RESUMO
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 (75)Se at 10 microM added Na(2)(75)SeO(3), 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 (75)Se uptake against 2-20 microM Na(2)(75)SeO(3) 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 microM, 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 microM and 4-fold at 50 microM in the Mim 0.1 group than those in the -Se-stressed group. Initial uptake levels with added mimosine up to 50 or 100 microM yielded nonlinear double-reciprocal plots; and kinetic analyses at 5 to 50 microM 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.
