Esterases and putative lipases from tropical isolates of Aureobasidium pullulans

Esterases and lipases have been studied in a number of fungi, though very little is known about esterases from Aureobasidium pullulans especially from the African tropics. In this study, forty-two Zimbabwean isolates were screened for lipase activity on tributyrin agar. Extracellular esterase activities of seven selected isolates were studied under varying conditions using para-nitrophenol acetate as substrate. Twenty isolates (48%) showed lipolytic activity; sixteen showed negative results for lipase activity while the rest showed weak activities. Esterase activities in broth cultures ranged from 0.011-0.223 mmol/microg protein/min while activities ranged from 1.5-12.8 U/ml under solid state fermentation. The esterases were optimally active at pH 7.6-8.0, showed a temperature optimum of 35 degrees C and retained more than 50% activity at temperatures up to 60 degrees C and at pH 4.0-7.0 after 150 min. Enzyme production was optimal after 5-6 days with diammonium hydrogen phosphate as nitrogen source. Isolates showed variations in preference for carbon source for esterase production. The A. pullulans esterases differed from most fungal esterases in that they are optimally active in alkaline conditions and are active over a broad pH range.