1. Rat liver microsomal preparations incubated in 1% Triton X-100 at 37°C for 1h released about 60% of the membrane-bound UDP-galactose–glycoprotein galactosyltransferase (EC 2.4.1.22) into a high-speed supernatant. The supernatant galactosyltransferase which was solubilized but not purified by this treatment had a higher molecular weight than the serum enzyme as shown by Sephadex G-100 column chromatography. 2. The galactosyltransferase present in the high-speed supernatant was purified 680-fold by an affinity-column-chromatographic technique by using a column of activated Sepharose 4B coupled with α-lactalbumin. The galactosyltransferase ran as a single band on polyacrylamide gels and contained no sialyltransferase, N-acetylglucosaminyltransferase or UDP-galactose pyrophosphatase activities. 3. The purified membrane enzyme had properties similar to serum galactosyltransferase. It had an absolute requirement for Mn2+ that could not be replaced by Ca2+, Mg2+, Zn2+ or Co2+, and was active over a wide pH range (6–8) with a pH optimum of 6.5. The apparent Km for UDP-galactose was 10.8μm. The protein α-lactalbumin modified the enzyme to a lactose synthetase by increasing substrate specificity for glucose in preference to N-acetylglucosamine and fetuin depleted of sialic acid and galactose. 4. The molecular weight of the membrane enzyme was 65000–70000, similar to that of the purified serum enzyme. Amino acid analyses of the two proteins were similar but not identical. 5. Sephadex G-100 column chromatography of the purified membrane enzyme showed a small peak (2–5%) of higher molecular weight than the purified serum enzyme. Inclusion of 1mm-ε-aminohexanoic acid in the isolation procedures increased this peak to as much as 30% of the total enzyme recovered. Increasing the ε-aminohexanoic acid concentration to 100mm resulted in no further increase in this high-molecular-weight fraction.

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