THE BINDING OF CORTISOL BY OVINE PLASMA PROTEINS

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Abstract
SUMMARY: Albumin was isolated from ovine plasma and its affinity for cortisol was determined by equilibrium dialysis at 37°. The value of Ka[σpa] for a 1 % (w/v) albumin solution was 0·275 which is similar to the value for human plasma albumin. The affinity constant of transcortin in ovine plasma was determined by equilibrium dialysis of diluted plasma at several concentrations of cortisol. The value found, Kt (37°) = 0·87 x 108 l./mole, is close to that found for human plasma transcortin by Mills (1962). The concentration of transcortin in ovine plasma, expressed as cortisolbinding capacity, was 6–49 μg. (mean 24 μg.) cortisol/l. These concentrations are much lower than those found in human plasma. The observation of Lindner (1964) that cortisol binding capacity did not increase during pregnancy in sheep has been confirmed. In sheep which were accustomed to handling, the mean concentration of cortisol in plasma was 17·8 μg./l. and of this amount 59% was bound to transcortin, 19 % to albumin and 22 % was not bound to protein.