Neuhaus H -Ekkehard, Maaß Uwe
Pflanzenphysiologie, Fachbereich Biologie/Chemie, Universität Osnabrück, Barbarastraße 11, D-49069, Osnabrück, Germany.
Planta. 1996 Apr;198(4):542-548. doi: 10.1007/BF00262640. Epub 2017 Mar 18.
Isolated amyloplasts from cauliflower (Brassica oleracea L. var botrytis) buds are able to export orthophosphate unidirectionally into the incubation medium. This orthophosphate transport appears to be protein-mediated, as indicated by the following observations: (i) low temperature and the presence of inhibitors of protein-mediated transport reduced the rate of orthophosphate export, and (ii) the rate of orthophosphate export became saturated with rising internal substrate concentrations. Micromolar concentrations of 4,4'-diisothiocyano-2,2'-stilbene disulphonic acid inhibited the rate of unidirectional orthophosphate export, thus indicating the involvement of the amyloplastic glucose-6-phosphate (Glc6P)translocator in the unidirectional export of orthophosphate. The effect of rising concentrations of orthophosphate upon the activity of ADP glucose pyrophosphorylase in desalted extracts was determined. Orthophosphate given in concentrations similar to those measured in the amyloplastic stroma under conditions of steady-state rates of Glc6P-dependent starch synthesis inhibited the activity of ADP-glucose pyrophosphorylase significantly. However, even under strong limiting substrate conditions the residual activity was sufficient to catalyze the flux of carbon into starch. The maximal rates of orthophosphate transport (in the counter-exchange mode) by isolated spinach (Spinacia oleracea L.) chloroplasts and by isolated cauliflower-bud amyloplasts were also determined. These rates were compared with the maximal rates of undirectional orthophosphate export by these plastids. From these measurements we can conclude that, compared with spinach chloroplasts, isolated amyloplasts of cauliflower exhibit a fivefold greater ratio of unidirectional orthophosphate transport to maximal rate of orthophosphate transport in the counter-exchange mode compared to spinach chloroplasts. The determined rate of maximal unidirectional orthophosphate export is sufficient to catalyze the release of additional inorganic phosphate liberated in the amyloplastic stroma during the process of Glc6P-dependent starch synthesis.
从花椰菜（甘蓝变种）芽中分离出的造粉体能够将正磷酸盐单向输出到培养液中。如下观察结果表明，这种正磷酸盐转运似乎是由蛋白质介导的：（i）低温以及蛋白质介导转运的抑制剂的存在降低了正磷酸盐的输出速率；（ii）随着内部底物浓度的升高，正磷酸盐的输出速率达到饱和。微摩尔浓度的4,4'-二异硫氰基-2,2'-芪二磺酸抑制了正磷酸盐的单向输出速率，这表明造粉体中的葡萄糖-6-磷酸（Glc6P）转运体参与了正磷酸盐的单向输出。测定了正磷酸盐浓度升高对脱盐提取物中ADP葡萄糖焦磷酸化酶活性的影响。在依赖Glc6P的淀粉合成稳态速率条件下，给予与造粉体基质中测得浓度相似的正磷酸盐，可显著抑制ADP-葡萄糖焦磷酸化酶的活性。然而，即使在底物严重受限的条件下，残余活性仍足以催化碳流入淀粉。还测定了分离的菠菜叶绿体和花椰菜芽造粉体的正磷酸盐最大转运速率（反向交换模式）。将这些速率与这些质体的正磷酸盐单向输出最大速率进行了比较。从这些测量结果我们可以得出结论，与菠菜叶绿体相比，分离出的花椰菜造粉体在反向交换模式下，正磷酸盐单向转运与正磷酸盐最大转运速率的比值比菠菜叶绿体高五倍。所测定的正磷酸盐最大单向输出速率足以催化在依赖Glc6P的淀粉合成过程中造粉体基质中释放的额外无机磷酸盐的释放。
