Soybean (Glycine max) urease: significance of sulfhydryl groups in urea catalysis.

The soybean urease (urea amidohydrolase; EC 3.5.1.5) was investigated to elucidate the presence of sulfhydryl (-SH) groups and their significance in urea catalysis with the help of various -SH group specific reagents. The native urease incubated with 5,5'-dithiobis (2-nitrobenzoic acid) (DTNB) showed exponential increase in the absorbance, thereby revealing the presence of -SH groups. A total of 34 -SH groups per hexamer enzyme molecule were estimated from the absorption studies which represents nearly six -SH groups per subunit. The time-dependent inactivation of urease with DTNB, p-chloromercuribenzoate (p-CMB), N-ethylmaleimide (NEM) and iodoacetamide (IAM) showed biphasic kinetics, where half of the enzyme activity was lost more rapidly than the other half. This study reveals the presence of two categories of "accessible" -SH groups, one category being more reactive than the other. The inactivation of urease by p-CMB was largely reversed on treatment with cysteine, which might be due to unblocking of -SH group by mercaptide exchange reaction. Finally, when NEM inactivated urease was incubated with sodium fluoride, a time-dependent regain of activity was observed with higher concentrations of fluoride ion.