Previous experiments in vitro indicated that yeast glycogen synthetase might be regulated in vivo through the antagonism between adenosine triphosphate and adenosine diphosphate as inhibitors and glucose 6-phosphate as activator. When nonrespiring yeast cells were incubated with glucose and salts, the sum of intracellular adenosine triphosphate and adenosine diphosphate remained relatively constant after an initial peak. The formation of glycogen proceeded steadily until, as glucose was being consumed, the concentration of glucose 6-phosphate fell under 0.4 mM; thereafter the rate of polysaccharide accumulation declined. When the glucose concentration of the medium was maintained constant, addition of ammonium ions brought the increase in glycogen to a sudden stop. At the same time glucose 6-phosphate decreased abruptly, while adenosine triphosphate and adenosine diphosphate showed only minor changes. Uridine diphosphate glu cose was practically constant throughout the experiment; therefore regulation of glycogen synthetase by variations in substrate concentration is very unlikely. It was also shown that the effect of ammonium ions could not be explained by an increased degradation of glycogen or by changes in the properties or amount of glycogen synthetase. In other experiments yeast capable of respiration was incubated with glucose and salts. The rate of glycogen increase was rapid at first, but soon declined, despite a constant concentration of glucose in the medium. At the same time the concentration of glucose 6- phosphate in the cells fell continuously, while adenosine triphosphate, adenosine diphosphate, and uridine diphosphate glucose did not undergo pronounced variations. Thus, the results obtained are in general agreement with the hypothesis advanced previously. A modified method for determination of glycogen in yeast is described. © 1969, American Chemical Society. All rights reserved.
