Effect and potential ecological significance of the interaction of humic acids with two aquatic extracellular proteases

1. The effects of humic acids and fulvic acids on an extracellular serine and metalloprotease purified from a strain of Aeromonas hydrophila isolated from a freshwater wetland were examined. The serine protease was inhibited by humic acids at pH and humic acid concentrations found naturally in the wetland where this strain of A. hydrophila was isolated. The metalloprotease was not inhibited by humic acids at any pH investigated. Fulvic acids had no effect upon either protease. 2. Inhibition of serine protease activity by humic acids was reversed by increasing the pH to 9, as well as increasing the ionic strength by addition of CaCl₂- It was concluded that the interaction between humic acids and the serine protease was primarily ionic. 3. The formation of a humic acid-serine protease complex increased the half-life of enzymatic activity in dilute solutions. Humic acids had no effect on the stability of the metalloprotease. 4. There was no clear effect of humic acids on the growth of A. hydrophila, indicating that either the serine protease is not involved in the rate-limiting step of growth or that sufficient activity exists even when the serine protease is inhibited by humic acids. 5. Increasing the enzymatic lifetime through association with humic acids may be an adaptive mechanism which could result in energy conservation due to a decreased requirement for protein synthesis.