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Reactions catalyzed by human milk N‐acetyl lactosamine synthetase in the presence and absence of ä‐lactalbumins have been investigated by steady‐state kinetics. N‐Acetyl lactosamine synthesis and lactose synthesis in the absence of α‐lactalbumin appear to proceed by an ordered sequential reaction, with substrates attaching in the order: Mn²⁺, UDP‐galactose and monosaccharide. Under the conditions used (pH 7.4, 37 °C) the attachment of Mn₂₊ is not at thermo‐dynamic equilibrium and it appears that the enzyme can accept either free UDP‐galactose or its Mn²⁺ complex as substrate. Evidence is presented which suggests that the Mn²⁺ complex of UDP may be the final product released from the enzyme. Reactions in the presence of α‐lactalbumin proceed by a similar ordered mechanism. Kinetic effects observed in the presence of human α‐lactalbumin with three different monosaccharide acceptors …