Distribution of lipases in the Gramineae. Partial purification and characterization of esterase from Avena fatua

The activity levels of esterase, lipid acylhydrolase and lipase were quantitatively screened in 23 species and cultivars of Gramineae. Their activity levels expressed as units g−1 seeds, were found to range from 10 to 123 for esterase, 0.28 to 7.67 for lipid acylhydrolase and 13.1 to 93.9 for lipase. Avena fatua, one of the grass species, exhibited the highest levels of esterase and lipase and could be potentially a good starting material for preparation of lipases. A. fatua esterase has been partially purified and characterized. Four isoenzymes, EI, EII, EIII and EIV, were separated by ion exchange chromatography. Esterases EII and EIII had Km values of 0.52 and 0.38 mM and a pH optimum at 9.0 with half maximal activities at pHs 8.5, 10 and 8, 10.5, respectively. Esterases EII and EIII had optimum activities at temperatures of 75°C and 65°C with activation energies of 3.3 and 4.3 kcal mol−1, respectively. The enzymes were thermally stable as esterases EII and EIII retaining 39% and 23% of their activities at 90°C, respectively. Esterases EII and EIII were stimulated by Ba2+ and Ca2+ but were inhibited by Mn2+ and Zn2+. A. fatua esterases exhibited optimum storage stability and were stable at high temperatures and alkaline pH. They possessed high affinity toward substrate and were resistant to inhibition by most divalent cations that were examined. These are important properties when considering the industrial application of these enzymes.