Specific Cortisol Binding Protein in Porcine Follicular Fluid

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Abstract
A specific cortisol binding protein has been demonstrated in the follicular fluid of the porcine ovary. The binding of cortisol, progesterone, testosterone, Δ4-androstenedione and estradiol-17β was studied by equilibrium dialysis at 4°C in follicular fluid obtained from ovaries of mature pigs (6-12-months-old). The binding capacity of the follicular fluid protein with each steroid was variable. Cortisol demonstrated a specific binding to follicular fluid. The mean association constants of cortisol binding to follicular fluid from large, medium and small follicles were 1.99 ± 0.64 (SE) x 109 M-1 (N = 8), and 1.30 ± 0.62 (SE) x 109 M-1 (N = 6) and 2.05 x 109 M-1 (N = 2), respectively and were not significantly different from each other. The corresponding binding capacities were 0.60 ± 0 19 (SD) x 10-7, 1.06 ± 0.45 (SD) x 10-7 and 0.83 x 10-7 moles/l of undiluted follicular fluid. Tritiated cortisol bound to follicular fluid protein was displaced by corticosterone, deoxycorticosterone, progesterone and testosterone with relative affinities compared to cortisol (100%) of corticosterone 24%, deoxycorticosterone 20%, progesterone 11% and testosterone ++), magnesium (0.05M Mg++), thioglycerol (0.01M), L-cysteine (0.01M) and dithiothreitol (0.01M) did not alter the cortisol binding properties.

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