Free and protein bound leptin are distinct and independently controlled factors in energy regulation.
OBJECTIVE
Leptin exerts important regulating effects on energy homeostasis and could have a central role in our understanding of obesity, diabetes mellitus and the metabolic syndrome. Leptin circulates in a free and protein bound form. The aim of the present study was to test whether both fractions of the leptin system can be selectively regulated and thus serve independent physiological roles.


METHODS
Using specific radioimmunoassays we measured both leptin components in relation to BMI in healthy subjects before and after weight reduction and in hyperthyroid patients during correction of thyrotoxicosis. In the latter group body composition and resting energy expenditure was monitored. In addition, we measured serum and cerebrospinal fluid concentrations of free and bound leptin in patients with neurological disorders.


RESULTS
Under all conditions free leptin concentrations reflected body fat mass. Bound leptin concentrations decreased during weight reduction but also after treatment of hyperthyroidism despite an increase in fat mass. Direct measurement of resting energy expenditure and bound leptin in hyperthyroid patients and under thyrostatic treatment showed a significant positive correlation of both variables. In contrast to free leptin whose transport into the cerebrospinal fluid appears to be saturated at low physiological concentrations of serum free leptin, bound leptin concentrations in the cerebrospinal fluid increased in parallel to serum concentrations over the whole physiologically relevant range.


CONCLUSIONS
Our data indicate a distinct role of free and bound leptin in the feedback regulating energy intake and expenditure and could have important implications for our understanding of the physiology and pathophysiology of leptin-dependent signalling.
Brabant
G
G
Department of Clinical Endocrinology, School of Medicine, Hannover, Germany.
Horn
R
R
von zur Mühlen
A
A
Mayr
B
B
Wurster
U
U
Heidenreich
F
F
Schnabel
D
D
Grüters-Kieslich
A
A
Zimmermann-Belsing
T
T
Feldt-Rasmussen
U
U
D000293:Adolescent; D000328:Adult; D000368:Aged; D001798:Blood Proteins / Q000378:metabolism*; D001823:Body Composition; D015992:Body Mass Index; D002648:Child; D004734:Energy Metabolism; D005260:Female; D006706:Homeostasis; D006801:Humans; D020738:Leptin / Q000134:cerebrospinal fluid / Q000378:metabolism*; D008875:Middle Aged; D011485:Protein Binding; D015430:Weight Gain; D015431:Weight Loss
D001798:Blood Proteins; D020738:Leptin