Properties of Galactosyltransferase‐Enriched Vesicles of Golgi Membranes from Lactating‐Rat Mammary Gland

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Abstract
Lactose was synthesised within the lumen of purified Golgi membrane vesicles, prepared from lactating rat mammary gland, from externally added glucose and UDP‐galactose. An apparent Km of 1.5 mM was shown towards glucose at anomeric equilibrium, but only β‐glucose was utilised. Two apparent Km values, about 17 μM and 112 μM, were shown towards UDP‐galactose. 5‐d‐Thioglucose, 6‐deoxy‐d‐glucose and 6‐deoxy‐6‐chloro‐d‐glucose were alternative substrates, acquiring α‐lactalbumin dependence when the vesicles were lysed with detergent. Substrates independent of α‐lactalbumin, or inhibited by it, included a wide range of, N‐acylated glucosamines as well as phenyl‐β‐glucoside. The galactosylation of these by vesicle preparations could be ascribed to a proportion of leaky vesicles. Suitably low concentrations of Triton X‐100 activated lactose synthesis by intact vesicles, indicating the membrane as a rate‐limiting feature of the system.

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