Humic substances and phosphatase activities in plant tissues

Humic acid and its fractions obtained by water- or acid-boiling inhibited phosphatase activity per se in beet, carrot and potato discs, pea roots and epicotyls and wheat roots, coleoptiles and leaves. In wheat roots the order of effectiveness of various humic acid fractions in inhibiting phosphatase activity was acid-boiled soluble > water-boiled soluble > acid-boiled insoluble > water-boiled insoluble > original humic acid. A number of synthetic humic acids were also effective inhibitors of enzyme activity but phenolic acids had no effect.The degree of inhibition was not related to C, H or N contents or to the total ash, carboxyl or phenolic contents of the humic acid samples. Magnesium ions enhanced phosphatase activity and decreased the inhibition of phosphatase activity produced by the humic acid fractions.Humic acid fractions did not affect the maximum temperature for enzyme activity or its pH optimum and had little effect on the Michaelis constant. They did, however, reduce the maximum velocity of the enzyme reaction thus producing a non-competitive inhibition of enzyme activity.It is suggested that the humic acid fractions inhibit phosphatase activity by combining with the enzyme, but not at the most active site of enzyme activity.