Pii: S0038-0717(00)00163-2

The aim of this work was to evaluate the effect of two different humic acid (HA) fractions, high molecular weight (HMW 100–300 kDa) and low molecular weight (LMW 10–20 kDa), extracted from peat on the activity and stability of Jack Bean urease. HMW HA significantly inhibited urease activity at pH 6.0 but did not influence the activity at pH 7.0 and 8.0. HMW HA stabilised urease activity over a period of 11 days to treatments with protease and with Cu and Hg (two powerful inhibitors of soluble urease activity). The LMW HA inhibited urease activity at pH 6.0, 7.0 and 8.0, and did not stabilise urease activity in the presence of protease. The residual activity of urease, at pH 7.0 and 8.0 in the presence of LMW HA and Cu, was 90 and 69%; in the presence of LMW HA and Hg it was 81 and 52%. The residual activity of urease, at pH 7.0 and 8.0, in the presence of HMW HA and Cu was 99 and 88%; in the presence of HMW HA and Hg it was 94 and 74%. These results showed that the two HA fractions influenced both the activity and stability of the urease differently. It is proposed that the inhibition of the urease by HMW and LMW HA is mainly due to the two heavy metals which, although immobilised on the HA, are still able to interact with the urease. q 2000 Elsevier Science Ltd. All rights reserved.