JOURNAL of Soil Biology and Ecology

Author: D.SRINIVAS AND P. CHANDRASEKHAR RAO

Abstract :

Alkaline phosphatases belong to the group of enzymes, phosphomonoesterases, which play a major role in the mineralization of soil organic P. Substrate concentration of enzyme is one of the factors that affect the enzyme kinetics.  To study the role of substrate concentration on soil enzyme alkaline phosphatase in vertisols, twenty-five surface soil samples were collected and assayed for the activity of soil alkaline phosphatase. The alkaline phosphatase activity as expressed in terms of µg of           4-nitrophenol released g-1 soil h-1 in these soils varied from 53.8 to 498.7 with a mean value of 187.4 in surface soils. Among them, ten soil samples were selected to study enzyme kinetics with thirteen substrate concentrations under laboratory incubation studies with three replications. Soil alkaline phosphatase increased with an increase in substrate concentration up to 20mM, and almost reached a plateau at a substrate concentration of more than 15mM. The enzyme activity remained almost constant in all the ten vertisols, with further increase in substrate concentration, minimal change in enzyme activity was observed. Characteristics of enzyme activities like maximum enzyme reaction velocity (Vmax) and Michaelis constant (Km) were determined using the Michaelis–Menten equation similar to those determined in a homogenous system. The maximum reaction velocity of soil alkaline phosphatase for soils under study was calculated (µg of 4-nitrophenol g-1 soil h-1) which varied from 87.6 to 197.5 with the soils S9, S2, S5 and S8 recorded higher values using Lineweaver – Burk plot. The values compared well with Hanes–Wolf transformation 75.5 to 129.9 and the soils S2, S9, S5, and S3 recorded higher values.  Under the Eadie–Hofstee transformation the values varied from 82.5 to 140.8 and S2, S5, S3 and S9 recorded higher values.  Michaelis constant (Km) of the soil alkaline phosphatase calculated using Line weaver–Burk transformation plot varied from 3.8 to 16.80 mM and the higher values were recorded by S9, S2, S8, S7 and S6. The values compared well with those obtained from Hanes–Wolf (1.10 to 7.80) and soils S9, S2, S8 and S6 recorded higher values. In Eadie–Hofstee plots the values ranged from (3.10 to 8.20) and higher values were found in the soils S9, S2, S4 and S8.  It was concluded that the linear transformation of Line-weaver Burk values was superior for estimating Km and Vmax values of enzymes in soil. However significant differences were noticed when Km and Vmax were calculated from the linear transformation of Michaelis plot.

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