Hydrolytic Proteins of Sugarcane: The Q Enzyme

Abstract

A polysaccharide branching factor, apparently identical with the Q enzyme of potatoes, has been extracted from sugarcane leaves and partly characterized. Procedures used successfully by other workers to isolate the Q enzyme from potato extracts failed to yield the branching system free from other catalysts of sugarcane. A preparation satisfactory for characterization studies was obtained by precipitating protein from water extracts with lead acetate, eluting the precipitate with bicarbonate solution, and fractionating the bicarbonate extract with ammonium sulfate. The Q enzyme was precipitated between 0 and 35-percent saturation, at pH 7.2 and 22°C. The 0 to 35-percent fraction was virtually free of acid phosphatase and ß-amylase. Traces of starch phosphorylase (P enzyme) were present but caused no difficulty in the absence of phosphate buffers. However, an amylase of the a type was present, which interferred seriously with Q enzyme measurements in acid media, particularly in the area of pH 4 to 5. Q enzyme was moderately active between pH 6.5 and 9.0, with optimum pH at approximately 7.5. The α-amylase was very weak at pH values higher than 6.5. Soluble potato starch was employed as substrate throughout these studies. Optimum concentration was 2.0 to 2.5 mg./ml. of digest,  α-amylase was most active against 0.5 mg./ml. of digest. Q enzyme experienced an 85-percent inhibition by lead ions, 0.5 mg./ml., when acting upon a substrate level of 0.25 mg./ml. of digest. Increasing the starch concentration to 2.5 mg./ml. partly removed the lead effects, indicating that the inhibition was competitive. Fluoride, 0.025 mg./ml., completely eliminated α-amylase activity at pH 7.5, but had little effect at pH 4.0. α-amylase was also suppressed by copper. Dialysis against distilled water had little effect upon Q enzyme, but strongly retarded amylase after 6 and 10 hours. Twenty-two-hour dialysis caused no further changes in either enzyme. The most favorable temperature tested with Q enzyme was 30°C, although the system was fairly active at temperatures as low as 7°C. α-amylase was not active at 7°C, but achieved the maximum rate measured at 26°C. A series of sugarcane tissues was tested for Q enzyme, including meristem, leaves +1 and +2, leaves +6 and +7, sheaths +1 and +2, 8 to 10 nodes, and 8 to 10 internodes. Activity was far greater in leaves +6 and +7 than any other tissue tested, α-amylase was most active in the meristem preparations. Electrophoresis studies showed that the 0 to 35-percent fraction was composed of at least three distinct protein constituents. These were presumed to represent Q enzyme, α-amylase, and starch phosphorylase. The significance of Q enzyme in sugarcane is briefly discussed. A diagram is presented to illustrate the role of the polysaccharide branching factor in relation to the starch -and sucrose-forming pathways.