D-phosphoarabinoisomerase and D-ribulokinase in Escherichia coli.

An enzyme, D-phosphoarabinoisomerase, which stoichiometrically interconverts D-arabinose 5-phosphate and Dribulose S-phosphate, was isolated from Escherichia coli. At equilibrium, 75% of D-arabinose-5-P and 25% of D-ribulose-5-P are found. This unstable enzyme does not show any requirement for added cofactor, and is inhibited by Mn++, Co++, Zn+f, Cd++, p-chloromercuribenzoate, and phosphate. The optimum pH is 8.0. The Km is 1.36 X 10u3 M for Darabinose-5-P and 5.40 X low4 M for D-ribulose-5-P. The enzyme lacks activity toward D-arabinose, D-ribulose, Dribose, and D-ribose-5-P. This enzyme not only accounts for the metabolic origin of the precursor for Z-keto-3-deoxyoctonate (KDO), a constituent in the cell wall lipopolysaccharide of E. coli and some other gram-negative bacteria, but also fits into the schema suggested for the utilization of D-arabinose-1-P generated in the metabolism of D-arabinosyl nucleosides. The enzyme is absent from yeast and mouse fibroblasts. D-Arabinose-5-P was synthesized by phosphorylating D-glucosamine followed by ninhydrin degradation. The product was contaminated with D-ribulose-5-P and was purified by precipitation of the latter with borate. D-Ribulose-5-P was obtained by phosphorylating D-ribulose with a D-ribulokinase isolated from E. coli. The product showed a distinct carbonyl absorption band in its infrared spectrum. Upon periodate oxidation, phosphoglycolaldehyde was detected as a major degradation product, as was expected for D-ribulose-5-P. However, this purified preparation of D-ribulokinase also phosphorylated L-fuculose at position 1.