Dependence of Substrate Irradiation Reaction Rate Stimulation on Lactic Dehydrogenase Source

Stimulation of LDH initial reaction rates by timed pre-irradiation of crystalline sodium pyruvate and lithium lactate is reported for enzymes isolated from rabbit muscle, pig heart, human erythrocytes and chicken heart. The phenomenon investigated is referred to as the Comorosan effect. For the mammalian source enzymes, the pyruvate irradiation stimulations occurred at irradiation times of 5 and 35 sec. and the lactate irradiation times at 15 and 45 seconds. In contrast, for the chicken heart enzyme, the pyruvate irradiation stimulations occurred at 15 and 35 sec., while those for lactate occurred at 5 and 20 sec. Thus, a shift in stimulatory irradiation times is found on going from the mammalian enzymes to the avian enzyme. A similar shift between mammalian and yeast enzymes has been established by Comorosan and co-workers. For the chicken heart LDH, the separation between successive irradiation times is different for the forward and reverse reactions. This is the first reported incidence of the separation not being the same. Introduction Comorosan and co-workers (Comorosan et al.1970a,b,c; 1971a,b) have reported experimental studies in which the rate of in vitro enzymic reactions could be enhanced by irradiating the enzyme substrate in the crystalline state prior to dissolution and incorporation in the reaction mixture. The reaction rate increases occurred only for certain timed exposures of the pure crystalline materials. Initially, an x-ray source was employed. This was soon replaced by a high pressure mercury lamp. With increasing precision in their kinetic assays, it became apparent that the special irradiation times, designated t*, that produced increases for a given enzyme – substrate reaction rate could be represented by the manifold t* = tm + nτ where tm is the shortest effective exposure time, τ is a constant and n is an integer (0, 1, 2, ...). All t* have been found to be a multiple of 5 sec. Results from over 50 enzyme – substrate reactions involving enzymes isolated from 6 different species have been reported (Comorosan et al., 1980). Of particular interest here are the relative values of t* found for forward and reverse reactions catalyzed by a given enzyme. For example, the enzyme lactic dehydrogenase will catalyze the conversion of pyruvate to lactate as well as the conversion of lactate to pyruvate. In all of the studies reported to date, t* values which stimulate the reaction in one direction have no effect on the reaction in the reverse direction. The tm values for the forward and reverse reactions have always