Cleavage Reaction Mechanis of Peroxydative Product in Biogeneration of Leaf Green Odour

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When 13-hydroperoxylinoleic acid is incubated with tea chloroplast, several products including geometrical isomers can be detected.Although the cleavage reaction performed by hydroperoxide lyase was extensively explored in our laboratry, whether this enzyme also progress isomerization or not had notbeen known yet. Investigation of this mechanism was performed with an alcohol derivative of 13-hydroperoxylinoleic acid, 13-hydroperoxylinoleyl alcohol (1). Incubation of synthesized 12-hydroxy-(3z)-dodecenal (2) with tea chloroplast revealed that 12-hydroxy-(2E)-dodecenal (3) was the product of isomerization reaction of 2. This isomerization was found to be caused also by incubation of 2 with heat inactivated chloroplast. Furthermore, this isomerization activity could be extracted with heat ehanol. These results indicate that isomerization is occured by heat-stable factor(s) but not by enzyme(s). This factor(s) was activated in alkaline condition but not affected with salt concentration. When 13-hydroperoxylinoleyl alcohol (1) was incubated with tea chloroplast under conditions which suppresses isomerization, 3 was formed only after cleavage reaction was almostly ceased. And when ^<18>O-(1) was incubated with tea chloroplast under this suppressed condition ^<18>O-(2) was effectively detected. These results indicated that the isomerization reaction was independent from cleavage reaction performed by hydroperoxide lyase. From this finding it is proposed that this enzyme retain cis-configuration of double bond at C-9 position of substrate (1) during cleavage reaction. In fact, when (9E, 11E)-(1) was incubated with this enzyme, there occurred no reaction Double bond at C-9 position of substrate must be recognized by some ligand around active centers of the enzyme but have no contribution on the reaction itself.