Standardisation of assay of L-glutaminase activity in arid and semi arid tropical soils of south India

L-glutaminase (L-glutamine amidohydrolase E.C. 3.5.1.2) is the soil enzyme that catalyzes the hydrolysis of L-glutamine and produces L-glutamic acid and ammonia. Thirty surface soil samples of varying physico-chemical properties representing, various cropping systems were collected from the Rajendranagar campus of Hyderabad. The procedure for assay of L-glutaminase activity in soils was standardized and quantification was compared between the modified indophenol blue method and the steam distillation method. A linear regression analysis (R² = 0.997) was carried out for the activity of enzymes obtained by the modified indophenol method against steam distillation. Assay its activity is described, which involves determination of the NH₄+ released by L-glutaminase activity when 10 grams of soil is incubated with L-glutamine, THAM buffer (pH 8) and toluene at 37°C for 4 h. Incubation was terminated by the addition of 2.5 M KCl containing a L-glutaminase inhibitor (100 ppm Ag₂SO₄). The NH₄+-N released soil suspension is determined modified indophenol method. Correlation analysis between the soil properties and L-glutaminase activity indicated that L-glutaminase was significantly and positively correlated with organic carbon, total nitrogen, clay content (and available nitrogen for red soils. Similarly, in black soils, these were positive and significant also, coefficient correlation with O.C, total N, clay content and available N. For all the soils (red and black soils together) highly significant correlation was with O.C, total nitrogen, clay content and available nitrogen. However, there was no significant correlation between L-glutaminase activity and pH, total soluble salt content.