Kinetic Mechanism of the Exchanges Catalysed by the Adenine‐Nucleotide Carrier

Abstract

Initial rates of the exchange ADP_in/ADP_out 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.