Hydrolytic Proteins of Sugarcane: The Acid Invertases

Abstract

Invertase has been extracted from meristem, leaf, sheath, node, and internode tissue of sugarcane. The meristem was the richest source for invertase acting under both acidic (pH 4.65) and neutral (pH 7.0) conditions. Acid invertase was extracted from meristem with water after the samples had been frozen, lyophilized, ground to a fine powder, and sonified in a powder-water suspension. Virtually all invertase was precipitated from solution with ammonium sulfate below 55-percent saturation. Acid invertase was precipitated primarily between 30- and 52-percent saturation. Within the acid-invertase preparation, two distinct enzymes were demonstrated, one, α-glucosidase, "taka-invertase", which attacks the glucose end of the sucrose molecule, and the other, ß-fructosidase, "yeast invertase", attacking the fructose end. ß-fructosidase is predominant by about 2 to 1. The possibility that α-glucosidase takes part in the degradation of glucosidically linked oligosaccharides, or products of polysaccharide hydrolysis, is discussed. Optimum pH for the acid-invertase preparation lay between 4.75 and 5.5. Optimum temperature was 44° C, and substrate concentration about 80 µmoles of sucrose per milliliter of digest. Invertase was inhibited by iodide, lead, and mercury at concentrations of 1.0, 0.5, and 0.0003 µmole/ml. of digest, respectively. Iodide inhibition was completely reversed by increasing substrate concentration, and lead inhibition was partly reversed. The inhibitory effects of mercury were not reversible. Arsenic and tungsten also inhibited invertase, but at relatively high concentrations, 5.0 and 10.0 µmoles/ml. of digest, respectively. Manganese doubled invertase activity at 0.5 µmole/ml. of digest, and as little as 0.005 µmole markedly stimulated the reaction. Prolonged dialysis (36 hours) against distilled water reduced invertase activity by about 95 percent. Added manganese revived the activity and stimulated the enzyme beyond predialysis levels. Activity was also revived by sucrose, maltose, galactose, glucose, and fructose, when these were added to the dialyzed enzyme. It was concluded that the active, acid-invertases are protein-sugar-manganese complexes, in which the protein constituent is virtually inactive in the absence of either manganese or sugar.