Subcellular compartmentation in the synthesis of the milk sugars lactose and ?-2,3-sialyllactose

Abstract

This paper assembles published and new material to trace out the compartmental aspects of milk sugar synthesis in the mammary gland. The generation of lactose, from glucose and UDP-galactose, in thetrans cisternae of the Golgi stack is well established from biochemical and electron microscopy work. New experiments show that in the rat essentially all this lactose is available for sialyllation to α-2,3-sialyllactose, which accumulates in the same compartment. UDP arising from utilization of UDP-galactose inhibits lactose synthetase, but is removed by nucleoside diphosphatase. Therefore lactose synthetase, sialyltransferase and nucleoside diphosphatase form a coupled enzyme system located on the luminal face of the Golgi membrane. Transport of UMP to the cytosol, followed by its re-conversion via UDP and UDP-glucose to UDP-galactose, defines the operation of a uridine nucleotide cycle that links Golgi and cytosol compartments in the support of milk sugar synthesis. Evidence for carrier-mediated uniport or antiport of UMP and UDP-galactose across the Golgi membrane is discussed, together with the evidence for transport of glucose and inositol by small water-filled pores that forbid back diffusion of lactose, galactinol or sialyllactose. Possible intra-Golgi signals, achieving short-term regulation of milk sugar synthesis, include glucose, submicromolar free Mn(II) ions and cationic activator proteins. Experimental evidence for intracellular and intra-Golgi glucose concentrations in the range 0.1–0.4 mM imply lack of saturation of lactose synthetase. It is shown that lactose synthetase within undamaged Golgi membrane vesicles is about 100 times more sensitive to Mn ions than in damaged or solubilized membranes. Both lactose synthetase and sialyltransferase are greatly stimulated by basic proteins, and similar lactose synthease-activating material is extractable from Golgi preparations. Development of these features may designate the sorts of control exerted upon these Golgi enzymes, and perhaps others. This may uncover the mechanisms by which hormonal and nutritional changes, and manipulation of mammary tissue in vitro, cause profound alterations in milk sugar production.