Effect of polyethylene glycol on the activity, intrinsic fluorescence, and oligomeric structure of castor seed cytosolic fructose-1,6-bisphosphatase.

The effect of polyethylene glycol (PEG) on the activity, intrinsic fluorescence, and oligomeric structure of homogeneous cytosolic fructose-1,6-bisphosphatase (FBPasec) from endosperm of germinating castor oil seeds has been examined. Increasing the PEG concentration in the FBPasec reaction mixture elicited a progressive 3-fold decrease in the enzyme's Km for fructose-1,6-P2. The presence of PEG also: (i) increased the extent of FBPasec inhibition by high levels of fructose-1,6-P2, (ii) enhanced the intensity of the enzyme's fluorescence emission spectra, and (iii) prevents dissociation of the active tetrameric native enzyme into inactive lower M(r) forms during gel filtration HPLC. It is concluded that the activity and structure of plant FBPasec is modified by extreme dilution, probably as a result of partial deaggregation of the native tetrameric enzyme.