Selenium (Se) deficiency in the experimental models, Coturnix coturnix japonica and Corcyra cephalonica, resulted in impaired mitochondrial substrate oxidations and lowered thiol levels. Studies with respiratory inhibitors confirmed reduced mitochondrial electron transport enzyme activities, especially at cytochrome c oxidase (COX), the terminal segment. Enhanced mitochondrial lipid peroxidation in Se deficiency was more pronounced in the heart tissue of the quail compared to other tissues. Glutathione peroxidase (GSH-Px) activity toward H2O2 and cumene hydroperoxide were generally low in the insect muscle tissue and activity toward H2O2 was maximal in the quail heart mitochondria that was not very sensitive to Se status. Lowered COX activity in Se deficiency was more directly correlated with the increased level of lipid peroxidation than with the GSH-Px activity measured, suggestive of Se mediated protective mechanisms independent of GSH-Px. Electron microscopic observations revealed structural changes such as loss of cristae with proliferative and degenerative changes of the mitochondria in Se deficiency. Involvement of Se in maintaining structure and functional efficiency of mitochondria is evident from the present study.

Kalyanaraman Lalitha

article

CELL ULTRASTRUCTURE

chicken

electron transport

Lipid peroxidation

MITOCHONDRIAL RESPIRATION

Mitochondrion

newborn

nonhuman

SELENIUM DEFICIENCY

CORCYRA CEPHALONICA

COTURNIX COTURNIX

COTURNIX JAPONICA

Gallus gallus

Insecta

JAPONICA

PHASIANIDAE

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IIT Madras

Evidence for Altered Structure and Impaired Mitochondrial Electron Transport Function in Selenium Deficiency

Biological Trace Element Research

Mimosine, a non-protein plant amino acid found in Mimosa pudica and certain species of Leucaena, was beneficial for the growth of seedlings of Vigna radiata germinated under selenium-deficient stressed condition (-Se stressed) despite the recognized toxicity of the allelochemical. Exposure of mimosine at 0.1 mM (Mim-0.1) promoted the growth of the seedlings and significantly enhanced mitochondrial functional efficiency. Growth-related parameters including root and shoot lengths and dry weight were increased by 44-58% in the Mim-0.1 group compared to that of the -Se-stressed group. Oxygen uptake by mitochondria of Mim-0.1 group, studied with different substrates, revealed enhanced State 3 respiratory rates with regulated State 4 rates, resulting in high respiratory control ratio (RCR) of 3.4 to 3.9 indicative of a high degree of oxidative coupling. Specific activities of mitochondrial electron transport enzymes, nicotinamide adenine dinucleotide (reduced form) (NADH)-cytochrome (cyt) c oxidoreductase, succinate dehydrogenase, and cyt c oxidase in the Mim-0.1 group were enhanced by 53% to threefold over those of the Se-stressed group. Marked decreases in the extent of mitochondrial lipid peroxidation ensued upon mimosine exposure, indicative of its antioxidant function. Mitochondrial 45Ca2+ uptake was notably augmented twofold in the Mim-0.1 group, compared to the Se-stressed group. Detailed kinetic analyses of Ca 2+ uptake revealed positive cooperative interactions in both -Se-stressed group and Mim-0.1 groups with Hill coefficient (nH) values of 1.7 and 2, respectively. The present study establishes the beneficial effects of mimosine exposure at 0.1 mM on the growth and mitochondrial function of the seedlings grown under selenium-deficient stressed condition and a significant physiological role can be ascribed to mimosine. © Humana Press Inc. 2007.

Mimosine mitigates oxidative stress in selenium deficient seedlings of Vigna radiata-Part I: Restoration of mitochondrial function

During the growth of selenium (Se)-deficient seedlings of Vigna radiata, exposure to mimosine [2-amino-3-(3-hydroxy-4-oxo-1H-pyridin-1-yl)-propanoic acid], a nonprotein plant amino acid, effectively mitigated stress at 0.1 mM, as reflected in enhancement of growth and efficiency of mitochondrial functions. Since the changes in the seedlings elicited by exposure to mimosine were similar to those effected by Se at an optimal exposure level of 0.75 ppm (Sreekala et al., Biol Trace Elem Res 70:193-207, 1999), the uptake of Se and that of mimosine itself was individually studied in the respiring mitochondria of Se-deficient seedlings (-Se-stressed group) in comparison with those exposed to mimosine during growth at 0.1 mM (Mim 0.1 group). In both groups, the mitochondrial uptake of 75Se at 10 μM added Na2 75 SeO3, increased linearly up to 2 min, attaining steady-state levels thereafter. Uptake levels were 2.3-fold higher in the Mim 0.1 group than in the -Se-stressed group. Double-reciprocal plots of mitochondrial 75Se uptake against 2-20 μM Na2 75 SeO3 in the medium were nonlinear and negative cooperative effects during the uptake were confirmed by Scatchard plots, whereas Hill coefficients were 0.8 and 0.85 for the two groups. Mitochondrial uptake of mimosine, at added levels of 25 or 50 μM, increased linearly up to 1 min and decelerated thereafter. Initial uptake levels of mimosine at 1 min were higher by 6.5-fold at 25 μM and 4-fold at 50 μM in the Mim 0.1 group than those in the -Se-stressed group. Initial uptake levels with added mimosine up to 50 or 100 μM yielded nonlinear double-reciprocal plots; and kinetic analyses at 5 to 50 μM revealed the prevalence of positive cooperativity in the -Se-stressed group and negative cooperativity in the Mim 0.1 group. Involvement of active thiol groups in the uptake of both Se and mimosine were indicated by inhibition studies. Evidence presented for mimosine mediated increase in mitochondrial Se uptake and cooperative interactions thereof underscores the metabolic significance of mimosine. © Humana Press Inc. 2007.

Mimosine mitigates oxidative stress in selenium deficient seedlings of Vigna radiata. Part II: Mitochondrial uptake of 75selenium and mimosine

Mitochondria from Trigonella foenum-graecum seedlings grown independently in the presence of either selenium (0.75 ppm) or mimosine (0.1 mM) exhibited respiration-stimulated energy-dependent uptake of Ca2+. Uptake studies were carried out independently at a series of Ca2+ concentrations at two different levels: (1) 1-20 μM and (2) 25-1500 μM. Levels of uptake were 50-100% higher in the mitochondria of seedlings of both the Se and mimosine groups. Detailed kinetic analyses revealed negative cooperative effects operative during uptake of Ca2+ at 25-1500 μM given in the medium. Hill coefficients for Ca2+ uptake by the mitochondria of different groups remained unchanged (nH, 0.75). Biphasic Scatchard plots were concave upward, suggestive of two classes of binding sites. High-affinity binding sites were estimated to be 16 nmol/mg protein with dissociation constant (KCa) of 2.5 × 109 L/mol. In contrast, graphical analyses of the uptake of Ca2+ in the range 1-20 μM in the medium revealed cooperative effects of positive nature. The present study demonstrates mixed cooperative effects during Ca2+ uptake by mitochondria from seedlings of T. foenum-graecum.

Uptake of 45Ca by mitochondria of Trigonella foenum-graecum as influenced by selenium and mimosine - Detailed kinetic analyses

Uptake of (75Se) added in vitro was followed in mitochondria isolated from Trigonella foenum-graecum seedlings grown under different Se status (0.5-1.0 ppm) and with added mimosine (0.1 mM). Uptake of 75Se followed with added Na2 75SeO3 up to 20 μM in the medium was nonlinear in all the groups. Kinetic analyses of the uptake of 75Se for 1 min were carried out for ali the groups. The results indicated a cooperative effect during Se transport. Graphical analyses using the Hill plot and Scatchard plot confirmed the existence of negative cooperativity during 75Se uptake. Scatchard plots were biphasic, suggesting the probable presence of two classes of binding sites. The presence of succinate or ATP in the incubation medium inhibited 75Se uptake by 40%. Studies with mitochondrial respiratory inhibitors indicated the uptake to be energy independent. A decrease in the uptake of 75Se by 40% effected by HgCl2, N-ethyl maleimide, and iodoacetate confirmed the interaction of active thiols in the process. The present study confirms the controlled nature of 75Se uptake by plant mitochondria.

Kinetic analyses of mitochondrial 75selenium uptake in Trigonella foenum-graecum seedlings exposed to selenium and mimosine

Adaptive alterations in glutathione (GSH) metabolism were studied during oxidative stress induced by selenium (Se) deficiency in germinating seedlings of Trigonella foenum-graecum grown for 72 h and the response to supplementation individually of Se or mimosine was explored. Growth enhancement with improved mitochondrial efficiency was elicited by supplementation of Se at 0.5-0.75 ppm or mimosine at 0.1-0.2 mM. Total thiol and protein levels of mitochondrial and soluble fractions, in general, did not vary significantly with supplementation of either Se or mimosine except that the mitochondrial protein levels in mimosine groups (0.1-0.2 mM) decreased by 20-30%. Mitochondrial glutathione peroxidase (GSH-Px) increased by twofold in activity toward H2O2, cumene hydroperoxide (CHP), and t-butyl hydroperoxide (tBHP) in Se groups, and by 50-60% increase toward H2O2 and CHP but by a twofold enhancement in enzyme activity with tBHP in mimosine groups. Soluble GSH-Px activity increased by 30-40% only in mimosine groups and remained unaltered in Se groups. Glutathione S-transferase activity (GST) in the soluble fraction of both Se and mimosine groups increased dramatically by fivefold to sixfold. Distinct differences were noted in the response of the stressed seedlings toward exposure to Se or mimosine and included a decline in glutathione reductase (GR) activity by 50-60% in both mitochondria and soluble fractions of Se groups and an increase in GR activity of the mitochondria by twofold and of the soluble enzyme activity by 30% in the mimosine groups. Mimosine exposure resulted in a dose-dependent decrease in the γ-glutamy transpeptidase levels, but, in contrast, a significant enhancement by 50% was noted in the Se group at 0.75 ppm. The results including the differential response of GR activity to Se or mimosine supplementation are reflective of an effective reductive environment in Se group, and increased turnover of GSH in the presence of mimosine.

Journal	Biological Trace Element Research
Publisher	Humana Press Inc.
ISSN	01634984
Open Access	No