Influence of the presence of OB-Re on leptin radioimmunoassay.

Leptin, a hormone derived from adipose tissue, regulates energy homeostasis and body weight. In the mouse, serum leptin levels, when measured by radioimmunoassay (RIA), increase by a factor of more than 50 times during pregnancy, compared with those in the non-pregnant state. It is well known that mouse placenta produces the secretory isoform of the leptin receptor, OB-Re. In order to investigate the issue of whether serum leptin levels are actually increased during pregnancy or whether the increased OB-Re concentration plays a role in this phenomenon, serum leptin levels were determined by the immunoprecipitation of leptin using anti-leptin antibody, and were found to be increased only by about ten times during pregnancy. To investigate the influence of OB-Re on leptin measurement by the RIA procedure, serum leptin levels were measured by the RIA after the addition of OB-Re to the serum. The apparent values of leptin levels increased in parallel with the amount of OB-Re added to the serum. Leptin levels, as determined by the RIA, might therefore provide artificially high values when serum levels of the secretory form of OB-R are high, in cases, for example, such as the last period of pregnancy in mice.