Participation and properties of lipoxygenase and hydroperoxide lyase in volatile C₆-aldehyde formation from C₁₈-unsaturated. Fatty acids in isolated tea chloroplasts

Isolated tea chloroplasts utilized linoleic acid, linolenic acid and their 13-hydroperoxides as substrates for volatile C₆-aldehyde formation. Optimal pH values for oxygen uptake, hydroperoxide lyase and the overall reaction from C₁₈-fatty acids to C₆-aldehydes were 6.3, 7.0 and 6.3, respectively. Methyl linoleate, linoleyl alcohol and γ-linolenic acid were poor substrates for the overall reaction, but linoleic and linolenic acids were good substrates. The 13-hydroperoxides of the above fatty acids and alcohol also showed substrate specificity similar to that of fatty acids. Oxygen uptakes (relative V_max) with methyl linoleate, linoleyl alcohol, linolenic acid, γ-linolenic acid and arachidonic acid were comparable to or higher than that with linoleic acid. In winter leaves, the activity for C₆-aldehyde formation from C₁₈-fatty acids was raduced to almost zero. This was due to the reduction in oxygenation. The findings presented here provide evidence for the involvement of lipoxygenase and hydroperoxide lyase in C₆-aldehyde formation in isolated chloroplasts.