Differential Subcellular Localization and Expression of ATP Sulfurylase and 5′-Adenylylsulfate Reductase during Ontogenesis of Arabidopsis Leaves Indicates That Cytosolic and Plastid Forms of ATP Sulfurylase May Have Specialized Functions

ATP sulfurylase and 5′-adenylylsulfate (APS) reductase catalyze two reactions in the sulfate assimilation pathway. Cell fractionation of Arabidopsis leaves revealed that ATP sulfurylase isoenzymes exist in the chloroplast and the cytosol, whereas APS reductase is localized exclusively in chloroplasts. During development of Arabidopsis plants the total activity of ATP sulfurylase and APS reductase declines by 3-fold in leaves. The decline in APS reductase can be attributed to a reduction of enzyme during aging of individual leaves, the highest activity occurring in the youngest leaves and the lowest in fully expanded leaves. By contrast, total ATP sulfurylase activity declines proportionally in all the leaves. The distinct behavior of ATP sulfurylase can be attributed to reciprocal expression of the chloroplast and cytosolic isoenzymes. The chloroplast form, representing the more abundant isoenzyme, declines in parallel with APS reductase during aging; however, the cytosolic form increases over the same period. In total, the results suggest that cytosolic ATP sulfurylase plays a specialized function that is probably unrelated to sulfate reduction. A plausible function could be in generating APS for sulfation reactions.

ATP硫酸化酶（ATP sulfurylase）与5'-腺苷酰硫酸（5′-adenylylsulfate, APS）还原酶催化硫酸盐同化通路中的两步反应。对拟南芥（Arabidopsis）叶片进行细胞组分分离分析后发现，ATP硫酸化酶的同工酶同时存在于叶绿体（chloroplast）与细胞质基质（cytosol）中，而APS还原酶仅定位于叶绿体。在拟南芥植株的发育过程中，叶片内ATP硫酸化酶与APS还原酶的总活性会下降至原水平的1/3。APS还原酶活性的下降可归因于单叶衰老过程中酶含量的减少：其活性在幼叶中最高，在完全展开的成熟叶片中最低。与之相对，所有叶片中的ATP硫酸化酶总活性均呈等比例下降。ATP硫酸化酶的这种独特行为，可通过叶绿体与细胞质基质同工酶的反向表达来解释：作为丰度更高的同工酶亚型，叶绿体型ATP硫酸化酶的表达水平随叶片衰老与APS还原酶同步下降，而细胞质基质型同工酶的表达水平在同一时期却呈上升趋势。综合上述结果提示，细胞质基质型ATP硫酸化酶可能具有与硫酸盐还原无关的特殊功能，一种合理的推测是，该酶可通过生成APS参与硫酸化反应。