Compromised energetics in the adenylate kinase AK1 gene knockout heart under metabolic stress

Rapid exchange of high energy carrying molecules between intracellular comp artments is essential in sustaining cellular energetic homeostasis, Adenyla te kinase (AK)-catalyzed transfer of adenine nucleotide beta- and gamma -ph osphoryls has been implicated in intracellular energy communication and nuc leotide metabolism. To demonstrate the significance of this reaction in car diac energetics, phosphotransfer dynamics were determined by [O-18]phosphor yl oxygen analysis using P-31 NMR and mass spectrometry. In hearts with a n ull mutation of the AK1 gene, which encodes the major AK isoform, total AX activity and beta -phosphoryl transfer was reduced by 94% and 36%, respecti vely. This was associated with up-regulation of phosphoryl flux through rem aining minor AK isoforms and the glycolytic phosphotransfer enzyme, S-phosp hoglycerate kinase, In the absence of metabolic stress, deletion of AK1 did not translate into gross abnormalities in nucleotide levels, gamma -ATP tu rnover rate or creatine kinase-catalyzed phosphotransfer. However, under hy poxia AK1-deficient hearts, compared with the wild type, had a blunted AK-c atalyzed phosphotransfer response, lowered intracellular ATP levels, increa sed P-i/ATP ratio, and suppressed generation of adenosine, Thus, although l ack of AK1 phosphotransfer can be compensated in the absence of metabolic c hallenge, under hypoxia AK1-knockout hearts display compromised energetics and impaired cardioprotective signaling, This study, therefore, provides fi rst direct evidence that AK1 is essential in maintaining myocardial energet ic homeostasis, in particular under metabolic stress.