Mechanisms of regulation of glycogen phosphorylase activity in Saccharomyces carlsbergensis

Botanisches Institut der Universität Bonn, Kirschallee 1, D-5300 Bonn 1, Federal Republic of Germany ABSTRACT Summary: The content of glycogen phosphorylase (1,4- - D -glucan:orthophosphate - D -glucosyl-transferase, EC 2.4.1.1 ) in yeast ( Saccharomyces carlsbergensis ) cells depended on the growth phase. Cells of the early exponential phase under carbohydrate-limited conditions showed low, but significant, phosphorylase activity; the activity markedly increased in the late exponential growth phase, concomitant with the appearance of measurable phosphorylase antigen. This pointed to an induction of the enzyme. During the interexponential phase (the slow proliferation phase during the diauxic growth of the culture, when the cells switch to utilization of accumulated ethanol) and the stationary growth phase, phosphorylase concentration remained constant while its specific molecular activity increased further, probably caused by conversion of the enzyme to an active form. During transition of stationary phase cells to growth, phosphorylase activity and concentration slowly decreased in the cells at a rate compatible with dilution by newly synthesized proteins. A residual activity always remained, which could be attributed to the presence of active phosphorylase, detectable by activity staining after gel electrophoresis in the presence of glycogen. No direct correlation could be detected between the specific molecular activity of phosphorylase and glycogen metabolism. This indicated that covalent modification of the enzyme regulated the total capacity of the enzyme available to the cell, rather than the actual activity limiting glycogen breakdown.