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Ectonucleotidase Activities Associated with Cholinergic Synaptosomes Isolated from Torpedo Electric Organ
Author(s) -
Grondal E. J. M.,
Zimmermann H.
Publication year - 1986
Publication title -
journal of neurochemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.75
H-Index - 229
eISSN - 1471-4159
pISSN - 0022-3042
DOI - 10.1111/j.1471-4159.1986.tb00692.x
Subject(s) - nucleoside , nucleoside triphosphate , biochemistry , atpase , gtp' , ouabain , chemistry , enzyme , atp hydrolysis , adenosine triphosphate , nucleotide , sodium , organic chemistry , gene
Intact synaptosomes isolated from the electric organ of the electric ray Torpedo marmorata contain, at their surface, enzyme activities for the hydrolysis of externally applied nucleoside phosphates. The diazonium salt of sulfanilic acid, as a low‐molecular‐weight, slowly permeating, covalent inhibitory agent, selectively blocks these enzyme activities and leaves intracellular lactate dehydrogenase intact. The ectoenzymes comprise both a nucleoside triphosphate and diphosphate phosphohydrolase, as well as a 5′‐nucleotidase. Activity of nonspecific ectophosphatases is absent. The nucleoside triphosphatase hydrolyzes almost equally well ATP, GTP, CTP, UTP, and ITP and is activated to a similar degree by Mg 2+ or Ca 2+ . It has a high affinity for ATP ( K m for ATP in the presence of Mg 2+ , 75 μ M ; in the presence of Ca 2+ , 66 μ M ). Maximal rates in the presence of Mg 2+ and Ca 2+ were very similar (34.8 and 32.5 nmol of P/min/mg of synaptosomal protein, respectively). Either Mg‐ATP or Ca‐ATP can act as a true substrate. ADP inhibits hydrolysis of ATP, but AMP is without effect. The nucleoside triphosphatase is not inhibited significantly by a number of inhibitors of mitochondrial Mg 2+ ‐ATPase or of Ca 2+ + Mg 2+ ‐ATPases. It is, however, considerably inhibited by filipin and quercitin. The capacity of intact synaptosomes to hydrolyze also extracellular ADP, GDP, AMP, GMP, and IMP suggests that the nucleoside triphosphatase is part of an enzyme chain that causes complete hydrolysis of the respective nucleoside triphosphate to the nucleoside. We conclude that the cholinergic nerve terminals of the Torpedo electric organ can hydrolyze ATP released on coexocytosis with acetylcholine via an ectonucleoside triphosphatase activity that is different from known endogenous nerve terminal ATPases. The final product of the hydrolysis, adenosine, can then be salvaged by the nerve terminal for resynthesis of ATP. Other possible physiological functions of the ectonucleotidases are discussed.