Purification by affinity chromatography and some properties of microsomal galactosyltransferase from pig thyroid

Abstract

Membrane-bound 4-β-galactosyltransferase (lactose synthase; UDP galactose: d-glucose 4-β-galactosyltransferase, EC 2.4.1.22) was purified 1500-fold to near homogeneity from pig thyroid microsomes with about 30% yield. The purified enzyme behaved as a lipophilic protein, rapidly losing activity and aggregating if not supplemented with either Triton X-100 or serum albumin (both of these were equally effective for long-term stabilization). The enzyme preparation showed an absolute requirement for Mn 2+, which could not be replaced by other cations. Catalytic properties were very similar to those reported for soluble forms of the enzyme in biological fluids. The purified galactosyltransferase showed a major protein band of approx. 74 000 daltons on sodium dodecyl sulfate gel electrophoresis. On gel filtration, enzyme activity was eluted at approx. 70 000 daltons. It is concluded that the membrane-bound thyroid galactosyltransferase is a monomeric protein significantly larger than the soluble forms of this enzyme described earlier; but it resembles recently reported galactosyltransferases from sheep mammary Golgi membranes and liver microsomes.