[Inhibition of isocitrate lyase in watermelon seedlings by white light]

The isocitrate lyase activity (E.C. 4.1.3.1.) from watermelon cotyledons (Citrullus vulgaris SCHRAD.) is inhibited by white light (Fig. 5). To exclude artefacts during enzyme preparation the following experiments were performed (Table 1 and 2): 1. Mixing of raw extracts from cotyledons of light and dark grown seedlings. 2. Joint homogenization a) of cotyledons from light and dark grown seedlings, b) of purified isocitrate lyase together with cotyledons from light or dark grown seedlings. The total activity corresponded to an amount which was expected for the sum of the individual activities. The results justify the conclusion that the inhibition of the isocitrate lyase by light is real, and that the measured enzyme activities are close to the true enzyme concentrations in the plant tissue. - The relatively slow inhibition of the enzyme activity caused by light seems to be correlated with the formation of the photosynthetic apparatus.