Desensitization of Yeast Phosphofructokinase to ATP‐Inhibition by a Protein Fraction from Yeast

A heat‐labile fraction from yeast is described which converts the ATP‐sensitive from of phosphofructokinase (Form s), inhibited by high concentrations of ATP, into a desensitized form (Form d), not inhibited by excess ATP. This desensitization reaction is time‐dependent. Forms s and d are different, relatively stable forms of phosphofructokinase. Retaining their characteristic sensitivity or insensitivity to ATP, they can be fractionated by ammonium sulfate and acetone. The heat‐labile fraction, desensitizing Form s to Form d, is neither dialysable nor ultrafiltrable. It is inactivated by trypsin treatment of the crude extract. It can be fractionated by ammonium sulfate and acetone. All these facts strongly suggest that the active principle is a protein. The protein fraction from crude yeast extract desensitizing Form s to Form d, was about 50‐fold purified by treatment with protamine sulfate and fractionation with ammonium sulfate and acetone. Besides the desensitizing protein, the conversion reaction is dependent on fluoride, AMP, ATP‐Mg ++ , and fructose‐6‐phosphate. Ado‐3′:5′‐ P has no effect.