Influence of oxygen on lovastatin biosynthesis by Aspergillus terreus ATCC 20542 quantitatively studied on the level of individual pellets.

Despite oxygen is believed to be the most important environmental factor for any aerobic microbial process, the quantitative studies of its influence on growth and metabolite formation on the level of individual pellets formed by filamentous fungi were seldom performed. Never was it made for lovastatin producer Aspergillus terreus ATCC20542. Thus, this work is a quantitative study of oxygen transfer into A. terreus pellets during lovastatin biosynthesis in the shake flask culture. The basic measurement tool was an oxygen microprobe allowing for obtaining oxygen concentration profiles in the pellets. The pellets of various sizes from 1,600 to 6,400 µm exerting different oxygen transfer conditions were studied. Also various initial concentrations of carbon source were applied to change the conditions of biological reaction running in the pellets. Effective diffusivities in A. terreus pellets ranged from 643 to 1,342 µm s(-1) dependent on their size and structure. It occurred that only the smallest pellets of diameter equal to about 1,400 µm were fully penetrated by oxygen. What is more, apart from the size of pellets, the appropriate lactose concentration was required to effectively produce lovastatin. Its value was correlated with oxygen concentration on the surface of the pellet and could not be either too high, as the aforementioned oxygen level tended then to zero, or too low, as despite high oxygen concentration no biological reaction ran in the pellet and no lovastatin was formed.

Effect of pH on biosynthesis of lovastatin and other secondary metabolites by Aspergillus terreus ATCC 20542.

The effect of the initial pH value of the cultivation medium on lovastatin (mevinolinic acid) biosynthesis by Aspergillus terreus ATCC20542 was studied. It was found that if the pH value of the broth is acidic, the direct chromatographic assay of mevinolinic acid leads to the underestimated values. Thus, the equilibrium curve was determined for the transformation of ß-hydroxy acid form of lovastatin (mevinolinic acid) into lovastatin lactone. The calculation of the equilibrium constant shows that when the pH value of the solution is 4.98, concentrations of both forms of lovastatin are equal to each other. This finding was next used to study mevinolinic acid formation at the various initial pH values of the medium. It occurs that even at pH lower than 5.5 mevinolinic acid is still, although inefficiently, produced and its presence remains unnoticed, unless the samples of the broth are alkalised prior to the assay. Mevinolinic acid is efficiently produced at the initial pH value of the medium equal to 7.5 and 8.5 and it correlates with the rapid utilisation of lactose by A. terreus. Additionally, other secondary metabolites were sought at the various initial pH values of the medium with the use of mass spectrometry. (+)-Geodin is only formed at pH 6.5, while monacolin L is found at the highest amount at pH 7.5.