Cardiac expression and location of hexokinase changes in a mouse model of pure creatine-deficiency.
Branovets J., Karro N., Barsunova K., Laasmaa M., Lygate CA., Vendelin M., Birkedal R.
Creatine kinase (CK) is considered the main phosphotransfer system in the heart, important for overcoming diffusion restrictions and regulating mitochondrial respiration. It is substrate limited in creatine-deficient mice lacking L-arginine:glycine amidinotransferase (AGAT) or guanidinoacetate methyltranferase (GAMT). Our aim was to determine the expression, activity and mitochondrial coupling of hexokinase (HK) and adenylate kinase (AK), as these represent alternative energy transfer systems. In permeabilized cardiomyocytes, we assessed how much endogenous ADP generated by HK, AK or CK stimulated mitochondrial respiration and how much was channeled to mitochondria. In whole heart homogenates, and cytosolic and mitochondrial fractions, we measured the activities of AK, CK and HK. Lastly, we assessed the expression of the major HK, AK and CK isoforms. Overall, respiration stimulated by HK, AK and CK was ~25, 90 and 80%, respectively, of the maximal respiration rate, and ~20, 0 and 25%, respectively, was channeled to the mitochondria. The activity, distribution and expression of HK, AK and CK did not change in GAMT KO mice. In AGAT KO mice, we found no changes in AK, but we found a higher HK activity in the mitochondrial fraction, greater expression of HK I, but a lower stimulation of respiration by HK. Our findings suggest that mouse hearts depend less on phosphotransfer systems to facilitate ADP flux across the mitochondrial membrane. In AGAT KO mice, which are a model of pure creatine-deficiency, the changes in HK may reflect changes in metabolism as well as influence mitochondrial regulation and reactive oxygen species production.