Binding of (45)Ca (2+) to particulate fractions of coleoptile tissue

Using recently developed techniques, we have investigated the binding of (45)Ca(2+) to membrane preparations from corn (Zea mays L) and oat (Avena sativa L) coleoptile tissue. Scatchard plot analysis reveals at least two Ca(2+)-binding sites in each tissue, a high affinity binding site (K m=7.7×10(-7) M, n=6.9×10(-10) mol·0.5 g f.w.(-1) in corn, K m=4.93×10(-6) M, n=2.29×10(-9) mol·0.5 g f.w.(-1) in Avena) and a low affinity binding site (K m=9.01×10(-5) M, n=5.4×10(-8) mol·0.5 g f.w.(-1) in corn; K m=1.03×10(-4) M, n=3.40×10(-8) mol·0.5 g f.w.(-1) in Avena). There is also some evidence of a third, lower affinity binding site in each tissue, especially corn.More detailed studies with corn coleoptile homogenates show that they contain a potent dialyzable inhibitor of Ca(2+) binding. Monovalent cations were observed to be ineffective as inhibitors of Ca(2+) binding in corn. However, of six divalent cations tested, all were capable of strong inhibition of Ca(2+)-binding and there appeared to be a relationship between size of the atomic radius of the ion and potency as an inhibitor of calcium binding.