Biosynthesis of marsupial milk oligosaccharides: characterization and developmental changes of two galactosyltransferases in lactating mammary glands of the tammar wallaby Macropus eugenii

Tammar wallaby (Macropus eugenii) mammary glands contain two galactosyltransferases of which the first, 4βGalT, is a UDP-galactose: N-acetylglucosaminyl β1 → 4-galactosyltransferase equivalent to the A protein of the lactose synthase of eutherian mammals. The second enzyme, 3βGalT, is a UDP-galactose:lactose β1 → 3-galactosyltransferase, not previously identified in mammary glands of any species, which catalyses the formation pf Galβ1 → 3Galβ1 → 4Glc from lactose. The two enzyme activities, as well as the lactose synthase activity, have been characterised with respect to the effects of pH, apparent Km values, effects of bovine and tammar α-lactalbumins, heat sensitivity and identity of products. Studies on the substrate specificity and heat sensitivity of the 3βGalT activity suggest that this enzyme may catalyse the β-galactosylation of Gal β1 3Galβ1 → 4Glc as well as of lactose. The activity of the 3βGalT, unlike that of the 4βGalT, changes dramatically during the course of lactation in parallel with similar changes in the carbohydrate content of tammar milk. © 1991.