Spectral changes arising from the action of spinach chloroplast ribosephosphate isomerase on ribose 5-phosphate

Following addition of spinach chloroplast ribosephosphate isomerase (RPI) to ribose 5-phosphate (R5P) buffered with neutral phosphate, the sequential appearance of compounds absorbing at 280, 308.5, and 285 nm was observed. The 280-nm absorbing compound has previously been identified as ribulose 5-phosphate (Ru5P). The 308.5-nm absorbing compound has a high molar extinction coefficient and was a transient species, appearing only after production of Ru5P and preceding the accumulation of the 285-nm absorbing compound. The 285-nm absorbing compound has been identified as 4-hydroxy-5-methyl-3(2H)-furanone (I). Following addition of rabbit skeleton muscle RPI to R5P buffered with phosphate only the 280-nm absorbing compound, Ru5P, could be observed. Preparations of RPI from photosynthetic tissues have invariably led to the rapid accumulation of I whereas preparations of RPI from nonphotosynthetic tissues produce only Ru5P. In solutions of R5P buffered with citrate, the transient band at 308.5 nm was not observed although I accumulates in solution in the presence of spinach chloroplast RPI. Prior to formation of I a compound with a low molar extinction coefficient and wavelength of maximum absorption at approximately 310 nm can be detected. These results suggest that purified preparations of RPI from spinach chloroplasts and other photosynthetic tissues possess an enzymatic activity not present in crude extracts obtained from non-photosynthetic tissues. This activity is in addition to the aldol-ketol activity. The product of this heretofore unrecognized enzymic activity is postulated to be 3,4-epoxy-Ru5P and could be derived from Ru5P by removing the elements of water from the hydroxyl groups at C-3 and C-4.