Genomic DNA fragmentation in red blood cells of the bullfrog during metamorphosis

Red blood cells (RBC) of the bullfrog ( Rana catesbeiana ) contain larval-type hemoglobin (Hb) during the larval period. At the beginning of metamorphosis, RBC containing adult-type Hb appear and two types of RBC coexist in the systemic circulation. During the metamorphic climax, RBC with larval-type Hb disappear from the circulation and, simultaneously, RBC containing adult-type Hb begin to circulate. These two types of RBC were separated by Percoll density gradient centrifugation to examine the molecular size of the genomic DNA of each population. DNA fragmentation was detected only in new RBC with adult-type Hb that appeared in the systemic circulation and remained throughout post-metamorphic life. Semiquantification of DNA on agarose gel showed that the degree of DNA fragmentation was highest at the metamorphic climax. As the existence of DNA fragments suggested endonucleolytic cleavage, nuclease activity was examined by an activity gel system and in vitro circular plasmid DNA digestion assays. The latter revealed that both types of RBC possess endonucleolytic activity throughout the pre- and post-metamorphic periods. Assays of endogenous endonucleolytic activities under different divalent ionic conditions suggested that mobilization of intracellular Ca²⁺-Mg²⁺ induces genomic DNA fragmentation in adult-type RBC.