BACKGROUND: Klotho, a single-pass transmembrane protein primarily expressed in 
the kidneys, parathyroid glands, and choroid plexus of the brain, has a short 
cytoplasmic tail and a long extracellular domain, which can be cleaved and 
released as a soluble form. However, information regarding the origins and 
kinetics of soluble serum Klotho remains poorly understood. We evaluated serial 
changes in serum Klotho levels among living donors before and after 
retroperitoneoscopic nephrectomy as well as in their renal transplant 
recipients.
METHODS: The levels of soluble Klotho in serum obtained from 10 living donors 
and their renal transplant recipients were determined using a sandwich 
enzyme-linked immunosorbent assay system.
RESULTS: Serum soluble Klotho was detectable in all subjects. The baseline serum 
Klotho concentrations in the living donors ranged from 726.4 to 1417.1 pg/mL 
(median, 909.8 pg/mL; interquartile ranges [IR], 754.8-1132.4), whereas that in 
the concomitant renal transplant recipients ranged from 397.5 to 1047.2 pg/mL 
(median, 613.0 pg/mL; IR, 445.9-750.8; P = .003). The levels of soluble serum 
Klotho measured 5 days after retroperitoneoscopic nephrectomy (median, 619.0 
pg/mL; IR, 544.6-688.5; P = .001) were significantly lower than the baseline 
values. Among the renal transplant recipients, no significant changes in serum 
Klotho levels were observed during the observation period.
CONCLUSION: Our data regarding soluble serum Klotho levels obtained from living 
donors support the idea that the kidneys are a major source of soluble serum 
Klotho in human subjects without a deterioration of renal function. In 
recipients, concomitant acute kidney injuries and immunosuppressive protocols 
might modulate the release of soluble Klotho from the grafts into the 
circulation.