L-Ribulose and l-Arabinose

Abstract

This chapter discusses the method for the enzymatic determination of L -ribulose and L -arabinose. This method is based on the principle that L -Arabinose isomerase catalyzes the reaction as described in the chapter. The equilibrium of the reaction lies in favor of L -arabinose, with an equilibrium constant of 7.33 at 34°C. In the presence of excess isomerase, 88% of the L -ribulose is converted to L -arabinose. L -Arabinose does not react in the cysteine–carbazole test. Therefore, if the color test is carried out before and after incubation with L -arabinose isomerase, the difference in color intensities is equivalent to 88% of the L -ribulose that is present in the sample. The reaction is standardized with L -ribulose-O-nitrophenylhydrazone. The same method can be used for the determination of L -arabinose, if the enzymatic reaction is carried out in borate buffer at pH 8.2 instead of tris buffer pH 7.5. At pH 8.2, the equilibrium of reaction is in favor of the ketopentose. Crystalline L -arabinose is used as a standard.