Keller F, Zimmermann H
Life Sci. 1983 Dec 26;33(26):2635-41. doi: 10.1016/0024-3205(83)90347-8.
Synaptosomes isolated from the electric organ of Torpedo marmorata contain activity of an ATPase which is located at the extracellular face of the plasma membrane. Ecto-ATPase activity can be stimulated independently and to a similar extent by either Ca-2+ or Mg-2+. Apparent Km-values for ATP are 79 microM and 53 microM for Ca-2+ and Mg-2+ respectively. Apparent Km-values for Ca-2+ and Mg-2+ at 1 mM ATP are 0.71 mM and 0.61 mM respectively. The enzyme is also activated by Mn-2+ and GTP can replace ATP as a substrate. Presence of 5'- nucleotidase activity suggests that adenosine is the final hydrolysis product. Thus hydrolysis of nucleotides released during exocytosis of synaptic vesicle contents and purine salvage must be a major role of this ecto-enzyme. We furthermore suggest that the ecto-ATPase may provide the key to understanding the storage of the high energy compound ATP in cholinergic synaptic vesicles. On depolarization of the nerve terminal and exocytosis, ATP represents the signal for activating the ATPase whereby concentrations of Ca-2+ and Mg-2+ are already saturating. Following depolarization induced Ca-2+ influx, a possible function of the ATPase may be the outward transport of Ca-2+ from the nerve terminal.
从电鳐的电器官分离出的突触体含有一种ATP酶活性，该酶位于质膜的细胞外表面。胞外ATP酶活性可被Ca²⁺或Mg²⁺独立且相似程度地刺激。Ca²⁺和Mg²⁺对ATP的表观Km值分别为79微摩尔和53微摩尔。在1毫摩尔ATP存在下，Ca²⁺和Mg²⁺的表观Km值分别为0.71毫摩尔和0.61毫摩尔。该酶也被Mn²⁺激活，并且GTP可作为底物替代ATP。5'-核苷酸酶活性的存在表明腺苷是最终水解产物。因此，突触小泡内容物胞吐过程中释放的核苷酸的水解以及嘌呤补救必定是这种胞外酶的主要作用。我们还认为，胞外ATP酶可能为理解胆碱能突触小泡中高能化合物ATP的储存提供关键线索。在神经末梢去极化和胞吐时，ATP代表激活ATP酶的信号，此时Ca²⁺和Mg²⁺的浓度已经饱和。在去极化诱导的Ca²⁺内流之后，ATP酶的一个可能功能可能是将Ca²⁺从神经末梢向外转运。
