Kinetics of the inactivation of glucoso-6-phosphate-dehydrogenase in solutions by low-frequency ultrasound

A comparative study of the kinetics of the inactivation of glucoso-6-phosphate-dehydrogenase (G6PDH) in solutions (pH 6-9.1, 36-50°C) by low-frequency ultrasound (20.8 kHz, 62 W/cm 2 ) and the thermal inactivation of the enzyme under the same conditions was performed. The inactivation of G6PDH was characterized by the effective first-order rate constants (min - 1 ): k i n , k* i n , and t i n (us) for the total, thermal, and ultrasound (US) inactivation, respectively. It was found that all these rate constants decrease drastically as the enzyme concentration decreases from 20 to 3 nM in an 0.1 M phosphate buffer (pH 7.4). The value of k i n increases as the US specific power increases from 53 to 65 W/cm 2 . In all cases, k i n > k* i n ; the constants increase with an increase in pH from 6 to 9.1. It was shown that all of the three rate constants of inactivation of G6PDH increase with temperature. The temperature dependence of k i n and k* i n shows a point of inflection at 44°C in the Arrhenius coordinates. This feature was attributed to the existence of two phases of inactivation. It was found that the activation energy E a c t for the inactivation of the enzyme by ultrasound is considerably lower than that for thermal inactivation of G6PDH at various initial concentrations of the latter (3-20 nM). The results obtained are consistent with the associative-dissociative scheme of the inactivation of suhunit G6PDH.