Adenine nucleotide pool size, adenine nucleotide translocase activity, and respiratory activity in newborn rabbit liver mitochondria.

The adenine nucleotide content (ATP+ADP+AMP) of newborn rabbit liver mitochondria was 6.0 +/- 0.5 nmol/mg mitochondrial protein at birth, increased rapidly to 14.5 +/- 1.7 nmol/mg protein by 2 h postnatal, peaked at 6 h, then decreased gradually to 7.8 +/- 0.6 nmol/mg protein by 4 days postnatal. There was a strong positive correlation (r = 0.82) between the total adenine nucleotide pool size and adenine nucleotide translocase activity in these mitochondria. In contrast, glutamate + malate-supported State 3 respiratory rates remained constant from birth through the first week of life. State 4 rates also remained constant, as did the respiratory control index and uncoupled respiratory rates. The following conclusions are suggested: (1) The maximum rate of translocase activity is limited by the intramitochondrial adenine nucleotide pool size. (2) In newborn rabbit liver mitochondria, the State 3 respiratory rate is not limited by either the adenine pool size or the maximum capacity for translocase-mediated adenine exchange. (3) In contrast to rat, rabbit liver mitochondria are fully functional at birth with regard to respiratory rates and oxidative phosphorylation. (4) The rapid postnatal accumulation fo adenine nucleotides by liver mitochondria, now documented in two species, may be a general characteristic of normal metabolic adjustment in neonatal mammals.