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Characteristics of Aldosterone Binding in Human Plasma^*

Section of Endocrinology, University of Southern California, Los Angeles County Medical Center Los Angeles, California 90033

Address requests for reprints to: Dr. Zipser, USC School of Medicine Room 18-632, 2025 Zonal Avenue, Los Angeles, California 90033.

[³H]Aldosterone and [¹⁴C]cortisol were incubated with either fresh human plasma or purified transcortin [corticosteroid- binding globulin (CBG)] and subjected to gel filtration and gel electrophoresis to investigate the possible presence of a specific aldosterone-binding globulin. Sephadex G-100, G-150, and G-200 columns isolated only one protein fraction that bound [³H]aldosterone, and this fraction also bound [₁₄C]cortisol. CBG incubated with either steroid ran parallel to this fraction. Excess aldosterone, cortisol, progesterone, and spironolactone completely displaced both labeled steroids from plasma protein binding to the unbound fraction, whereas steroids that do not bind to CBG, such as estrone, androsterone, and 17-isoaldosterone, were without effect. Polyacrylamide gel electrophoresis of the peak G-200 fractions and of whole plasma incubated with labeled steroids revealed a common protein fraction that bound both steroids. CBG incubated with both steroids migrated to the identical band. Quantitation of [³H]aldosterone binding by equilibrium dialysis demonstrated that 36 ± 3% of aldosterone was unbound, 47 ± 1% was bound to albumin, and 17 ± 2% was bound to nonalbumin proteins. Binding to purified CBG in the same system was almost identical (17 ± 2%) to the nonalbumin protein binding. No evidence of a specific aldosterone-binding protein was detected. We conclude that CBG accounts for essentially all of the nonalbumin plasma binding of aldosterone.

^* This work was supported by grants from the NIH (HL-21112 and RR-43) and a Research Award from the American Heart Association- Greater Los Angeles Affiliate (567-Cl).

Received May 1, 1979.