Phosphofructokinase
Abstract Purified rabbit muscle phosphofructokinase required K+ ion for maximal activity and was inactive at pH 7 unless the Mg++ concentration exceeded the total adenosine triphosphate concentration. Enzymatic activity was proportional to sulfhydryl reduction, and was reversibly lost by sulfhydryl oxidation with oxidized glutathione. The molecular weight of enzyme crystallized in the presence of adenosine triphosphate was determined. The smallest fully active form had a molecular weight of 3.8 x 105 and could be reversibly dissociated into units of either one-half or one-quarter this size; 95 to 98% of the enzymatic activity was lost by dilution at pH 6.7, by urea treatment at pH 5.8, or by lowering the pH to 5.0, all of which decrease the molecular weight to 1.92 x 105. Protein of this size had an intrinsic activity 0.02 to 0.05 times that of the higher molecular weight form and was completely reactivated by reaggregation.