ESTIMATION OF L-GLUTAMINASE AND L-ASPARAGINASE ACTIVITIES IN SOILS BYTHE INDOPHENOL METHOD

A simple and rapid method was developed for determining L-glutaminase and L-asparaginase activities in soils. It involved colorimetric estim ation of NH4+ released by the activity of L-glutaminase and L-asparagi nase when soil was incubated with 0.25 M L-glutamine or L-asparagine i n 0.1 M phosphate buffer (pH 7.6) and toluene used as a germicide. The indophenol method was applied to the assay of NH4+ for soil L-glutami nase and L-asparaginase activities. Color development in the indopheno l method required one hour. The color development was stable for 2 d a nd was not appreciably affected by the presence of L-glutamine, L-glut amic acid, L-asparagine, and L-aspartic acid. In addition, no turbidit y nor precipitate appeared in the presence of a high concentration (10 mu g) of NH4+-N. The L-glutaminase activity was estimated in three so il samples differing in pH, organic-C content, and land use. The L-glu taminase activity in the three soils ranged from 28 to 1,470 mU (10(-9 ) mol min(-1) g(-1) of dry soil at 30 degrees C). The optimum pH for t he L-glutaminase activity in soils was about 7.5 in phosphate buffer. The incubation time of L-glutaminase and L-asparaginase can be shorten ed, because the sensitivity of the indophenol method is about 24 times as high as that of the Nessler method.