The partial purification and properties of pig brain glycogen synthase

Both the I (independent of glucose 6-phosphate) and D (dependent on glucose 6-phosphate) forms of glycogen synthase (UDP-glucose:glycogen alpha-4-glucosyltransferase EG 2.4.1.11) have been partially purified from pig brain and the kinetic constants of the enzymes have been examined. The Km for UDP-glucose for the I form increased from 0.11 to 0.5 mM when the temperature was raised from 25 to 37 degrees. When glucose 6-phosphate was present, the Km for UDP-glucose was decreased to 0.03 and 0.08 mM at 25 and 37 degrees, respectively. The amount of glucose 6-phosphate required to produce half-maximal stimulation decreased with increasing UDP-glucose concentration at both temperatures but increased with increasing temperature. The Km for glucose 6-phosphate at 0.03 and 0.20 mM UDP-glucose was 0.13 and 0.10 mM, respectively, at 25 degrees. At 37 degrees and 0.125 and 4.0 mM UDP-glucose the Km for glucose 6-phosphate was 0.32 and 0.04 mM, respectively. The Km for UDP-glucose for the D form at 0.75, 2.0, and 10 mM glucose 6-phosphate was 0.71, 0.50, and 0.42 mM at 25 degrees. At higher temperatures the apparent affinity for the substrate was decreased; at 37 degrees, the Km for UDP-glucose at 0.75 and 2.0 nM glucose 6-phosphate was 5.75 and 1.42 mM, respectively. The requirement for glucose 6-phosphate was decreased when UDP-glucose concentrations were increased; at 0.5 and 5.0 mM UDP-glucose concentrations, the Km for glucose 6-phosphate was 22.7 and 1.82 mM at 25 degrees. As was the case with the I form, the apparent Km for glucose 6-phosphate increased at higher temperatures. At 37 degrees, the Km for glucose 6-phosphate at 0.5 and 5.0 mM UDP-glucose was 43.5 and 6.15 mM. The temperature coefficient for the maximum velocity was 10.1% per degree for synthase I and 8.5% per degree for synthase D between 25 and 37 degrees. The D form of synthase was calculated to be virtually inactive under normal physiological conditions with the substrate concentrations found in the brain. The enzymatic activity calculated for synthase I correlates well with the observed rate of incorporation of UDP-[U-14C]glucose into brain glycogen.