Enzymatic Formation of Plant Cerebroside: Properties of UDP-glucose: Ceramide Glucosyltransferase in Radish Seedlings

The activity of cerebroside synthase (UDP-glucose: ceramide glucosyltransferase) was found in the microsomal fraction of radish hypocotyls, and was studied. One % of the radioactivity due to UDP-[ 3 H]glucose added to the membrane fraction was incorporated into cerebroside within 60 min. Optimum pH and temperature of the activity were pH 7.8 and 30°C, respectively. No metal ions enhanced the cerebroside synthase activity, dissimilar to that in animal tissues. The apparent K m for UDP-glucose was approximately 200 μM. The same activity was also observed in radish roots and cotyledons, but proportions of UDP-[ 3 H]glucose incorporation were slightly lower than that in hypocotyls. Exogenous ceramide species having trihydroxy sphingoid bases, which were major ceramide components of radish cerebrosides, usually stimulated cerebroside formation in microsomal fractions from radish seedlings, while any cerami de types having dihydroxy sphingoid bases were ineffective on the glucosylation reaction. It was assumed, therefore, that cerebroside synthase in radish seedlings would have substrate selectivity for ceramide species.