Regulation of enzymes of energy metabolism – AMP deaminase and creatine kinase – in an anoxia tolerant turtle.
The red-eared slider turtle (Trachemys scripta elegans) is one of the few vertebrate species that can survive long term oxygen deprivation. Maintenance of viable cellular energetics is key to anoxia survival and, in response to low oxygen, most anoxia tolerant animals show a drop in total adenylate levels while energy charge remains stable. To better understand how turtles regulate their energy metabolism when deprived of oxygen, the present studies focused on the control of two important enzymes in muscle energy metabolism: AMP deaminase (AMPD) and creatine kinase (CK). AMPD activity increased under anoxia in turtle skeletal muscle and the effects of ATP∙Mg and ions indicated that allosteric controls are part of the mechanism of AMPD regulation. In vitro incubations to stimulate the actions of endogenous protein kinases and phosphatases showed that AMPD is a phosphoenzyme and suggested that reversible phosphorylation has a central role in AMPD regulation under aerobic versus anoxic conditions. CK from turtle heart is also a phosphoprotein and anoxia-induced metabolic rate depression was accompanied by a strong increase in the fraction of dephosphorylated CK that showed increased affinity for creatine. Incubation studies implicated selected protein kinases (PKA, PKG, and AMPK) and phosphatases (PP1) as responsible for heart CK regulation. However, anoxia-responsive changes in kinetic properties of skeletal muscle CK did not appear to be caused by a change in phosphorylation state. Regulation of muscle CK under anoxia may be linked with changes in the binding of CK with myofibrils and the effects of binding on enzyme properties.