Occurrence of DNase gamma-like apoptotic endonucleases in hematopoietic cells in Xenopus laevis and their relation to metamorphosis.

DNase gamma has been suggested to be an endonuclease responsible for thymic apoptosis in mammals. Using the frog, Xenopus laevis, we examined whether any hematopoietic cells other than thymocytes have DNase gamma activity. The activity gel assays and HeLa cell nuclear assays for DNase revealed that nuclei from red blood cells and cells in liver (an erythropoietic organ) contain DNase gamma-like activities (molecular mass of 38 and 36 kDa) indicated by their biochemical features (internucleosomal DNA cleavage, requirement of Ca2+ and Mg2+, and sensitivity to Zn2+). Distribution of these two (38 and 36 kDa) forms of enzyme was determined by cell fractionation analysis of liver cells: the 38 kDa form of enzyme was found in erythrocytes/erythroblasts and hepatocytes, whereas the 36 kDa form was in lymphocytes/macrophages. Interestingly, these two enzyme activities increased during metamorphic climax, at which time larval-type cells are converted to adult ones. It thus appears that these gamma-type DNases are involved in programmed cell death during metamorphosis.