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

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.