Planteose Synthesis in Seeds of Sesamum indicum L.

An enzyme, UDP-galactose : sucrose 6^fru-α-galactosyltransferase, which synthesizes planteose (6^fru-α-galactosylsucrose), has been purified 55-fold from seeds of Sesamum indicum L. (Pedaliaceae) using ammonium sulfate and protamine sulfate precipitation. The partially purified enzyme fraction showed only very low α-galactosidase and β-fructosidase activity but significant UDP-glucose 4-epimerase, phosphodiesterase, phosphatase and sucrose synthase activity which interfere with planteose synthesis. UDP-glucose 4-epimerase activity could be inhibited by about 50% by the addition of 10 mmol/l Mn²⁺, whereas phosphodiesterase activity could not be reduced by the addition of ATP, NAD or UDP-glucose, potentially alternative substrates for UDP-galactose. The optimal activity of the planteose synthesizing enzyme was obtained at pH 6.2. The enzyme was stable for at least six months at -20°C. It did not need sulfhydryl reagents for full activity. The enzyme displayed a high specificity for UDP-galactose (K_m 0.2–0.5 mmol/l), the galactosyl donor, and for sucrose (K_m 3.6–6.0 mmol/l), the acceptor. 5′-UMP was inhibitory, while galactose 1-phosphate, glucose l-phosphate and UDP-glucose did not affect the enzyme activity.