Rat adipose tissue glycogen synthase. Evidence for multiple discrete kinetic species and their interconversion

• Published in: The Journal of biological chemistry 1976-04, Vol.251 (8), p.2316-2322
• Main Author: Eichner, R D
• Format: Article
• Language: English
• Subjects: Adipose Tissue - enzymology; Animals; Cyclic AMP - pharmacology; Diabetes Mellitus - chemically induced; Diabetes Mellitus - enzymology; Enzyme Activation - drug effects; Glycogen Synthase - metabolism; Kinetics; Macromolecular Substances; Magnesium - pharmacology; Male; Molecular Weight; Rats; Streptozocin
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1016/S0021-9258(17)33588-3

Rat adipose tissue glycogen synthase has been kinetically characterized. The classical D form has an apparent Km for UDP-glucose of 0.7 mM and 0.4 mM in the absence and presence of glucose 6-phosphate, respectively. The apparent Ka for glucose 6-phosphate is 0.6 mM. The effect of glucose 6-phosphate on the D form is to enhance the Vmax 7-fold. The I form is also affected by glucose 6-phosphate (Ka, 0.025 mM) but the Vmax is increased only by 20%; apparent Km values for UDP-glucose are 0.4 mM and 0.045 mM in the absence and presence of glucose 6-phosphate, respectively. In addition, two new kinetically distinguishable forms have been observed. The first, designated glycogen synthase Q, arises from an Mg2+ATP-dependent deactivation of the I form. The apparent Km values of glycogen synthase Q for UDP-glucose are identical with those of the I form; however, the apparent Ka for glucose 6-phosphate (0.2 mM) is 8-fold higher than that for the I form and one-third that for the D form. Preparations from fasted or diabetic rats contain a form of glycogen synthase, designated glycogen synthase X, that has a much lower affinity for glucose 6-phosphate than the D form (apparent Ka, 3 mM); the apparent Km values for UDP-glucose are similar to those of the D form (0.7 mM and 0.3 mM in the absence and presence of glucose 6-phosphate, respectively). In preparations from fasted rats a stepwise Mg2+-dependent conversion was demonstrated of synthase X to D to Q to I; this sequential conversion was reversed on incubation with Mg2+ATP. In preparations from fed rats, synthase Q could be generated either by limited activation (from the D form) or, after conversion to the I form, by deactivation with Mg2+ATP. However, even prolonged incubation with Mg2+ATP failed to generate the D (or X) form.