Transport of Adenine Nucleotides to and from Mitochondria

Although the mechanism of ATP synthesis is not fully understood, there is no doubt that the ATP is synthesized in the matrix of the mitochondrion. Mitochondrial ATP is then exported to the cytoplasm. A specific carrier (translocase Mr = 40,000) is involved in the simultaneous transport of 

ATP out of and ADP into the mitochondrion. Translocation is inhibited by two well-known toxins, atractyloside and bongkrekic acid, the former a glucoside found in the Mediterranean thistle and the latter produced by a Pseudomonas bacterium. 

The translocator moves the two nucleotides in either direction. However, ATT is transported as ATP4- and ADP as ADP3-. Thus the equilibrium position of the exchange is dependent on the electrochemical potential difference across the membrane. When the electrical potential difference is 160 mV, the ratio of ATP/ADP in the medium outside the mitochondrion is 125/1. 

A suspension of mitochondria was incubated under anaerobic conditions in the presence of NADH. Oxidative phosphorylation was initiated by a pulse of oxygen, and it was found that the ATP concentration in the suspension increased by 1.5 X 10 4mol L-1, with a corresponding decrease in the absorbance of the suspension of 0.30 at 340 nm, when using a 1-cm path length in a spectrophotometer. Given that the absorbance coefficient of NADH at 340 nm is 6.2 x 103 L mol-1 cm-1, calculate the number of ATP molecules produced per molecule of NADH oxidized. 

Comparison of this value with the change in [ATP] (1.5 x 10 4 mol L 1 -5- 4.84 x 10-5 mol L-1) gives a value of 3.1 ATP produced per NADH oxidized this is close to the theoretically predicted value of 3.0. 

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