The reversible formation of starch from glucose-1- phosphate catalysed by potato phosphorylase

Abstract

An enzyme (phosphorylase), which catalyses the reversible conversion of starch and inorganic phosphate to glucose-1-phosphate, has been found to occur in a num ber of higher plants. A m ethod is described for obtaining from potato tubers highly active preparations of phosphorylase which are free from certain enzymes which catalyse alternative reactions. The reversibility of the transformations: starch + inorganic phosphate ^ glucose-1-phosphate, is shown by the fact that the reaction proceeds in either direction until th e ratio of inorganic orthophosphate/glucose-1-phosphate attains a value which is not significantly altered by wide variations in the concentrations of the reactants or the enzyme. The equilibrium value of this ratio is not detectably affected by alterations in the concentration of starch but varies considerably with alterations in the concentration of hydrogen ion. This is shown to be due to the effects upon the extents of dissociation of the inorganic and esterified phosphate, the di-valent ions alone determ ining the equilibrium ; thus the ratio (HPO 4 )=/(C 6 H II O 5 . O . PO 3 )= remained approximately constant at a value of 2-2 when the pH value was varied from 5 to 7. When glucose-1-phosphate is added to the enzyme there is an initial lag in the liberation of phosphate and the formation of starch. This induction phase is abolished, and the reaction velocity greatly increased, by small additions of starch or maltose. The preparation on a large scale of glucose-1-phosphate (in the form of the crystalline potassium salt) by allowing crude potato juice to act in starch is described. Observations on the polysaccharide formed from pure glucose-1 -phosphate under the action of purified phosphorylase indicate that it resembles closely the so-called amylo-amylose fraction of natural potato starch. The mechanism of this reversible interconversion and its bearing on problems of carbohydrate metabolism in the plant are considered.