Enzymes of Adenylate Metabolism and Their Role in Hibernation of the White-Tailed Prairie Dog, Cynomys leucurus

AMP deaminase (AMPD) and adenylate kinase (AK) were purified from skeletal muscle of the white-tailed prairie dog, Cynomus leucurus, and enzyme properties were assayed at temperatures characteristic of euthermia (37°C) and hibernation (5°C) to analyze their role in adenylate metabolism during hibernation. Total adenylates decreased in muscle of torpid individuals from 6.97 ± 0.31 to 4.66 ± 0.58 μmol/g of wet weight due to a significant drop in ATP but ADP, AMP, IMP, and energy charge were unchanged. The affinity of prairie dog AMPD for AMP was not affected by temperature and did not differ from that of rabbit muscle AMPD, used for comparison. However, both prairie dog and rabbit AMPD showed much stronger inhibition by ions and GTP at 5°C, versus 37°C, and inhibition by inorganic phosphate, NH4Cl, and (NH4)2SO4 was much stronger at 5°C for the prairie dog enzyme. Furthermore, ATP and ADP, which activated AMPD at 37°C, were strong inhibitors of prairie dog AMPD at 5°C, with I50 values of 1 and 14 μM, respectively. ATP also inhibited rabbit AMPD at 5°C (I50 = 103 μM). Strong inhibition of AMPD at 5°C by several effectors suggests that enzyme function is specifically suppressed in muscle of hibernating animals. By contrast, AK showed properties that would maintain or even enhance its function at low temperature. Km values for substrates (ATP, ADP, AMP) decreased with decreasing temperature, the change in Km ATP paralleling the decrease in muscle ATP concentration. AK inhibition by ions was also reduced at 5°C. The data suggest that adenylate degradation via AMPD is blocked during hibernation but that AK maintains its function in stabilizing energy charge.