Is the first site of phosphorylation operative in rat brain mitochondria in early neonatal life? A critical re-evaluation.

The age-dependent changes in oxidative phosphorylation in rat brain mitochondria were studied in order to ascertain if the efficiency of phosphorylation with NAD(+)-linked substrates increases during the first month after birth. The state 3 respiration rates with all substrates tested increased with age but with distinctive developmental profiles for each substrate; the extent of increase was also substrate-specific. The ADP/O ratios obtained with all substrates, including NAD(+)-linked ones were comparable with the value for adult mitochondria right from the first postnatal week. The intramitochondrial cytochrome contents also increased with age; the different classes of cytochromes exhibiting different developmental patterns. Simultaneously there was an elevation in the serum concentrations of thyroid hormones up to the third postnatal week following which they declined. The increase in respiration rates was paralleled by decrease in the ratio of T4/T3 in the serum, thus suggesting that this ratio is a sensitive index for developmental profiles of substrate oxidation.