THE REDUCTIVE MODULATION OF CHLOROPLAST FRUCTOSE-1,6-BISPHOSPHATASE BY TRIBUTYLPHOSPHINE AND SODIUM-BOROHYDRIDE

The cleavage of disulfide bonds is the major modification of chloropla st fructose-1,6-bisphosphatase when the light-mediated ferredoxin-thio redoxin system enhances the activity of the enzyme. In vitro, only thi ol-bearing compounds are functional in the stimulation of fructose 1,6 -bisphosphate hydrolysis. This investigation was undertaken to determi ne the effectivity of other reductants for enhancing the catalytic cap acity. In the presence of 1 mM fructose 1,6-bisphosphate and 0.1 mM Ca 2+, the five-fold activation triggered by 3.5 mM tributylphosphine is further potentiated by 15% (v/v) 2-propanol. When the enzyme is incuba ted in the presence of 0.15 M sodium trichloroacetate in place of the cosolvent, NaH4B initially stimulates the activity but subsequently ca uses the inactivation of the enzyme. A model developed to analyze this dual effect suggests that the concerted action of fructose 1,6-bispho sphate, Ca2+ and trichloroacetate yields an enzyme form that is slight ly activable by reduction (t(0.5) = 28 min.). However, chloroplast fru ctose-1,6-bisphosphatase becomes highly sensitive to trichloroacetate inactivation (t(0.5) = 5 min.) when NaH4B reduces fructose 1,6-bisphos phate. Hence, the thiol/disulfide exchange constitutes a particular ca se of reductive mechanisms that stimulate the activity of chloroplast fructose-1,6-bisphosphatase.