Guanine nucleotide transport by atractyloside-sensitive and -insensitive carriers in isolated heart mitochondria

In previous work (McKee EE, Bentley AT, Smith RM Jr, and Ciaccio CE, Bioche m Biophys Res Commun 257: 466- 472, 1999), the transport of guanine nucleot ides into the matrix of intact isolated heart mitochondria was demonstrated . In this study, the time course and mechanisms of guanine nucleotide trans port are characterized. Two distinct mechanisms of transport were found to be capable of moving guanine nucleotides across the inner membrane. The fir st carrier was saturable, displayed temperature dependence, preferred GDP t o GTP, and did not transport GMP or IMP. When incubated in the absence of e xogenous ATP, this carrier had a V-max of 946 +/- 53 pmol.mg(-1).min(-1) wi th a K-m of 2.9 +/- 0.3 mM for GDP. However, transport of GTP and GDP on th is carrier was completely inhibited by physiological concentrations of ATP, suggesting that this carrier was not involved with guanine nucleotide tran sport in vivo. Because transport on this carrier was also inhibited by atra ctyloside, this carrier was consistent with the well-characterized ATP/ADP translocase. The second mechanism of guanine nucleotide uptake was insensit ive to atractyloside, displayed temperature dependence, and was capable of transporting GMP, GDP, and GTP at approximately equal rates but did not tra nsport IMP, guanine, or guanosine. GTP transport via this mechanism was slo w, with a V-max of 48.7 +/- 1.4 pmol.mg(-1).min(-1) and a K-m = 4.4 +/- 0.4 mM. However, because the requirement for guanine nucleotide transport is l ow in nondividing tissues such as the heart, this transport process is neve rtheless sufficient to account for the matrix uptake of guanine nucleotides and may represent the physiological mechanism of transport.