The Conversion of External Sugars to Internal Sucrose in Morning-glory Callus

Abstract

The conversion of external sugars to internal sucrose in morning-glory callus cells was investigated by administration of ¹⁴C-glucose, ¹⁴C-fructose and ¹⁴C-sucrose in the external medium, subsequent determinations of sugars and invertase in the medium and assays for intracellular distribution of some enzymes involved in sucrose biosynthesis.
1) ¹⁴C-glucose administration:
In callus cells, the label first appeared in glucose, which remained at relatively low level. Sucrose was subsequently labeled to continue a remarkable steady increase. Fructose was labeled very weakly, suggesting that free fructose might not be involved in sucrose biosynthesis. Labels also appeared in sucrose phosphate, glucose phosphate and fructose phosphate. In addition equal amounts of label appeared in the glucose and fructose moieties of sucrose. It was, therefore, inferred that sucrose was synthesized via sucrose phosphate.
2) ¹⁴C-fructose administration:
In further support of the above inference, the labeling patterns were similar to those in, i. e., ¹⁴C-glucose administration, except that fructose was more strongly labeled than glucose.
3) ¹⁴C-sucrose administration:
In callus cells, sucrose was labeled subsequent to the monosaccharides and monosaccharide phosphates, it suggested that its direct penetration, if any, preceded by hydrolysis.
4) Intracellular distribution of the enzymes involved in sucrose metabolism:
In cell wall fraction, acid invertase and acid phosphatase were presented. In cytoplasma fraction, acid and neutral invertase, hexokinase, glucosephosphate isomerase, UDPG-ppase, sucrose synthetase, sucrose-6-phosphate synthetase, and acid and alkaline phosphatase were present. Sucrose phosphorylase was not detected in both fractions.
These results suggested that internal sucrose was synthesyzed via sucrose-6-phos-phate from external sugars and that, prior to sucrose penetration, external sucrose was hydrolized by cell wall-bound invertase. On a basis of these findings, a possible scheme for sucrose metabolism in callus cells was proposed.