Sephadex G-100 chromatography of rat liver supernatant after addition of [125I]T3 revealed four peaks of protein-bound radioactivity in the void volume, albumin, ligandin, and Z-containing regions, respectively. The peaks were identified by cochromatography of BSP and [125I]T3 and immonodiffusion with antiratligandin IgG and antirat Z IgG. Binding of [125I]T4 to rat liver supernatant occurred in void volume, albumin, and Z regions only. Studies in vivo reveal a pattern of [125I]T3 binding to rat liver supernatant fractions quantitatively different from that observed in vitro. [125I]T4 binding to liver supernatant fractions in vivo occurred in all four peaks. BSP or bilirubin added to liver supernatant decreased T3 and T4 binding by each fraction. Flavaspidic acid inhibited binding of T3 and T4 to albumin, ligandin, and Z protein. Phenobarbital pretreatment of rats increased binding of T3 by ligandin and of T4 by albumin-containing fractions. Circular dichroism studies with purified rat liver ligandin suggest that T3 and T4 bind competitively to the same site as does bilirubin; the association constants of T3 and T4 for ligandin are 10(6) and 10(5) M-1, respectively. T4 was bound only by purified ligandin and not by ligandin in liver supernatant. To determine whether unconjugated bilirubin interferes with hepatic uptake of T3, [125I]T3 was administered to icteric homozygous and phenotypically normal heterozygous Gunn rats. Hepatic uptake and supernatant binding [125I]T3 were significantly reduced in homozygous Gunn rats. Hepatic uptake of [125I]T3 was also reduced in vivo by infusion of BSP with or without flavaspidic acid. BSP infusion abolished [125I]T3 binding to ligandin; BSP and flavaspidic acid abolished binding to ligandin and Z. These observations suggest that ligandin and Z protein are thyroid hormone binding proteins in rat liver cytosol and may influence the net flux of iodothyronies from plasma into the liver.

Rabbits, rats and guinea-pigs were treated with di(p-aminophenyl) sulphoxide and their urines exdmined by an analytical method which permits the simultaneous determination of this compound and of dapsone [di(p-aminophenyl) sulphone] which is a possible product of metabolic oxidation. The method gives for each drug the total of free compound plus acid-labile conjugates. All three species excreted unchanged drug together with dapsone. With rats and guinea-pigs about 33% of the excretion is dapsone, but with rabbits only 6 to 12%. The rate of combined excretion is much greater in rabbits than in the other two species. These results are discussed in relation to the significance of di(p-aminophenyl) sulphoxide as a drug in the treatment of leprosy.Di(p-aminophenyl) sulphoxide has received favourable preliminary reports in clinical trials against leprosy (Buu-Hoi, Khuyen, and Xuong, 1955;Davey, Kissaun, and Moneta, 1957;Laviron, Lauret, Kerbastard, and Jardin, 1957), and there has been a suggestion that its action might be in some ways different from that of dapsone, di(p-aminophenyl) sulphone. Various workers have shown the sulphoxide to be almost without action in vitro against micro-organisms including pneumococci, haemolytic streptococci, and Mycobacterium tuberculosis H37Rv, in contrast to the considerable activity of dapsone in these species. However, the sulphoxide has definite activity in vivo against these same organisms in the mouse and the guinea-pig. These findings have led to the suggestion that the sulphoxide is activated by conversion to the sulphone in the animal body (Jensen and Schmith, 1943;Youmans, Feldman, and Doub, 1946;Wagner and Kimmig, 1946).In order to evaluate the specific usefulness of the sulphoxide in leprosy it is desirable to know the extent of its conversion to dapsone in the human body. The metabolic fate of sulphoxides varies with different compounds and in different species. A number of instances are known where thioethers are oxidized to sulphoxides and are excreted in this form rather than as sulphones, for example, in the metabolism of phenothiazine (Whitten, Filmer, and Clare, 1947), chlorpromazine (Salzman andBrodie, 1956), and a pyrazolidinedione containing an alkylthio group (Burns, Yu, Ritterband, Perel, Gutman, and Brodie, 1957); the oxidation of di(p-aminophenyl) sulphide by a guinea-pig liver microsome preparation is said to go to the sulphoxide but no further (Gillette, Kamm, and Brodie, 1959). On the other hand, sulphides can be oxidized in vivo to the sulphone, for example, p-methylthioaniline (Rose and Spinks, 1948), and the catabolism of ethanethiol probably follows this route after methylation (Snow, 1957;Lowe, 1960 Francis and Spinks (1950) to be applied to both components.Procedure.-Urine samples (5 ml.) are acidified with N HC1 (2.5 ml.); they are left for 1 hr. at 20 to 250 to allow for hydrolysis of acid-labile conjugates. After neutralization with N NaOH (2.5 ml.) the samples are diluted to contain 3 to 40 fig. /ml. of either sulphoxide or sulphone. For es...