title: Kinetic mechanism of the exchanges catalysed by the adenine-nucleotide carrier.
abstract: Initial rates of the exchange ADPin/ADPout catalysed by the adenine-nucleotide carrier of rat-heart mitochondria have been studied under conditions where internal and external ADP may be varied. The initial rate was measured within 1 s by the carboxyatractyloside-stop method, using a rapid-mixing technique. The double-reciprocal plots v0(-1) versus [ADP]out-1 at different internal-ADP concentrations and v0(-1) versus [ADP]in-1 at different external-ADP concentrations exhibit straight-line relationships having a common point of intersection on the axis of ordinates. These results demonstrate the essential role of a ternary complex and thus exclude the ping-pong mechanism generally accepted. The kinetic equation implies a strong positive cooperativity in the binding of the two substrates. Two models are proposed: (a) the ternary complex performs the exchange and the transport of the substrates in a single step; (b) the carrier is mobile and transports the substrates one by one, the formation of a ternary complex being needed to release the first product.
mesh_terms: D000244:Adenosine Diphosphate / Q000378:metabolism; D000818:Animals; D066298:In Vitro Techniques; D007700:Kinetics; D008297:Male; D008929:Mitochondria, Heart / Q000201:enzymology; D000226:Mitochondrial ADP, ATP Translocases / Q000378:metabolism*; D008954:Models, Biological; D008956:Models, Chemical; D009713:Nucleotidyltransferases / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains
chemical_list: D000244:Adenosine Diphosphate; D000226:Mitochondrial ADP, ATP Translocases; D009713:Nucleotidyltransferases