Studies on the Ribose-5-Phosphatase of Sugarcane

Abstract

A study has been made of ribose-5-phosphatase in sugarcane. The enzyme catalyzes the hydrolysis of ribose-5-phosphate to yield free ribose and phosphoric acid. The enzyme was extracted from fresh leaf tissues of 7-month old sugarcane grown in sand culture. Fractionation of cane extracts with ammonium sulfate showed that most of the enzyme was precipitated between 35 to 60 percent saturation. The richest source was spindle tissue. Leaves + 1 and + 2 were employed for studying the enzyme's properties. Dialysis up to 72 hours against several changes of distilled water had no appreciable effect on the phosphatase activity. Optimum pH was 5.5 and the K_m was 2.5 X 10^-3
 moles of ribose-5-phosphate per liter. Good resolution of the enzyme and other protein constituents was obtained by gel filtration on Sephadex G-200. Molybdenum and tungsten inhibited at low concentrations. Molybdenum at 0.1 µmole and tungsten at 1 µmole per ml. severely inhibited the enzyme's action. Inhibition was reversed at higher concentrations. Molybdenum action was specific. Inhibition was lost during prolonged dialysis. It is proposed that ribose-5-phosphatase is a distinctly different enzyme than the other acid phosphatases previously reported for sugarcane. It appears to be functional in the CO₂ fixation pathway elucidated by Calvin.