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 proper ties were assayed at temperatures characteristic of euthermia (37 degrees C ) and hibernation (5 degrees C) to analyze their role in adenylate metaboli sm during hibernation. Total adenylates decreased in muscle of torpid indiv iduals from 6.97 +/- 0.31 to 4.66 +/- 0.58 mu mol/g of wet weight due to a significant drop in ATP but ADP, AMP, ZMP, 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. Ho wever, both prairie dog and rabbit AMPD showed much stronger inhibition by ions and GTP at 5 degrees C, versus 37 degrees C, and inhibition by inorgan ic phosphate, NH4Cl, and (NH4)(2)SO4 was much stronger at 5 degrees C for t he prairie dog enzyme. Furthermore, ATP and ADP, which activated AMPD at 37 degrees C, were strong inhibitors of prairie dog AMPD at 5 degrees C, with I-50 values of 1 and 14 mu M, respectively. ATP also inhibited rabbit AMPD at 5 degrees C (I-50 = 103 mu M). Strong inhibition of AMPD at 5 degrees C by several effecters suggests that enzyme function is specifically suppres sed in muscle of hibernating animals. By contrast, AK showed properties tha t would maintain or even enhance its function at low temperature. K-m value s for substrates (ATP, ADP, AMP) decreased with decreasing temperature, the change in K-m ATP paralleling the decrease in muscle ATP concentration. Al l inhibition by ions was also reduced at 5 degrees C. The data suggest that adenylate degradation via AMPD is blocked during hibernation but that AK m aintains its function in stabilizing energy charge. (C) 2000 Academic Press .