Phosphatases in aflatoxin-producing and non-producing strains of Aspergillus flavus Link.

摘要

The nonspecific acid and alkaline phosphatases and ATPase of toxinogenic and non-toxinogenic Aspergillus flavus strains were investigated to establish a relationship, if any, between the activities of these enzymes and the concentrations of inorganic phosphate as well as aflatoxins produced by the toxinogenic strains. The inorganic phosphate concentrations in cell-free extracts and the activities of the enzymes were determined at different incubation periods (3, 5, 7, 9 and 11 days). The results showed that there was no marked difference in inorganic phosphate levels at different incubation periods between the toxinogenic and non-toxinogenic strains, but the rate of inorganic phosphate concentration build-up was higher in the toxinogenic strain than in the non-toxinogenic strain at day 3, and it decreased towards the start of aflatoxin biosynthesis (day 5). The activities of both acid and alkaline phosphatases in the non-toxinogenic strain showed an inverse relationship with inorganic phosphate level up to day 9. In the toxinogenic strain, the changes in the activities of these enzymes were independent of inorganic phosphate concentration, however the activity of acid phosphatase decreased from day 5 to day 9 with increase in aflatoxin concentration. In contrast, the activity of alkaline phosphatase exhibited a sharp increase up to day 5 and remained relatively stable during the period of aflatoxin biosynthesis (days 5-9), followed by a slight decrease on day 11. In the non-toxinogenic strain, there was a gradual increase in the specific activity of ATPase up to day 11 irrespective of inorganic phosphate levels. In the toxinogenic strain, an increase in ATPase activity was observed up to day 5 (start of aflatoxin biosynthesis), followed by a gradual decrease up to day 9 (aflatoxin concentration was maximum), and again an increase on day 11 (sharp decrease in aflatoxin concentration). It is concluded that inorganic phosphate inhibits the activities of acid and alkaline phosphatases in non-toxinogenic A. flavus, and lower levels of inorganic phosphate favour aflatoxin synthesis in the toxinogenic strain. In addition, acid phosphatase and ATPase are sensitive to inhibition by increased aflatoxin concentrations.