[Mechanism of the reaction catalyzed by glycogen synthetase I in rabbit skeletal muscle]. / K voprosu o mekhanizme reaktsii, kataliziruemoi glikogensintazoi I skeletnykh myshts krolika.

1.5-Gluconolactone was shown to exert a strong inhibiting effect on the activity of rabbit skeletal muscle glycogen synthase I. The Ki values determined according to Dixon (0.13 mM) and Chuang and Bell (0.14 mM) coincide with the Km value for UDPG. Within the pH range of 5.4-7.0, N-ethyl-N'-(3-dimethylaminopropyl) carbodiimide (less than or equal to 3 mM) specifically inhibits the carboxyl group, which was supported by the reactivation of the enzyme under mild alkaline conditions. The reversible competitive inhibitor of glycogen synthase and the UDP reaction product as well as 1.5-gluconolactone afford an effective protective effect. It is supposed that the reaction catalyzed by rabbit skeletal muscle glycogen synthase I results in the formation of an intermediate carbonium ion. An essential role in the enzyme activity belongs to the carboxylic group of the active center.

[Covalent modification of glycogen synthase I in rabbit skeletal muscle by 5'-fluorosulfonylbenzoyl derivatives of uridine]. / Kovalentnaia modifikatsiia glikogensintazy I skeletnykh myshts krolika 5'-ftorsul'fonilbenzoil'nymi proizvodnymi uridina.

Preincubation of rabbit skeletal muscle glycogen synthase I with 5'(p-fluorosulfonylbenzoyl)uridine (p-FSBU) or 5'-(m-fluorosulfonylbenzoyl)uridine (m-FSBU) results in a decrease of the enzymatic activity with time. UDP, the product of the glycogen synthase reaction, protects the enzyme against inactivation. It was shown that the covalent binding of the enzyme with each of its inhibitors is preceded by the formation of a reversible complex with Ki = 0.285 and 1.820 mM for p-FSBU and m-FSBU, respectively. This reaction is of pseudo-first order with respect to the inhibitors. The K2 values for p-FSBU and m-FSBU are nearly identical, i. e. 0.050 and 0.042 min-1, respectively, which suggests modification of the same functional group of the enzyme. The number and reactivity of the SH-groups of glycogen synthase I were determined, using 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB). The total number of SH-groups determined in the presence of 8 M urea is six as calculated per enzyme monomer. In the native enzyme DTNB titrates two SH-groups which according to their reactivity can be subdivided into 2 groups. The more reactive SH-group is localized in the active center of the enzyme. p-FSBU induces covalent blocking or screening of this SH-group during specific interaction with the active site of glycogen synthase I.