The isomerization of 2,5- and 9,12-octadecadienoic acids by an extract of Butyrivibrio fibrisolvens.

A cell-free particulate preparation from Butyrivibrio fibrisolvens was used to study the relative rates of isomerization of all cis,cis-methylene-interrupted isomers of octadecadienoic acid. Only two isomers were found to be substrates, the 9,12-isomer was isomerized at 41 +/- 4 mumol/min per mg protein, and the 2,5-isomer at 11 +/- 1 mumol/min per mg. The product of the isomerization of the 2,5-isomer had an ultraviolet absorption maximum at 233 nm indicating that it was the 3,5-isomer. The isomerization of the 2,5-isomer was studied in detail. Its rate of isomerization was linear with protein concentration up to 0.047 mg/ml, and was linear with substrate concentration up to 48 muM. The pH optimum was 6.8. Below pH 6, the substrate was also subject to spontaneous isomerization. The inhibition of isomerization of the 9,12-isomer by the other isomers was studied. Those isomers in which the double bonds are close to the carboxyl group were the most effective inhibitors. The preparation was also found capable of hydrogenating the conjugated diene product from the 2,5-isomer to a monoene after prolonged incubation.