Evidence for Leptin Binding to Proteins in Serum of Rodents and Humans: Modulation With Obesity

Many hormones circulate bound to serum proteins that modulate ligand bioactivity and bioavailability. To understand the biology of leptin action, we investigated the presence of leptin binding proteins in serum. ¹²⁵I-labeled leptin binds competitively to at least three serum macromolecules with molecular masses of ∼ 85, ∼ 176, and ∼240 kDa in rodents and ∼176 and ∼240 kDa in humans. The ability to bind appears to involve sulfhydryl/disulfide interactions because it is inhibited under reducing conditions. When serum is added to recombinant ¹²⁵I-leptin, there is a shift in sedimentation of 125I-leptin as analyzed by sucrose gradient centrifugation from ∼ S1.9 to ∼ S4.3. This shift is markedly attenuated in serum from obese mice (ob/ob, db/db, brown-fat ablated, gold-thioglucose treated, high-fat fed) compared with that from nonobese controls. The size distribution of endogenous serum leptin as determined by radioimmunoassay (RIA) after sucrose gradient centrifugation is also consistent with saturation of binding in hyperleptinemic obesity. In humans, free leptin increases with BMI. Thus, in lean rodents and humans a large proportion of leptin circulates bound to several serum proteins. Free leptin is increased in serum of obese subjects, which may alter leptin bioactivity, transport, and/or clearance.