Differences in primary cellular factors influencing the metabolism and distribution of 3,5,3'-L-triiodothyronine and L-thyroxine.

Administration of phenobarbital, which acts exclusively on cellular sites, results in an augmentation of the liver/plasma concentration ratio of L-thyroxine (T4) in rats but no change in the liver/plasma concentration ratio of L-triiodothyronine (T3). Whereas phenobarbital stimulates the fecal clearance rate both of T3 and T4, it increases the deiodinative clearance rate of T4 only. These findings suggest basic differences in the cellular metabolism of T3 and T4. Further evidence pointing to cellular differences was obtained from a comparison of the distribution and metabolism of these hormones with appropriate corrections for the effect of differential plasma binding. The percentage of total exchangeable cellular T4 within the liver (28.5) is significantly greater than the corresponding percentage of exchangeable cellular T3 within this organ (12.3). Extrahepatic tissues bind T3 twice as firmly as T4. The cellular metabolic clearance rate (= free hormone clearance rate) of T3 exceeds that of T4 by a factor 1.8 in the rat. The corresponding ratio in man, 2.4, was determined by noncompartmental analysis of turnover studies in four individuals after the simultaneous injection of T4-(125)I and T3-(131)I. The greater cellular metabolic clearance rate of T3 both in rat and man may be related to the higher specific hormonal potency of this iodothyronine.

Inability to demonstrate hydroxylation of tyrosine by murine melanoma "tyrosinase" (L-DOPA oxidase), using the tritiated water assay technique.

Validity of the tritiated water assay technique for tyrosine hydroxylase activity as a qualitative method was demonstrated with mushroom tyrosinase. Using this method, isolated murine melanoma "tyrosinase" (L-dopa oxidase) showed no tyrosine hydroxylase activity. This finding supports previous studies in our laboratory which used a variety of histochemical and biochemical methods. The nonenzymatic production of tritiated water caused by tritium exchange with hydrogen peroxide complicates the use of the tritiated water assay technique with crude systems, since hydrogen peroxide is generated by a variety of oxidase reactions. For this reason, previous studies using the tritiated water assay technique with crude systems are ambiguous.