Formation of isoprostane F2-like compounds (phytoprostanes F1) from α-linolenic acid in plants

Isoprostanes F2 are biologically active prostaglandin F2-like compounds formed by free radical–catalyzed oxidation of arachidonic acid (C20:4). Here, we show that a series of dinor isoprostanes F1, which we term phytoprostanes F1 (PPF1s), are formed by nonenzymatic oxidation of linolenate (C18:3) in plants. Identification and quantification of PPF1s were achieved by a negative ion chemical ionization gas chromatography–mass spectrometry method using oxygen 18–labeled PPF1s as internal standards. PPF1s were found in leaves, flowers, and roots of taxonomically distinct plant species at concentrations ranging from 43 to 1380 ng/g of dry weight. In addition, esterified PPF1s were found at 10- to 150-fold higher concentrations. During the drying and storage of various plant organs, endogenous PPF1 levels increased dramatically by 15- to 263-fold. Because the structurally related prostaglandin F2α and isoprostanes F2 exert potent biological activities (i.e., broncho- and vasoconstriction) in the nanomolar range, PPF1s could potentially exert similar biological activities. Notably, fresh birch pollen, which can easily be inhaled, contains exceedingly high concentrations (32,440 ng/g) of free PPF1s.