Fatty Acid Toxicity and Methyl Ketone Production in Aspergillus niger

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Abstract
Vegetative hyphae of Aspergillus niger rapidly converted caproic acid into 2-pentanone. More caproic acid was required for maximal ketone production at alkaline as compared to acidic p H values. Further increases in caproate concentrations at each p H value tested (4.5, 5.5, 6.5, 7.5, and 8.5) resulted in inhibition of ketone production and O 2 uptake. At alkaline p H values (8.5 and 7.5), oxygen uptake above the endogenous level and the production of 2-pentanone were parallel. This relationship did not hold at acidic p H values. At these p H values, ketone production continued ( p H 6.5) or attained a maximum ( p H 5.5 and 4.5) at caproate concentrations at which oxygen uptake was inhibited below endogenous levels. These data indicate that endogenous oxygen uptake was not inhibited by caproate at alkaline p H values at concentrations which did inhibit caproate oxidation and 2-pentanone production. Conversely, at acidic p H values, endogenous oxygen uptake was vigorously inhibited by caproate at concentrations at which exogenous fatty acid oxidation and 2-pentanone production were less affected. Simon-Beevers plots of these data showed that the undissociated acid was the permeant form of caproic acid. The fatty anion appeared to be the active or inhibitory form of caproate within the cell. Vegetative hyphae of A. niger were poorly buffered. Once the hyphae were washed and resuspended in phosphate buffer, they were well buffered towards inhibitory concentrations of caproic acid. These findings suggest that the primary mechanism(s) by which caproate inhibits oxygen uptake and ketone formation does not involve a change in the intracellular p H.