Spinach ferredoxin is a calcium-binding protein

Spinach-leaf ferredoxin was identified as a calcium-binding protein by (45)Ca autoradiography on nitrocellulose membranes and with the cationic carbocyanine dye 1-ethyl-2-[3-(1-ethylnaphtho[1,2-d]thiazolin-2-ylidene)-2-methylpropenyl] naphtho[1,2-d]thiazolium bromide ("stains-all"). Binding of (45)Ca was observed at pH 6.8 and pH 7.8 and in the presence of 5 mM and 20 mM MgCl2. At the higher MgCl2 concentration the Ca(2+)-binding capacity is reduced. Only micromolar concentrations of LaCl3, however, are required to achieve a similar effect. Both the oxidized and reduced forms of ferredoxin bind calcium.