RESUMO
The effect of intact diphtheria toxin and of its fragment A on protein synthesis in mouse liver mitoplasts (digitonin-treated mitochondria) was studied. Fragment A inhibited protein synthesis in intact mitoplasts to the same extent as the uncoupler, carbonylcyanide p-trifluoromethoxyphenylhydrazone, but similar effects were not observed in lyzed mitoplasts. Intact diphtheria toxin was without effect in either case. Fragment A strongly stimulated mitochondrial ATPase activity. At concentrations which efficiently inhibited mitochondrial protein synthesis and stimulated ATPase activity, fragment A had no effect on the intramitochondrial concentration of nicotinamide adenine dinucleotides. Moreover, it did not catalyze ADP ribosylation of mitochondrial proteins. The results indicate that the effects observed did not involve the NAD+-glycohydrolase activity of fragment A. [125I]-Labelled fragment A was bound to mitoplasts to about the same extent as the labelled intact diphtheria toxin. The present results suggest that fragment A of diphtheria toxin is capable of inhibiting the energy coupling in mitoplasts, thereby inhibiting protein synthesis. The detailed mechanism of the uncoupling and its possible physiological significance remains to be elucidated.