Isophosphorylase

Abstract

BOTH pure potato phosphorylase¹ and crystalline muscle phosphorylase² produce from glucose-1-phosphate a non-branched polysaccharide containing only α-1,4-glucosidic links and resembling amylose. By the action of extracts of yeast³, of heart, brain or liver⁴, branched polysaccharides are obtained containing up to 10 per cent of α-1,6-glucosidic linkages and resembling glycogen. In 1942, Meyer and Bernfeld⁵ reported the presence of an enzyme in yeast which brings about the phosphorolysis of the terminal α-1,6-glucosidic links of residual dextrin (the final degradation product of amylopectin by β-amylase). A purified potato phosphorylase was not able to effect this reaction. It was therefore concluded⁵ that there are two different phosphorylases, one of which effects the fission or synthesis of α-1,4-glucosidic links, the other the α-1,6-glucosidic links involved in branching. Cori⁶ has found an enzyme in liver extract and also in heart extract Which he calls the ‘branching factor', and which is able to produce glycogen by simultaneous action with the crystalline muscle phosphorylase. Haworth, Peat and Bourne⁷ have described a thermolabile factor which they term the 'Q' and to which they attribute the property of synthesizing the α-1,6-glucosidic links in amylopectin. Their 'Q enzyme' has, in addition, an amylatic action.