title: Adenosine metabolism in the guinea pig heart: the role of cytosolic S-adenosyl-L-homocysteine hydrolase, 5'-nucleotidase and adenosine kinase.
abstract: This study was conducted to elucidate the role of S-adenosyl-L-homocysteine (SAH) hydrolase, 5'-nucleotidase and adenosine kinase in the production and removal of adenosine in the isolated guinea pig heart during normoxic (95% O2) and hypoxic (30% O2) perfusion. Using an adenosine kinase inhibitor (5'-amino-5'-deoxy-adenosine; 50 microM) and an adenosine deaminase inhibitor (EHNA; 5 microM) the total steady-state production rate of adenosine in the heart was estimated to be greater than 1.2 nmol.min-1 per g wet wt., during normoxia. Most (95%) of the SAH-derived adenosine is salvaged by adenosine kinase action. The rate of adenosine phosphorylation increased 3-fold when isolated hearts were perfused with hypoxic medium, suggesting that adenosine kinase is not substrate-saturated under normoxic conditions. The steady-state production of adenosine was also estimated during hypoxia (5.9 nmol-min-1 per g wet wt.) and compared with previously determined transmethylation rate during hypoxia (1.12 nmol.min-1 x g wet wt.). In an attempt to assess the in-vivo activity of cytosolic 5'-nucleotidase, the 5'-AMP pool was labelled by perfusing the isolated hearts with tricyclic nucleoside (TCN) which became phosphorylated (TCN-P). The release rate of both adenosine and TCN in the post-labelling phase was increased by hypoxic perfusion, suggesting that the increased rate of 5'-AMP hydrolysis may be due to increased availability of substrate, as well as activation of 5'-nucleotidase. Our findings suggest that during normoxic perfusion a significant amount of adenosine is derived from an apparently oxygen-independent mechanism (cellular transmethylation) whereas during hypoxic perfusion hydrolysis of adenine nucleotides to adenosine prevails.
authors.affiliation: Physiologisches Institut, Heinrich Heine Universität, Düsseldorf, Germany.
authors.forename: H G
authors.initials: HG
authors.lastname: Lloyd
authors.forename: J
authors.initials: J
authors.lastname: Schrader
mesh_terms: D015720:5'-Nucleotidase / Q000378:metabolism*; D000241:Adenosine / Q000378:metabolism*; D000248:Adenosine Kinase / Q000378:metabolism*; D043383:Adenosylhomocysteinase; D000818:Animals; D003326:Coronary Circulation / Q000502:physiology; D004734:Energy Metabolism; D006168:Guinea Pigs; D006867:Hydrolases / Q000378:metabolism*; D006868:Hydrolysis; D000860:Hypoxia / Q000201:enzymology / Q000378:metabolism; D066298:In Vitro Techniques; D009206:Myocardium / Q000201:enzymology / Q000378:metabolism*; D012435:S-Adenosylhomocysteine / Q000378:metabolism
chemical_list: D012435:S-Adenosylhomocysteine; D000248:Adenosine Kinase; D006867:Hydrolases; D015720:5'-Nucleotidase; D043383:Adenosylhomocysteinase; D000241:Adenosine