Effects of thiols on the activity of soybean (Glycine max) urease

The various thiols namely, β-ME, DTT and L-cysteine were studied for their effect on soybean urease. All thiols were shown to enhance urease activity at low concentration, while at higher concentration the inhibitory effects were observed. The β-ME and DTT exhibited the enhancement effects over a wide range of concentration while Lcysteine showed some enhancements only at low concentrations. All thiols showed two or three peaks of activity enhancement at low concentrations (0.01-3 mM). The β-ME was found to be the most effective enhancer of urease activity, being two times more efficient than DTT and three times than L-cysteine. L-cysteine showed inhibition beyond 3 mM. From these studies it was observed that the order of effectiveness as enhancer of urease activity followed the sequence β-ME > DTT > L-cysteine. The observed activation of urease by various thiols was suggestive of the protection of the SH groups of the enzyme by added thiol. Further with time dependent studies the enhancement effects were shown to occur in two different phases; the initial fast phase, in which the rate increases almost instantly and linearly (first-order kinetics) and the later slow phase, in which no significant enhancements were observed (zero-order kinetics).