title: The reductive modulation of chloroplast fructose-1,6-bisphosphatase by tributylphosphine and sodium borohydride.
abstract: 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.
authors.affiliation: Instituto de Investigaciones Bioquímicas Fundación Campomar, F.C.E.N.-U.B.A., CONICET, Buenos Aires, Argentina.
authors.forename: M A
authors.initials: MA
authors.lastname: Ballícora
authors.forename: A C
authors.initials: AC
authors.lastname: D'Alessio
authors.forename: S
authors.initials: S
authors.lastname: Mora-García
authors.forename: R J
authors.initials: RJ
authors.lastname: Rodríguez-Suarez
authors.forename: R A
authors.initials: RA
authors.lastname: Wolosiuk
mesh_terms: D001894:Borohydrides / Q000378:metabolism*; D002736:Chloroplasts / Q000201:enzymology*; D004789:Enzyme Activation; D006597:Fructose-Bisphosphatase / Q000378:metabolism*; D010084:Oxidation-Reduction; D010720:Phosphines / Q000378:metabolism*; D018724:Spinacia oleracea / Q000201:enzymology*
chemical_list: D001894:Borohydrides; D010720:Phosphines; C043255:tri-n-butylphosphine; C025364:sodium borohydride; D006597:Fructose-Bisphosphatase