Metabolism of L-thyroxine (T4) and L-triiodothyronine (T3) by human fibroblasts in tissue culture: evidence for cellular binding proteins and conversion of T4 to T3.

Abstract

The metabolism of L-thyroxine (T4) and L-triiodothyronine (T3) by human fibroblasts in tissue culture was studied. Fibroblasts were incubated in serumless medium and in media containing various concentrations of normal human serum (HS) or fetal calf serum (FCS) enriched with both T4-125I and T3-131I, and the simultaneous cell uptake of both tracers and the T4 to T3 conversion determined. Interference from the preferential cellular uptake of T3 impurities in T4 was corrected by determination of the 125I/131I ratio migrating as T3 on bidimensional paper chromatography of medium and cells. In the absence of serum in the medium, cell to medium (C/M) concentration gradient was 90 to 100 for T3 and 49 to 58 for T4. With increasing amounts of HS, the fractional uptake of T3 and T4 diminished and the C/M gradient for T4 was less than 1. The cellular T3/T4 ratio increased to a maximum of 11.4 in the presence of 5% HS. The T3/T4 ratio in the dialysable fraction was 1.01 for serumless medium and 5.8 for medium conta...