Evidence of free leptin in human seminal plasma

Leptin is an adipose tissue-secreted hormone that actively participates in the regulation of energy homeostasis. Besides this principal role, leptin has been implicated in a large variety of neuroendocrine, paracrine, and autocrine actions involved in the regulation of reproductive function in both experimental animals and humans. Although the participation of leptin in female reproduction is well established, any role in male reproductive function is at best tenuous. The aim of this study was to ascertain whether true leptin is present in human seminal fluid and the tissue of its production. Pooled human seminal plasma obtained from healthy donors showed by direct radioimmunoassay (RIA) the presence of radioimmunoassayable leptin. Serial dilutions of unextracted semen paralleled the RIA standard curve, also devoid of interference in the assay. To prove that this activity was true leptin, seminal plasma was subjected to size-exclusion chromatography, which showed that leptin immunoreactivity eluted with the same partition coefficient as cold leptin, I-125-leptin, and I-125-leptin preincubated with seminal plasma. The results demonstrate that true leptin was present in semen in a free form, i.e., without binding proteins. The presence of leptin charge variants in seminal plasma was assessed by anion-exchange chromatography, which showed two peaks of leptin inmunoreactivity, while I-125-leptin eluted as a single peak. Preincubation of I-125-leptin with seminal fluid converted the single peak into a double peak, indicating that components of the seminal fluid introduce a charge variation in leptin. Leptin levels in seminal plasma of 40 healthy men were 0.95 +/- 0.19 ng/mL while in 5 vasectomized men the levels were 0.92 +/- 0.25 ng/mL, suggesting that testicular tissues were not the source of seminal leptin. No correlation was observed between leptin concentrations in semen and the physical characteristics of semen samples or physical characteristics of spermatozoids, such as concentration, motility, vitality, or morphology. In conclusion it was unambiguously demonstrated that human leptin is present in seminal fluid, with at least two charge variants and no binding proteins, the most likely source being either seminal vesicles or prostate tissue. The role of seminal fluid leptin in the male reproductive function or sperm capacitation is at present unknown.