Rat liver alpha-tocopherol binding protein.

1. The properties of rat liver cytoplasmic alpha-tocopherol binding protein have been studied. 2. The binding protein sedimented in the 3 S region of sucrose density gradients, and gel filtration indicated an approximate molecular weight of 30 500. 3. Of the tissues examined by the present assay, binding was detectable only in the liver. 4. Optimal binding was achieved by incubation at 26 degrees C for 4 h and was independent of pH between 7.4 and 9.0. 5. Pronase completely abolished binding. The binding protein was, however, almost completely resistant to trypsin, and unaffected by RNAase, DNAase, triacylglycerol lipase, and phospholipase C. 6. A variety of tocopherol analogues and other lipid-soluble compounds were tested for their ability to compete for binding. Only alpha-tocopherol and to a lesser extent alpha-tocotrienol and gamma-tocopherol exhibited competition. alpha-Tocopherol acetate, alpha-tocopherol quinone and 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid had no effect on binding. 7. Tocopherol binding was reversible, and the tocopherol was not metabolized during incubation.