Some Biochemical Properties of Soluble and Bound Potato Tuber Peroxidase

摘要

ABSTRACTSoluble and bound forms of peroxidase were extracted from potato tubers using 0.05M sodium phosphate buffer (pH 6.0) and the same buffer containing 0.811 KC1 (pH 6.0), respectively. The soluble and bound potato tuber peroxidase (PTP) fractions represented about 60 and 40, respectively, of the total peroxidase extracted. Soluble PTP consisted of high levels of protein and low peroxidase activity relative to bound PTP, with the specific activity of the former being about 15‐fold lower than that of the latter. The biochemical properties of soluble and bound PTP in both the crude and the partially purified form were studied. The pH curves of soluble PTP and bound PTP were similar with a broad pH optima around 5.0–6.0. The stability of both forms of peroxidases to heat was also similar, with about 50 activity being lost after heating ffor 5 min at 70°C. The isoenzyme profile of equal amounts of activity of soluble PTP and bound PTP was different; anodic gel electrophoresis yielded ten and three isoenzymes, respectively, while cathodic runs showed the same number of isoenzymes in either fraction but with the isoenzymes of bound PTP being detected faster than those of soluble PTP. The contents of proteins and carbohydrates were much higher in the soluble PTP than in the bound PTP fraction. Partial purification of either forms of the enzyme was achieved by column chromatography on Sephadex G‐75 or on Sepharose 6B. Chromatography on Sepharose 6B resolved soluble PTP into one major peak (II) and one minor peak (I). Under identical conditions, bound PTP was resolved into two peaks with 80 and 20 of the activity in peaks II and III, jespectively. Column chromatography did not aid in resolving the isoperoxides. The molecular weight of peak II of soluble PTP and of bound PTP was estimated to be about 45,000, while that of peak III of bound PTP was 30,000. From analysis of the 30—90 ammonium sulfate fraction or of the partially purified enzyme on Concanavalin A‐Sepharose, it was concluded that the isoenzymes of soluble PTP and of bound PTP differ in their carbohydrate contem and/or composition or in the structure of their carbohyd