In Vitro Effects of Silicon on the Action Patterns of Sugarcane Acid Invertase

Abstract

In vitro studies of invertase action patterns were conducted in the presence of silicon (Si). This element is known to inhibit the enzyme both in living tissues and cell-free preparations. Substrates included sucrose, raffinose, stachyose, and turanose. Sugarcane acid invertase was prepared from lyophilized immature storage tissue and was partially purified by salt fractionation, dialysis, and gel filtration. Enzyme products were studied by paper chromatography. Two products, fructose and glucose, were quickly obtained in large quantity from sucrose. Against raffinose the reaction proceeded more slowly but yielded a total of four products. This suggests a powerful work potential of cane invertase since complete acid hydrolysis of raffinose gives only three products. It is proposed that both 1 —> 6 and 1 —> 2 linkages are broken. Fructose appears by direct hydrolysis of raffinose and by secondary attacks upon the intermediate product sucrose. Use of stachyose as substrate gave additional evidence for hydrolysis of the 1 —> 6 linkage. Three products were obtained in the presence of only one 1 —> 2 fructosidic linkage. Incorporation of Si into invertase digests abruptly terminated the reactions against sucrose, raffinose and stachyose. The effective Si concentration was slightly more than 3 µmoles of Si per milliliter of digest. Significance of the invertase inhibition is mentioned both from the standpoint of increasing sucrose yield and as an analytical indicator of silicic acid content of plant tissues. When the disaccharide turanose was employed as substrate, increasing Si levels gave additional products rather than suspended enyme action. Four products appeared in the presence of 9 µmoles of Si. The substrates sucrose and raffinose yielded masses of variably-staining products at Si levels above 27 µmoles per milliliter. The latter products were of low chromatographic mobility and resembled fragments of hydrolyzed starch. To account for Si action on invertase it is proposed that a silicic acid gel forms around the enzyme in direct proportion to Si concentration. Inhibition of sucrose hydrolysis does not stem from severe protein structural changes. Rather, the hypothetical, gel-encased enzyme might cease to function against one substrate while retaining or increasing its capacity to attack others.