The reductive modulation of chloroplast fructose-1,6-bisphosphatase by tributylphosphine and sodium borohydride

The cleavage of disulfide bonds is the major modification of chloroplast fructose-1,6-bisphosphatase when the light-mediated ferredoxin-thioredoxin system enhances the activity of the enzyme. In vitro, only thiol-bearing compounds are functional in the stimulation of fructose 1,6-bisphosphate hydrolysis. This investigation was undertaken to determine the effectivity of other reductants for enhancing the catalytic capacity. In the presence of 1 mM fructose 1,6-bisphosphate and 0.1 mM Ca2+, the five-fold activation triggered by 3.5 mM tributylphosphine is further potentiated by 15% (v/v) 2-propanol. When the enzyme is incubated in the presence of 0.15 M sodium trichloroacetate in place of the cosolvent, NaH4B initially stimulates the activity but subsequently causes the inactivation of the enzyme. A model developed to analyze this dual effect suggests that the concerted action of fructose 1,6-bisphosphate, Ca2+ and trichloroacetate yields an enzyme form that is slightly activable by reduction (t0.5 = 28 min.). However, chloroplast fructose-1,6-bisphosphatase becomes highly sensitive to trichloroacetate inactivation (t0.5 = 5 min.) when NaH4B reduces fructose 1,6-bisphosphate. Hence, the thiol/disulfide exchange constitutes a particular case of reductive mechanisms that stimulate the activity of chloroplast fructose-1,6-bisphosphatase.