To gain some insight into the mechanism of cell photostimulation by laser light, measurements were made of the rate of ADP/ATP exchange in mitochondria irradiated with the low power continuous wave Helium Neon laser (energy dose 5 Joules/cm2). To do this a method has been developed to continuously monitor ATP efflux from phosphorylating mitochondria caused by externally added ADP, by photometrically following the NADP+reduction which occurs in the presence of glucose, hexokinase, glucose-6-phosphate dehydrogenase and effluxed ATP. The NADP+reduction rate shows hyperbolic dependence on ADP concentration (Km and Vmax values 8.5 +/- 0.87 μM and 20.7 +/- 0.49 nmoles NADP+reduced/min × mg mitochondrial protein, respectively), and proves to measure the activity of the ADP/ATP translocator as shown by Inhibition experiments using atractyloside, powerful inhibitor of this carrier. Irradiation was found to enhance the rate of ADP/ATP antiport, with externally added ADP ranging between 5 and 100 μM. As a result of experiments carried out with mitochondria loaded with either ATP or ADP, the increase in the activity of the ADP/ATP translocator is here proposed to depend on the increase in the electrochemical proton gradient which occurs owing to irradiation of mitochondria. © 1988 Academic Press, Inc.
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