Inhibition of plant and microbial ureases by phosphoroamides

Enzyme kinetic studies of inhibition of plant (jackbean) and microbial (Bacillus pasteurii) ureases by eight phosphoroamides [phenylphosphorodiamidate, 4-chlorophenylphosphorodiamidate, phosphoric triamide, N-(diaminophosphinyl) benzamide, N-(diaminophosphinyl) benzeneacetamide, 4-chloro-N-(diaminophosphinyl) benzamide, N-(4-nitrophenyl) phosphoric triamide, N-(diaminophosphinyl)-3-pyridinecarboxamide] demonstrated that these compounds are slow, tight-binding inhibitors of urease enzymes. Measurement of the dissociation constants (Ki*) of the enzyme-inhibitor complexes (E . I*) formed by interaction of the ureases and phosphoroamide inhibitors studied showed that these inhibitors had a much higher affinity (i. e., a lower Ki*) for plant urease than for microbial urease. Measurement of rate constants for formation (kon) and decay (koff) of E · I* showed that, whereas k on varied greatly with the different inhibitors and ureases, k off was constant for the phosphoroamides tested and had a characteristic value for each urease. The half-life of E · I* (30°C; pH 7 THAM buffer) for the plant urease was much longer than that for the microbial urease, and this difference largely accounted for the much higher values of Ki* (koff/kon) observed with microbial urease.