Structural and replicative forms of mitochondrial DNA from human leukocytes in relation to age

The structure and replication of human leukocyte mitochondrial DNA (mtDNA) was investigated in healthy young adult males (23--37 years old), middle-aged males (42--52 years old) with secondary polycythemia, and elderly males (80--89 years old) who exhibited different degrees of age-related disease syndromes. The distribution of the various cell types within the white cell population was within normal limits in all samples. Total mtDNA was isolated in ethidium bromide--CsCl gradients and examined by electron microscopy after spreading by the aqueous and formamide techniques. The individual frequencies of catenated forms ranged from 2 to 6% but showed relatively little change (declining slightly) with age. The individual frequencies of circular dimers varied from 0 to 0.1% in the young adult and polycythemic groups and in 10 out of 12 elderly individuals. One elderly individual had a circular dimer frequency of 0.3% (including a circular molecular of tetramer size) and another had 4.5%. This finding suggest that agerelated cellular pathology may exist in the blood-forming system in some cases. The mode of replication of leukocyte mtDNA agrees well with that described for mouse L cells. There was no evidence of aberrant mtDNA replication as a result of aging.