Dextran acceptor reaction of Streptococcus sobrinus glucosyltransferase GTF-I as revealed by using uniformly C-13-labeled sucrose

A sucrose glucosyltransferase GTF-I from cariogenic Streptococcus sobrinus transferred the uniformly C-13-labeled glucosyl residue ([U-C-13]Glc) from [U-C-13]sucrose to exogenous dextran T500 at the nonreducing-end, mostly by alpha-(1 -->6) linkages and partially by alpha-(1 -->3) linkages, as revea led by the C-13-C-13 NMR coupling pattern. With increasing amounts of [U-C- 13]sucrose, transfer of [U-C-13]Glc to the alpha-(1 -->3)-linked chain beca me predominant without increase in the number of chains. The transfer of [U -C-13]Glc to an isomaltopentaose acceptor occurred similarly to its transfe r to T500. alpha-(1 --> 3)-branches in the [U-C-13]dextran, specifically sy nthesized from [U-C-13]sucrose by a Streptococcus bovis dextransucrase, wer e not formed by GTF-I, as judged by the observation that a newly-formed alp ha -1,3,6-banched [U-C-13]Glc was not detected, which could have been forme d by transferring the unlabeled Glc from sucrose to the internal alpha-(1 - -> 6)-linked [U-C-13]Glc at C-3. The C-13-C-13 one-bond coupling constants ((1)J) were also recorded for the C-1-C-6 bond of the internal alpha-(1 --> 6)-linked [U-C-13]Glc and of the nonreducing-end [U-C-13]Glc. (C) 2001 Els evier Science Ltd. All rights reserved.