Leptin binding activity (LBA) in plasma of nondiabetic and diabetic adolescents and obese children: Relation to auxologic and hormonal data

Leptin circulates in serum bound to high molecular weight proteins. Hypothe sizing that leptin binding proteins may regulate the functional efficiency of leptin, we characterized auxologic and hormonal factors that influence l eptin binding in three disparate groups: normal adolescents, obese children , and teenagers with type 1 diabetes mellitus (IDDM), Specific leptin bindi ng activity (sLBA) was assessed by column chromatography after incubation o f serum with I-125-leptin in the presence and absence of excess unlabeled l eptin, Mean sLBA was 17.0 +/- 7% (SD) in the healthy adolescents (n=41), 6. 6 +/- 4.3% in the obese children (n=26), and 14.9 +/- 7.3% in the diabetic teenagers (n=17), At any value of sLBA, obese children had higher serum lep tin levels than non-obese adolescents or diabetic teenagers, consistent wit h "leptin resistance" in the obese group. sLBA was higher in males than in females only in those with diabetes (18.6 +/- 7.3 vs 10.9 +/- 5.1%, p<0.05) . sLBA correlated inversely with serum insulin-like growth factor-I values in the normal group (r= -0.45, p<0.01) and with insulin in the obese childr en (r= -0.53, p<0.01), There was no correlation between sLBA or serum lepti n values and HbA(lc) in the diabetic group, The serum leptin concentration was the principal determinant explaining the total variability of sLBA in a ll three cohorts. However, body mass index (BMI = weight/ height(2)) accoun ted for more of the total variability of percent specific binding in the he althy adolescents than in the other groups. We conclude that sLBA reflects circulating leptin levels, body composition, and hormonal milieu. Thus,in a ddition to leptin, qualitative and quantitative characteristics of leptin b inding may play a physiological role in the regulation of appetite and in t he "leptin resistance" of obesity.