Studies on UDPG: alpha-glucan transferase in frog muscle

Abstract

The enzyme system responsible for the biosyinthesis of glycogen in frog muscle is a D form of UDPG : a-i,4-glucan a-4- glucosytransferase with special characteristics.

As in other D forms studied, this enzy­me is competitively inhibited by S04= and PO4= on the activation site of the glucose-6-P.

Mg++ activation initially observed is kinetically explained buy a 10-fold in­ crease in the affinity of the allosteric activator glucose-6-P for its center. This effect is also seen by using a nonmetabolizable, very poor activator, 2-deoxy- glucose-6-P.

Other divalent cations such as Ca++ produce similar activation. This suggests a role in glycogen biosynthesis for these cations together with PO4= and hexosaphosphates in certain cells.

The ATP-Mg effect observed strongly supports the hypothesis of an inactive form of the enzyme in this species, pre­cursor of the D form, wich would be more phosphorylated than the active form. A phosphatase system, activated or protected by mercaptoethanol, would give rise to the D form during preincubation of the enzyme.