Comparative measurements of plasma binding capacity and concentration of human sex hormone binding globulin

Comparisons are made of the plasma binding capacity and concentration of sex hormone binding globulin. Concentration was measured by electroimmunodiffusion standardised in terms of mass of the protein and binding capacity by two methods measuring the binding of 5 alpha-dihydrotestosterone. Isolation of steroid bound by SHBG was by either ammonium sulphate precipitation or cellulose filter discs. Both binding methods correlate highly with electroimmunodiffusion indicating they respond similarly to changes in the plasma concentration of the protein. However, they do not equally reflect the actual concentration. Estimates of the molecular mass of the protein of 188000 and 100000 from the precipitation and disc methods respectively, suggest the former measures less of the protein present than does the latter. A parallel reduction in binding capacity and concentration is seen in obese post-menopausal females. This previously unreported finding suggests that the reduced plasma binding capacity of sex hormone binding globulin in obesity is not due to altered or impaired steroid binding.