Biosynthesis of fluorinated secondary metabolites by Streptomyces cattleya

Abstract

The biosynthesis of organofluorine compounds by Streptomyces cattleya NRRL 8057 was examined using 19F NMR spectroscopy. The organism produced 12 mM fluoroacetate and 05 mM 4-fluorothreonine as secondary metabolites when cultured for 28 d on a chemically defined medium containing 2 mM fluoride. Cell suspensions from batch cultures harvested at the growth maximum of 4 d were not capable of fluoride uptake or fluorometabolite biosynthesis, but by 6 d had developed an efficient fluoride-uptake system and biosynthesized the two fluorometabolites in almost equal proportions. As the harvest age increased, the proportion of fluoroacetate to 4-fluorothreonine formed by cell suspensions rose progressively so that 16-d-old cells showed a ratio of 7626 for the two compounds. Fluoride uptake and fluorometabolite production by cell suspensions were highly dependent on pH, with both processes showing a maximum rate at pH 60 but declining rapidly at higher pH values. This decrease was particularly marked in the case of fluoroacetate biosynthesis which was barely detectable at pH 75. Fluoroacetate and 4-fluorothreonine showed only low levels of interconversion by cell suspensions, suggesting that the carbon skeleton of neither was derived by metabolism of the other. The limited interconversion observed is explicable in terms of a small degree of biological defluorination occurring with each compound, followed by reincorporation of the resulting fluoride ion into the organic form by the active fluorinating system, a phenomenon also noted on incubation of cell suspensions with a number of other fluorinated biochemical intermediates. Cell suspensions were supplemented with a variety of amino acids and tricarboxylic acid cycle intermediates to determine the identity of the carbon substrate of the fluorinating system. No compound tested significantly increased the total amount of fluorometabolites formed or altered their relative proportions. However, in studies with 14-labelled precursors, the highest level of incorporation into fluoroacetate by cell suspensions was recorded with [U-14C]glycolate, suggesting that this compound or an activated derivative may be the substrate for the fluorinating system in the biosynthesis of fluoroacetate.