1. Human skin epithelium and human skin were found to absorb both free bilirubin and serum-bound bilirubin from an aqueous buffered medium. The serum-bound bilirubin thus absorbed was readily released when human skin epithelium or human skin were transferred to media containing no bilirubin. 2. The K m values for serum-bound bilirubin were 1.8×10 −3 m and 2.2×10 −3 m respectively for human skin epithelium and human skin; corresponding K m values for free bilirubin were 3.0×10 −4 m and 5×10 −4 m . The V max. for bound and free bilirubin was of the same magnitude, the apparent V max. being 1.0 and 1.66μmol/g of tissue for human skin epithelium and human skin respectively. 3. When human skin that had acquired a yellow tinge by absorbing bilirubin was incubated in a buffered medium and exposed to a mercury-vapour light, the yellow colour disappeared and decomposition products of bilirubin accumulated in the medium. 4. Experiments with [ 3 H]bilirubin indicated that the pigment absorbed by skin was photo-oxidized to products that were soluble in water and the quantity and number of such products increased with the time of exposure of human skin to the light-source. Under similar conditions [ 3 H]bilirubin alone in buffered medium was also oxidized and gave products which by paper chromatography appeared to be different from those released by human skin that had absorbed bilirubin. 5. The results suggest that by virtue of its large surface area human skin can act as a matrix for the degradative action of light on bilirubin.
1. Skin epithelium of albino rat, mouse and guinea pig was shown to accumulate bilirubin from a medium containing free or bound bilirubin. 2. The K m values for bound bilirubin were 2.22×10 -3 , 1.33×10 -3 and 9.5×10 -4 m for rat, mouse and guinea pig respectively and the corresponding K m values for free bilirubin were 5.2×10 -4 , 4.0×10 -4 , 1.8×10 -4 m ; the V max. values of bound and free bilirubin were unchanged. 3. The uptake showed saturation kinetics. Bound bilirubin was released together with serum proteins. Free bilirubin bound to skin was not released into the medium. 4. Freezing and thawing of skin epithelium did not cause any significant lowering of the uptake of bilirubin but heat-denatured skin epidermis took up only 50% of the bound bilirubin or free bilirubin taken up by control unheated skin epithelium. 5. The uptake of free and bound bilirubin was prevented by HgCl 2 but not by sodium arsenate, NaCN, NaF, cycloheximide, 2,4-dinitrophenol or iodoacetate. 6. Most of the free bilirubin was bound to the lipids or lipoprotein fraction of skin epithelium and could be extracted by solvents. 7. Rat skin showed the highest accumulation and efflux of bilirubin.
Induction of rat liver tyrosine aminotransferase by l -tyrosine and tryptophan oxygenase by l -tryptophan was studied in groups of rats fed on diets containing 18 or 5% protein. The basal activity of hepatic tyrosine aminotransferase of rats receiving 5% protein gradually increased with the age of the animals but that of rats receiving 18% protein did not. l -Tyrosine induced hepatic tyrosine aminotransferase in rats receiving 18% protein when tested at ages from 4 to 20 weeks. When induction by l -tyrosine was carried out in rats receiving the 5% protein diet, significant induction of tyrosine aminotransferase occurred only in 4- or 6-week-old rats. Induction by l -tryptophan of tryptophan oxygenase in liver or the basal activity of this enzyme in liver did not differ between the groups fed on 5 and 18% protein. On changing the diet from 0 to 18% protein, the above-mentioned effects on the induction of hepatic tyrosine aminotransferase were reversed.