The Occurrence and Properties of Soluble UDP-Galactose:Glycoprotein Galactosyltransferase in Human Amniotic Fluid

Abstract

Human amniotic fluid obtained by amniocentesis during the third trimester of pregnancy has been found to contain the enzyme for transferring galactose from UDP-galactose into various exogenous acceptors. The enzyme galactosyltransferase is present in soluble form and does not require Triton for activity. The optimum pH (pH 6.8), divalent cation requirement (Mn2+, 12.5 mM), optimum temperature (37 °C), and linearity with time and enzyme concentrations have been established. The amniotic fluid enzyme catalyzed the transfer of 60 nmol of galactose to fetuin acceptor per milliliter of amniotic fluid per hour and its apparent Km for UDP-galactose is 9.1 × 10−5 M. The enzyme is stable to storage at −20 °C for at least 2 months. Acid hydrolysis and β-galactosidase treatment of the labelled reaction product released all of the radioactivity as 14C-galactose. Uridine nucleotides inhibited the soluble enzyme whereas the adenine and cytidine nucleotides had no appreciable effect. The addition of α-lactalbumin in the assay caused the appearance of lactose synthetase activity and inhibition of N-acetyllactosamine synthetase activity. The specific activity of the enzyme in the amniotic fluid in human and rat is 30- to 40-fold higher than in their serum. Placenta, fetal liver, and fetal lungs of rat also showed considerable enzyme activity.Enzymes that transfer N-acetylglucosamine, sialic acid, and N-acetyl-4-galactosamine from their respective nucleotide sugars to exogenously added acceptors are also present in human amniotic fluid.