P less than 0.06); activity of manganese superoxide dismutase increased (P less than 0.05) but liver manganese did not change. The addition of arginine to casein diets had little effect on manganese bioavailability. Phytic acid affected neither manganese absorption nor biologic half-life in two experiments, but it depressed liver manganese in one experiment. These results suggest that neither arginine nor phytic acid was the component in soy protein which made manganese more available from soy protein diets than casein diets.