EVIDENCE OF FREE AND BOUND LEPTIN IN HUMAN CIRCULATION - STUDIES IN LEAN AND OBESE SUBJECTS AND DURING SHORT-TERM FASTING

Little is known about leptin's interaction with other circulating prot eins which could be important for its biological effects. Sephadex G-1 00 gel filtration elution profiles of I-125-leptin-serum complex demon strated I-125-leptin eluting in significant proportion associated with macromolecules. The I-125-leptin binding to circulating macromolecule s was specific, reversible, and displaceable with unlabeled leptin (ED (50): 0.73+/-0.09 nM, mean+/-SEM, n = 3). Several putative leptin bind ing proteins were detected by leptin-affinity chromatography of which either 80- or 100-kD proteins could be the soluble leptin receptor as similar to 10% of the bound I-125-leptin was immunoprecipitable with l eptin receptor antibodies. Significantly higher (P < 0.001) proportion s of total leptin circulate in the bound form in lean (46.5+/-6.6%) co mpared with obese (21.4+/-3.4%) subjects. In lean subjects with 21% or less body fat, 60-98% of the total leptin was in the bound form, Shor t-term fasting significantly decreased basal leptin levels in three le an (P < 0.0005) and three obese (P < 0.005) subjects while refeeding r estored it to basal levels. The effects of fasting on free leptin leve ls were more pronounced in lean subjects (basal vs. 24-h fasting: 19.6 +/-1.9 vs, 1.3+/-0.4 ng/ml) compared with those in obese subjects (28. 3+/-9.8 vs. 14.7+/-5.3). No significant (P > 0.05) decrease was observ ed in bound leptin in either group. These studies suggest that in obes e individuals the majority of leptin circulates in free form, presumab ly bioactive protein, and thus obese subjects are resistant to free le ptin. In lean subjects with relatively low adipose tissue, the majorit y of circulating leptin is in the bound form and thus may not be avail able to brain receptors for its inhibitory effects on food intake both under normal and food deprivation states.