PLASMA CONCENTRATION AND PROTEIN BINDING OF OESTRIOL AND ITS CONJUGATES IN PREGNANCY

Abstract

ABSTRACT Oestriol (OE3), oestriol-3-sulphate-16-glucosiduronate (OE3-3S,16Gl), oestriol-3-glucosiduronate (OE3-3Gl), oestriol-16-glucosiduronate (OE316Gl) and oestriol-3-sulphate (OE3-3S) concentrations were measured in a pool of plasma collected from 38 healthy pregnant women at 32 to 36 weeks of gestation. Using tritiated internal standards, OE3 and its 4 conjugates were isolated by countercurrent distributions and, following enzyme hydrolysis and chromatography, quantitated by fluorometry: OE3=1.48 μg/100 ml (13.8%), OE3-3S,16Gl = 3.78 μg/100 ml (35.2%), OE3-3Gl3-3Gl = 1.09 μg/100 ml (10.2%), OE3-16Gl= 1.59 μg/100 ml (14.8%) and OE3-3S = 2.79 μg/100 ml (26%). OE3 and its conjugates were found to bind to serum proteins in Sephadex G-25 gel filtration and equilibrium dialysis studies. Considerably more OE3-3S and OE3-3S,16Gl than unconjugated OE3, and particularly OE3-16Gl and OE3-3Gl was bound at corresponding serum protein concentrations. When serum was fractionated by starch block electrophoresis and gel filtration on Sephadex G-200, OE3 conjugates were found to be bound by protein(s) present in the albumin fraction. The difference in plasma protein binding between OE3-3S/OE3-3S,16Gl and OE3-16Gl/OE3-3Gl explains, in part, the preponderance of OE3 sulphoconjugates in plasma and their paucity in urine, as well as the rapid renal excretion of OE3-16Gl and OE3-3Gl. Protein binding thus appears to play an important role in OE3 conjugate transport, metabolism and excretion.