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 b ioactivity and bioavailability. To understand the biology of leptin ac tion, we investigated the presence of leptin binding proteins in serum , I-125-labeled leptin binds competitively to at; least three serum ma cromolecules with molecular masses of similar to 85, similar to 176, a nd similar to 240 kDa in rodents and similar to 176 and similar to 240 kDa in humans, The ability to bind appears to involve sulfhydryl/disu lfide interactions because It is inhibited under reducing conditions. When serum is added to recombinant I-125-leptin, there is a shin in se dimentation of I-125-leptin as analyzed by sucrose gradient centrifuga tion from approximately S1.9 to approximately S4.3. This shift is mark edly attenuated in serum hom obese mice (ob/ob, db/db, brown-fat ablat ed, gold-thioglucose treated, high-fat fed) compared with that from no nobese controls. The size distribution of endogenous serum leptin as d etermined by radioimmunoassay (RIA) after sucrose gradient centrifugat ion is also consistent with saturation of binding in hyper-leptinemic obesity. In humans, free leptin increases with BMI. Thus, in lean rode nts and humans a large proportion of leptin circulates bound to severa l serum proteins. Free leptin is increased in serum of obese subjects, which may alter leptin bioactivity, transport, and/or clearance.