Pentose phosphate isomerase and epimerase from animal tissues

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Abstract
Preparations of pentose phosphate isomerase (PPI) were made from rabbit skeletal muscle, horse erythrocytes and Krebs II ascites carcinoma cells of the mouse. The enzyme was partially purified, and its properties studied. The enzyme preparation from erythrocytes is virtually free from aldolase and contains very little phosphoketo-pentoepimerase (epimerase) activity. The pentulose phosphates formed from D-ribose 5-phosphate (R 5-P) were identified as ribulose 5-phosphate (Ru 5-P; 90%) and xylulose 5-phosphate (Xu 5-P 10%). This enzyme provides a convenient preparation of nearly pure Ru 5-P. The similar preparation from muscle is rich in epimerase and yields a mixture of pentulose phosphates containing 55% of Xu 5-P and 45% of Ru 5-P. The free ketopentoses liberated by phosphate hydrolysis were separated by column adsorption and characterized as D-xylulose and D-ribulose by optical rotation and preparation of crystalline derivatives. The carcinoma-cell "equilibrium" mixture of pentulose phosphates is similar to that from muscle. Both these preparations also contained aldolase. The extent of conversion of R 5-P into pentulose phosphate at 57[degree] was about 40%, a higher value than that (25%) reported for other PPI preparations. Possible reasons for this are discussed. With the aid of preparations containing both PPI and epimerase acting on R 5-P, the rate of transformation of the first product, Ru 5-P, into Xu 5-P was studied. The presence in the final product of a 3-pentulose, as described by Ashwell and Hickman (1955) for spleen extracts, was observed for the muscle preparation also; the amount present is, however, very small (1.3%).