KINETICS OF THERMAL INACTIVATION AND MOLECULAR ASYMMETRY OF UREASE FROM DEHUSKED PIGEONPEA (CAJANUS-CAJAN L.) SEEDS

The purified urease from pigeonpea was moderately stable at -10 degree s C. The shelf-life of the enzyme on storage in 0.1 M Tris-acetate buf fer, pH 6.5, at -10 degrees C showed a single exponential decay with a t(1/2) of approx. 30 days. In the presence of additives like 5mM dith iothreitol the t(1/2) increased to 223 days at the same temperature, i n a single exponential decay. The Arrhenius plot of the kinetics of th e pigeonpea urease catalysed urea hydrolysis over the temperature rang e 27 to 77 degrees C, was linear. The Q(10) value was found to be 1.46 . The energy of activation calculated from the Arrhenius equation was found to be 5.1 kcal/mole active site. The thermal denaturation of pig eopea urease at 65 and 70 degrees C was found to obey biphasic kinetic s in which half of the activity is destroyed faster than the remaining half. The time course of thermal inactivation can be described by the following equation, consisting of two first order terms: A(t) = A(fas t).e(-k)fast(-t) + A(slow.e)(-k)slow(.t) where, A(t) is the residual a ctivity at time t, A(fast) and A(slow), k(fast) and k(slow) are the am plitudes and the first-order rate constants of the fast and the slow p hases, respectively. The data suggests the existence of site-site hete rogeneity in oligomeric urease molecule from pigeonpea.