Compromised Energetics in the Adenylate Kinase AK1Gene Knockout Heart under Metabolic Stress*

Rapid exchange of high energy carrying molecules between intracellular compartments is essential in sustaining cellular energetic homeostasis. Adenylate kinase (AK)-catalyzed transfer of adenine nucleotide β- and γ-phosphoryls has been implicated in intracellular energy communication and nucleotide metabolism. To demonstrate the significance of this reaction in cardiac energetics, phosphotransfer dynamics were determined by [18O]phosphoryl oxygen analysis using31P NMR and mass spectrometry. In hearts with a null mutation of the AK1 gene, which encodes the major AK isoform, total AK activity and β-phosphoryl transfer was reduced by 94% and 36%, respectively. This was associated with up-regulation of phosphoryl flux through remaining minor AK isoforms and the glycolytic phosphotransfer enzyme, 3-phosphoglycerate kinase. In the absence of metabolic stress, deletion of AK1 did not translate into gross abnormalities in nucleotide levels, γ-ATP turnover rate or creatine kinase-catalyzed phosphotransfer. However, under hypoxia AK1-deficient hearts, compared with the wild type, had a blunted AK-catalyzed phosphotransfer response, lowered intracellular ATP levels, increased Pi/ATP ratio, and suppressed generation of adenosine. Thus, although lack of AK1 phosphotransfer can be compensated in the absence of metabolic challenge, under hypoxia AK1-knockout hearts display compromised energetics and impaired cardioprotective signaling. This study, therefore, provides first direct evidence that AK1 is essential in maintaining myocardial energetic homeostasis, in particular under metabolic stress.