Characterization of serum SHBG isoforms in prepubertal and pubertal girls

OBJECTIVE SHBG is a circulating glycoprotein that binds dihydrotestosterone , testosterone and oestradiol with high affinity and low capacity. In girls , serum concentrations of SHBG gradually decrease with age due to a true fa ll in concentration and not to a change in the binding characteristics. The aim of our study was to determine the pattern of serum SHBG isoforms in no rmal girls in early childhood (ECh), late childhood (LCh) and puberty (P), SUBJECTS Fifteen normal girls were studied. They were divided into three gr oups according to their age: ECh: 3.7 +/- 0.9 years (mean +/- SD, n = 5); L Ch: 6.4 +/- 0.5 years (n = 5); and P: 13.4 +/- 1.5 years (n = 5). METHODS AND MEASUREMENTS Preparative isoelectric focusing was used to isola te SHBG isoforms according to their isoelectric point (pl),Three groups of isoforms were isolated: S-I: pI 5.2-5.4; S-II: pI 5.4-5.6 and S-III: pI 5.6 -5.8. Steroid levels in serum were determined by RIA. RESULTS The relative distribution of St-IBG isoforms (% of the total SHBG r ecovered, mean +/- SD) in the three groups of girls studied was: ECh: S-I: 25.8 +/- 9.9, S-II: 53 +/- 10.5 and S-III: 21.2 +/- 1.6; LCh: S-I: 8.8 +/- 3.1, S-II: 58.8 +/- 12.2 and S-III: 31.8 +/- 8.6; P: S-I: non-detectable; S -II: 51.6 +/- 12.6 and S-III: 48.4 +/- 12.6. CONCLUSION These results indicate that serum SHBG is more heterogeneous bef ore puberty. A considerable proportion of acidic isoforms are present early in life; they decrease during the prepubertal period and disappear when se xual development is completed. After puberty the glycoprotein is more homog eneous and an important proportion of more basic isoforms is present. At pu berty serum SHBG not only falls in concentration but also has an altered si alic acid content which modulates its circulating half-life.