Sex steroids and their relationship to binding proteins in the serum of the marmoset monkey (Callithrix jacchus).

A sex hormone binding globulin (SHBG) similar to human SHBG was identified in marmoset serum based on its gel electrophoretic mobility, isoelectric point and steroid binding properties. Levels of serum SHBG were measured in immature and mature males, immature females and females during the luteal phase and pregnancy; serum progesterone, 5 alpha-dihydrotestosterone (5 alpha-DHT), testosterone, oestradiol-17 beta and oestrone were also measured. Mean (+/- S.E.M.) concentrations of SHBG in immature males (336 +/- 19 nmol/l) were higher (P less than 0.01) than those in mature males (251 +/- 13 nmol/l), whereas values in the groups of females were similar (359 +/- 12, 395 +/- 17, 397 +/- 39 nmol/l in immature, non-pregnant and pregnant females respectively). There was an inverse relationship between SHBG and the levels of testosterone (r = -0.67) and 5 alpha-DHT (r = -0.86) in males, but the correlation was significant (P less than 0.05) only for 5 alpha-DHT. There was no correlation between levels of SHBG and oestrogens in males or between levels of SHBG and any of the steroids measured in females. Equilibrium dialysis was used to assess the percentage of steroid in serum in the unbound form. Mean percentage values for unbound testosterone and 5 alpha-DHT were lower in immature males than in mature males (P less than 0.01) and negatively correlated with levels of SHBG (r = -0.78, testosterone; r = -0.56, 5 alpha-DHT).