UDP-N-acetyl-5-thio-galactosamine is a substrate of lactose synthase

Uridine 5′-(N-acetyl-5-thio-galactosaminyl diphosphate) (UDP-5SGalNAc) was synthesized from an N-acetylgalactosamine derivative via ring opening-recyclization approach. UDP-5SGalNAc was active as a donor substrate for lactose synthase, the complex of galactosyltransferase and lactalbumin, giving the disaccharide mimic (5SGalNAcβ[1→4]GlcNAc) which has a sulfur in the ring of the non-reducing sugar. The initial rate of the formation of the disaccharide mimic was 0.23 % that for the natural disaccharide (GalNAcβ[1→4]GlcNAc).