Rice bean acid phosphatase (Vigna umbellata Thunb.): substrate specificity and inactivation studies

Rice bean acid phosphatase exhibited a broad range of substrate specificity. The highest activity was observed with p-nitrophenylphosphate while, substrates like phenylphosphate, phosphoenolpyruvate, adenosine diphosphate, and 1-naphthyl phosphate showed % activities between 100 and 40%. Other phosphate esters displayed little activity (15–40%); no activity was observed with phytic acid. The specificity constant values for phenylphosphate and 1-naphthylphosphate are maximum and equal. Others follow the decreasing sequence: p-nitrophenylphosphate, phosphoenolpyruvate, and adenosine diphosphate, respectively. Thermal inactivation of acid phosphatase carried at 55, 60, and 65 °C with the t1/2 values 45.45, 9.708, and 1.54 min, respectively. The D-values were 151.5, 32.4, and 5.175 min; the Z-value was 6.82 °C. The thermal inactivation carried at 60 °C in the presence of additives brought an increase in the t1/2 values; citric acid (11.278 min), polyethylene glycol (11.278 min), and bovine serum albumin (16.48 min), respectively. The energy of activation in the enzyme inactivation was 309.7 kJmol−1. The value of ∆H and ∆G at 60 °C were 306.93 and 100.478 kJ mol−1. The ∆S was + 0.62 kJmol−1 K−1. There was a complete loss in activity when the enzyme incubated for an hour in the presence of 10 M urea or 5 M guanidine hydrochloride. The inactivation kinetics was carried out in the presence of 9 M urea and 4 M guanidine hydrochloride; the t1/2 values with both urea and guanidine hydrochloride were 55.55 min.