大麦根细胞质膜Ca^(2+)-ATP酶和Ca^(2+)转运系统的特性

Characteristics of Ca^(2+)-ATPase and Ca^(2+)-translocation of Barley Root Plasma Membrane

用大麦质膜微囊研究细胞质膜 Ca^(2+)转运过程,发现质膜 Ca^(2+)—ATP酶在反应系统中不存在Mg^(2+)时可正常表现活性。跨膜Ca^(2+)转运按其对Mg^(2+)的需求可分为两个过程,一个是不需Mg^(2+)的、具高Ca^(2+)亲和力和较低的转运能力;另一个则是需Mg^(2+)的、具低Ca^(2+)亲和力和较高的转运能力。前者的动力学特征与Ca^(2+)—ATP酶相近,而后者则相差很大。据此推测,大麦根细胞质膜上除Ca^(2+)—ATP酶外,还存在另一个不同的Ca^(2+)转运系统。由两者分别承担的Ca^(2+)转运过程在细胞钙信使系统中可能起着不同的作用。

Plasma membrane--enriched vesi-cles were prepared from barley root bysucrose gradient centrifugation. BothCa^(2+)- and Mg^(2+)-ATPase on the plas-ma membrane were identified. Ca^(2+)-ATPase was dependent on Ca^(2+), butindePendent of Mg^(2+). A 2.3--foldstimulation of Ca^(2+)--ATPase bycalmodulin prepared from porcinebrain was detected when the prepara-tion was purified further by two--phasepartitioning in the presence of EGTA. Though Mg^(2+) was a necessarycomponent in general assay medium,Ca^(2+)--uptake activity independent ofMg^(2+) and dependent on Ca^(2+) had beendetected in our preparations. In the ab-sence of Mg^(2+), Ca^(2+)--ATPase andCa^(2+)--uptake were both dependent onCa^(2+) and their enzymological be-haviours were almost identical. Theyhad similar K_m values for ATP andHill coefficient, and both had a greateraffinity for Ca^(2+). They had the sameoptimum pH and a similar sensitivity toerythrosin B and calmodulin antago-nists (R24157 and compound 48/80).On the other hand, in the presence ofMg^(2+), Ca^(2+) --uptake had differentcharacteristics. The results suggest that there betwo Ca^(2+)-uptake systems located onplasma membrane of barley root. Oneis Ca^(2+)-ATPase which is independentof Mg^(2+), has a higher affinity forCa^(2+) and a lower capacity for Ca^(2+)transport. The mechanism of the othersystem is unclear so far. It is depen-dent on Mg^(2+), had a lower affinity forCa^(2+) and a higher capacity for Ca^(2+)transport. The two systems probablyplay different roles in the second mes-senger system of plant cells.