The ATP-generating activity of mitochondria and mitochondrial creatine kinase was studied as a function of the isotopy of the magnesium pool of the incubation medium. The objects of investigation were in vitro systems containing preparations of isolated mitochondria and the enzyme from the myocardium of rats given a single injection of 1-methylnicotinamide to a dose of 1/2DL₅₀. It was shown that the presence of paramagnetic cations of the ²⁵Mg isotope leads to significant compensation for the intramitochondrial ATP deficiency caused by suppression of oxidative phosphorylation by 1-methylnicotinamide. This effect is virtually unachievable in systems whose magnesium pool comprises isotopes with zero nuclear spin (²⁴Mg and ²⁶Mg). The restoration of the mitochondrial ATP synthesis under induced inhibition of NAD/NADP-de-pendent reactions by 1-methylnicotinamide involves creatine kinase, whose activity is not suppressed by 1-methylnicotinamide. The high efficiency of restoration of this process is a spin-selective phenomenon and is reached mainly in the presence of ²⁵Mg²⁺ cations. The significance of the obtained data for further investigation of the mechanisms of regulation of enzymatic catalysis was discussed.

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