INTRAMITOCHONDRIAL PROTEIN-SYNTHESIS IS REGULATED BY MATRIX ADENINE-NUCLEOTIDE CONTENT AND REQUIRES CALCIUM

The hypothesis that fluctuations in matrix adenine nucleotide content (ATP + ADP + AMP) may regulate intramitochondrial protein synthesis wa s investigated in newborn and adult rat liver mitochondria. Protein sy nthesis in mitochondria from 0-h-old newborns, which contain 3.4 +/- 0 .3 nmol adenine nucleotide/mg protein, was >90% lower than protein syn thesis in mitochondria from 4-h-old newborns, which contain 9.1 +/- 0. 2 nmol adenine nucleotide/mg protein. If 0-h newborn mitochondria were preincubated to accumulate adenine nucleotides to 16.8 nmol/mg protei n in vitro, the protein synthesis rate increased 25-fold compared to c ontrol. Adult rat liver mitochondria normally contain 12-14 nmol adeni ne nucleotide/mg protein and exhibit a brisk rate of protein synthesis . Following a preincubation to deplete adenine nucleotides in vitro do wn to 3 nmol/mg protein, protein synthesis in adult liver mitochondria was nearly abolished. Conversely, when adult mitochondria were preinc ubated to super-load adenine nucleotides (to 29 nmol/mg protein), the rate of protein synthesis was doubled. Protein synthesis was also inhi bited when the matrix ATP/ADP ratio was lowered by adding FCCP or by o mitting phosphate. In adult mitochondria, protein synthesis was inhibi ted by 0.5 mM EGTA and was increased in proportion to buffered free ca lcium between 0 and 20 mu M. The rate of intramitochondrial RNA synthe sis was not inhibited by EGTA nor affected by variations in matrix ade nine nucleotide content. The results show that intramitochondrial tran slation requires matrix calcium and is regulated by changes in the mat rix adenine nucleotide content that affect the matrix ATP concentratio n. The matrix adenine nucleotide content is controlled by the ATP-Mg/P -i carrier. In newborns, the matrix adenine nucleotide content increas es 3-fold within 2-4 h after birth, stimulating mitochondrial translat ion 10-fold and probably contributing to the onset of postnatal mitoch ondrial biogenesis and adaptation to aerobic metabolism. (C) 1995 Acad emic Press, Inc.