Inhibition of Urease from Seeds of Water-melon (Citrullus vulgaris) by Heavy Metal Ions

Urease from the seeds of water melon was found to be inhibited by heavy metal ions like copper, lead, nickel and cobalt. The order of effectiveness of these metals as inhibitors was Cu²⁺ > Pb²⁺ > Ni²⁺ > Co²⁺. The inhibition by these ions was non-competitive. Time - dependent interaction of urease with nickel and cobalt exhibited a biphasic inhibition behaviour in which approximately half of the initial activity was lost rapidly (within 2 min) and remainder in a slow phase. The inhibition was largely irreversible, hence could not be reversed by dialysis. These observations are suggestive of half-and-half distribution of - SH groups on the native enzyme resulting urease into asymmetric oligomeric molecule.