Selective binding of (3H)gibberellin A1 by protein fractions from dwarf pea epicotyls

A homogenate from dwarf pea seedlings that had been treated with [(3)H]gibberellin A(1) was separated by Sephadex G-200 into two protein fractions, of high and intermediate molecular weight, with affinity for [(3)H]gibberellin A(1). Neither fraction bound [(3)H]gibberellin A(8), the only metabolite of [(3)H]gibberellin A(1) detected in significant quantity in extracts of pea seedlings. These proteinhormone complexes were noncovalently bound, as evidenced by their rapid disruption in ethanol. Approximate molecular weights of the fractions were 500,000 and 60,000. In experiments designed to detect competition for the binding proteins between [(3)H]gibberellin A(1) and closely related [(3)H]gibberellin molecules, [(3)H]gibberellin A(8) and pseudo [(3)H]gibberellin A(1) were inactive, but keto [(3)H]gibberellin competed favorably. Equilibrium dialysis experiments revealed that both high-molecular-weight and intermediate-molecular-weight protein-hormone complexes could undergo hormone exchange with nonlabeled gibberellin A(1).