REGULATION OF NADP-MALATE DEHYDROGENASE IN C-4 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 attr ibutable to the presence of non-active site thiol groups. Unlike thior edoxins from other sources, maize leaf thioredoxin m was susceptible t o inactivation by heating and this was associated with polymerisation of the molecule. Both these effects of heating were prevented or rever sed by adding thiol compounds such as dithiothreitol. We concluded tha t, on heating, the free SH groups in the oxidised thioredoxin m molecu le react with disulfide groups of other molecules to form polymeric co mplexes linked by disulfide bonds. We also observed the formation of a stable complex between the oxidised forms of thioredoxin m and NADP-m alate dehydrogenase (NADP-MDH) under certain conditions. Evidence that in maize chloroplasts thioredoxin in and NADP-MDH occur in a 1:1 mola r ratio suggested that they may exist as a complex in vivo. However, r atios of thioredoxin m to NADP-MDH varied widely in other species but always with thioredoxin m in excess. Furthermore, the complex we obser ved in vitro was shown to be the result of intermolecular disulfide bo nd formation and apparently occurred only with a nonphysiological form of oxidised thioredoxin m. We could not demonstrate any non-covalent binding between thioredoxin m and NADP-MDH.