Studies on the Properties of Carboxylating Enzymes in the Epidermis ofCommelina communis

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Abstract
A spectrophotometric assay has been used to measure the activity of PEP carboxylase and RuBP carboxylase in the epidermal and mesophyll tissue of Commelina communis. On both a chlorophyll and protein basis the PEP carboxylase activity was always greater in the epidermis than in the mesophyll, whereas RuBP carboxylase activity was always highest in the mesophyll. PEP carboxylase activity in epidermal extracts was lost very slowly and its pH optimum was a broad one in the range 7·5–8·0. The Km values for PEP carboxylase in the epidermis and mesophyll obtained from light- and dark-treated plants were not very different although its Vmax was much lower in dark-treated tissue. These data are discussed in relation to the possible role of PEP carboxylase in guard cell metabolism.