Regulation of NADP-Malate Dehydrogenase in C4 Plants: Activity and Properties of Maize Thioredoxin M and the Significance of Non-Active Site Thiol Groups

Some unusual properties of purified maize leaf thioredoxin m were attributable to the presence of non-active site thiol groups, Unlike thioredoxins from other sources, maize leaf thioredoxin m was susceptible to inactivation by heating and this was associated with polymerisation of the molecule. Both these effects of heating were prevented or reversed by adding thiol compounds such as dithiothreitol. We concluded that, on heating, the free SH groups in the oxidised thioredoxin m molecule react with disulfide groups of other molecules to form polymeric complexes linked by disulfide bonds. We also observed the formation of a stable complex between the oxidised forms of thioredoxin m and NADP-malate dehydrogenase (NADP-MDH) under certain conditions. Evidence that in maize chloroplasts thioredoxin m and NADP-MDH occur in a 1: 1 molar ratio suggested that they may exist as a complex in vivo. However, ratios of thioredoxin rn to NADP-MDH varied widely in other species but always with thioredoxin m in excess. Furthermore, the complex we observed in vitro was shown to be the result of intermolecular disulfide bond formation and apparently occurred only with a non-physiological form of oxidised thioredoxin m. We could not demonstrate any non-covalent binding between thioredoxin m and NADP-MDH.