Studies on the Mechanism of Substrate Protection of Enolase and Lactic Dehydrogenase against Ionizing Radiation

Beef heart lactic dehydrogenase (LDH) and rabbit muscle enolase were used to study the protective effect of substrates against ionizing radiation. The substrate of enolase, D-glyceric acid 2-phosphate, protected enolase activity against ionizing radiation, but the protection occurred in the absence of magnesium ions, which are required to bind the substrate molecule to the enzyme molecule. The substrate of enolase also protected LDH. Reduced nicotinamide adenine dinucleotide (NADH) and nicotinamide adenine dinucleotide (NAD) protected LDH and also enolase. In both cases NADH gave better protection. NADH was oxidized to NAD by radiation both in the presence and in the absence of protein molecules. Cytidine-2′,3′-cylic phosphate, a substrate for ribonuclease, gave partial protection to both enolase and LDH. From the results of this study, it is concluded that specific "substrate protection" is not the primary mechanism involved. The data suggest that a "radical scavenger" mechanism would best account for the observed results.