Effect of protein binding on testosterone extraction by human cirrhotic liver: evidence for a dissociation-limited uptake.

Testosterone (T) is a protein-bound substance, the hepatic extraction of which largely exceeds the free plasma fraction. In this study we attempted to determine if the dissociation of T from plasma proteins is the limiting factor for testosterone hepatic uptake in patients with cirrhosis. For this purpose we measured the hepatic uptake of T and the peripheral plasma concentrations of the different fractions of the hormone (total, free, albumin-bound, and sex hormone-binding globulin (SHBG)-bound) in 12 men with alcoholic cirrhosis. The hepatic extraction of T (mean = 42%) greatly exceeded the non-SHBG-bound fraction of T (free T plus albumin-bound T: mean = 13%). Thus, a substantial amount of SHBG-bound T must have entered the liver. A theoretical extraction ratio was calculated based upon the dissociation rate constants of T from albumin and SHBG and upon an estimate of sinusoidal transit time of plasma through the liver. The similarity between the measured and expected values indicates that the limiting step in hepatic uptake of T might be SHBG binding.