Among the many enzymes that constitute the glycosylation pathway of the endoplasmic reticulum and Golgi compartments, /31,4-galactosyItransferase clearly undergoes regulation in connection with its apparently rate-limiting role in the synthesis of the milk sugars lactose and sialyllactose in the mammary gland [ l , 21. Large and rapid changes in lactose synthesis follow nutritional change, yet the mechanism of galactosyltransferase regulation is uncertain [3-51. Indeed, its location within the Golgi trans-cisternal lumen raises the possibility that it does not come into contact with the common metabolic signals of the cytosol because the Golgi membrane is not readily permeable to calcium ions, nucleotides or many other small molecules for which no specific carrier exists. The one established regulator of mammary galactosyltransferase is alactalbumin, a protein synthesized within the mammary secretory cells that passes via the Golgi into the milk. It specifically evokes the galactosylation of free glucose, yielding lactose, without impairing the ability of the enzyme to galactosylate terminal GlcNAc residues of glycoproteins [6]. Alpha-lactalbumin has a high-affinity binding site for calcium, and also one for zinc [7, 81. The significance of these sites is, however, unclear. The surge of newly synthesized a-lactalbumin at the onset of lactation evidently initiates most of the lactose synthesis at that time; there is no evidence, however, that alactalbumin mediates the nutritional control of lactose synthesis during established lactation. Attention therefore focuses upon the activation of galactosyltransferase by manganese (11) ions, as reported over 30 years ago [9].Many glycosyltransferases, although by no means all, are activated by millimolar concentrations of bivalent metal ions, of which Mn" is the commonest and often the most efficient (see [lo]). Galactosyltransferase has two types of Mn'+-activated site. Occupation of the high-affinity site I is essential for activity, which is best evoked by MnL+ (apparent K,, of approximately 10-20 ,ml) and less well by FeL+, Coz+ or ZnL+. Two high-affinity sites per 40 k h subunit have been reported [ l l ] . Activity is further enhanced when the low-affinity site is also occupied by Mn" (apparent K,,, of approximately 800 p~) or less well by M$+, CaL+, SrL+ or Ha'+. The enzyme is not active when only site I1 is occupied [ 12-15]. Hoth of these activationsare of unknown mechanism, and raise a problem in that their Mn" requirements probably exceed likely physiological [MnL+] by 100-fold and 10 000-fold, respectively. The suggestion that CaL+ may be the natural site I1 activator is not well supported by the evidence. Indeed, the suitable use of chelators in assaying galactosyltransferase within intact Golgi membrane vesicles permeabilized to metal ions with ionophore A23 187 virtually rules out bivalent metal ions as site I1 activators.A possible solution to the problem of site I1 lies in its stimulation by basic proteins such as histone, clupeine, pancreatic ribonuclease...
