INACTIVATION OF THE NA,K-ATPASE BY RADIATION-INDUCED FREE-RADICALS - EVIDENCE FOR A RADICAL-CHAIN MECHANISM

Free radicals produced by water radiolysis were used to study the inactivation of the enzymatic activity of the Na,K-ATPase. A decrease of the activity to virtually zero with a mono-exponential dependence on the radiation dose was observed. The inactivation process is initiated by hydroxyl radicals. This was shown by the effect of appropriate radical scavengers such as t-butanol, formate and vitamin C. In all cases a significant increase in the characteristic D-37 dose of inactivation was observed. Inactivation was found to show a so-called inverse dose-rate effect, i.e. the sensitivity of the enzyme to radical attack is increased if the dose rate is reduced. The data were found to agree with the relationship 1/D(37)similar to 1/d(1/2), which is known to be a strong indicator of a radical chain mechanism. This means that the inactivation, after initiation by single radicals, is amplified by a subsequent chain mechanism.