Further studies on human plasma aldosterone-binding globulins: isolation, purification, and evidence for heterogeneity.

The present study further characterizes the aldosterone-binding globulin (ABG) in human plasma. Isolations were performed by ultrafiltration, Blue Sepharose CL-6B, or ion-exchange chromatography and gel filtration. New ABG species were identified: interconvertible with ABG in plasma is a thermolabile (MW less than 70,000) and a thermostable ABG macromolecule (MW 27,500). All forms were extensively purified to electrophoretic homogeneity (SDS-PAGE) showing microheterogeneity in isoelectric focusing (pI 4.76 and 4.8) and reversible high-affinity binding for aldosterone. Chromatography of lipoprotein-free plasma in DEAE-Sephadex A-50 gel (ionic gradient) revealed the presence of a dimer of ABG (MW 55,000) which was found to be interconvertible with ABG and was the main carrier of aldosterone in normal fresh plasma. Selective adsorption of albumin from lipoprotein-free plasma on Blue Sepharose CL-6B permitted in a single novel step the complete resolution of bound aldosterone (binding ABG) and bound cortisol (binding corticosteroid-binding globulin). Both ABG and the dimer can be separated from corticosteroid-binding globulin by Blue Sepharose chromatography. A new homeostatic mechanism in which ABG participates in blood pressure regulation by interacting with aldosterone is postulated.