Levels of the mitochondrial endonuclease during rat cardiac development implicate a role for the enzyme in repair of oxidative damage in mitochondrial DNA.

Mammalian heart mitochondria (MT) contain a potent Mg(2+)-dependent DNA endonuclease that becomes soluble once isolated mitochondria are disrupted using detergent. The level of this endonuclease was previously found to be markedly elevated in adult rat heart compared to other adult rat tissues. Among tissues, the level of the MT endonuclease does not appear to be correlated with the rate of MT DNA replication but rather with the rate of oxidative metabolism [Houmiel, K.L., Gerschenson, M. and Low, R.L., 1991. Biochimica Biophysica Acta 1079: 197-202]. In the present study, the level of the endonuclease has been quantitated both during rat cardiac development, from gestational day 18 through adulthood, and in cultured rat heart myoblasts. Surprisingly, the specific activity of the MT endonuclease in fetal and newborn mitochondria is high. The values are greater than 50% of that seen in the adult even though the mitochondria at this period of heart development are few and structurally disorganized. Remarkably, there is a burst of endonuclease activity at day 2 which accompanies a similar, transient elevation of respiratory complex I and IV activities. At later times, the endonuclease activity gradually increases until adulthood and correlates with steady increases in MT DNA and DNA polymerase-gamma. In cultured myoblasts, the level of the endonuclease increases about seven-fold as the growing cells reach confluency and differentiate into myotubes. These variations in the specific activity of the endonuclease, when considered along with other properties of the enzyme suggest that the endonuclease may serve a role in the removal of oxidative damage in MT DNA incurred from respiration.