Effects of neem leaf extract on production of aflatoxins and activities of fatty acid synthetase, isocitrate dehydrogenase and glutathione S-transferase in Aspergillus parasiticus

Abstract

The relationship between the activities of 3 cytosolic enzymes with aflatoxin biosynthesis in Aspergillus parasiticus cultured under different conditions has been investigated in order to find out the role of each enzyme in aflatoxin biosynthesis. Basically the activity of isocitrate dehydrogenase (IDH) was higher in non-toxigenic strains as compared to its counterpart toxigenic fungi (p < 0.05). In contrast, the activities of fatty acid synthase (FAS) as well as glutathione S-transferase (GST) were higher (P < 0.05) in toxigenic strains than that of the non-toxigenic fungi. Aflatoxin production was inhibited in fungi grown in presence of various concentrations of neem leaf extract. Aflatoxin was at its lowest level (>90% inhibition) when the concentration of neem extract was adjusted to 50% (v/v). No significant changes in FAS and IDH activities were observed when aflatoxin synthesis was under restraints by neem (Azadirachta indica) leaf extract. During a certain period of time of culture growth, when aflatoxin production reached to its maximum level, the activity of FAS was slightly induced in the toxigenic strains fed with a low concentration (1.56% v/v) of the neem leaf extract. At the time (96 h) when aflatoxin concentration reached to its maximum levels, the activity of GST in the toxigenic fungi was significantly higher (i.e., 7-11 folds) than that of non-toxigenic strains. The difference was highest in mycelial samples collected after 120 h. However unlike FAS and IDH, GST was readily inhibited (approximately 67%) in mycelia fed with 1.56% v/v of the neem extract. The inhibition reached to maximum of 80% in samples exposed to 6.25-12.5% of the extract. These results further substantiate previous finding that there is a positive correlation between GST activity and aflatoxin production in fungi.