Completion of the thioredoxin reaction mechanism: kinetic evidence for protein complexes between thioredoxin and fructose 1,6-bisphosphatase.

The activation of chloroplast fructose 1,6-bisphosphatase from spinach and soybean leaves by the two chloroplast thioredoxins isolated from the same plants has been studied. The thioredoxin saturation characteristics (Vmax: 0.15-103.2 mumol Pi/min per mg enzyme; K0.5: 0.0048-0.84 microM; Hill coefficient n: 1.02-3.80) indicate that in addition to the reductive activation by thioredoxin specific complex formation between thioredoxin and fructose 1,6-bisphosphatase is responsible for fine regulation of the enzyme activity. This complex formation has been inserted into the thioredoxin mechanism and the physiological consequences discussed. Obviously, physiologically relevant investigations of the thioredoxin-dependent regulation of fructose 1,6-bisphosphatase activity can only be performed in homologous enzyme-thioredoxin combinations. Dithiothreitol and E. coli thioredoxin are no complete substitutes in regulatory studies.