The transport of hemin and protoporphyrin across the plasma membrane of chick embryo liver cells in culture.

Primary monolayer cultures of chick embryo hepatocytes can be cultured in a chemically defined medium (Ham F-12) containing insulin. The absence of serum from the medium permitted a study of the effects of added serum proteins on the transport of hemin and protoporphyrin across the plasma membrane of the hepatocyte. As the criterion of hemin uptake we used its unique and selective activity in repressing the induced synthesis of delta-aminolevulinate synthetase by various chemicals. Movement of hemin into the cells is rapid and does not require added serum proteins. Hemin represses the induced synthesis. The repression by hemin is decreased 50% when the molar ratio of hemin to human serum albumin (6.5 muM) is 1 :2, i.e., where the calculated concentration of dissociated hemin is 10(-8) M. Apparently serum albumin does not enter the cells; it decreases entry of hemin into the cells by virtue of its high affinity for hemin. Compared to human serum albumin, bovine serum albumin and chicken serum albumin, under the same conditions, have a much lower affinity for hemin and scarcely influence the repression effect by hemin. Protoporphyrin can be specifically caused to accumulate in the cytosol, and uroporphyrin in the nucleus of the hepatocytes by the use of different inducers of delta-aminolevulinate synthetase. Protoporphyrin, but not uroporphyrin, is released rapidly from the cells when the moles of human serum albumin added to the medium are 5 times that of porphyrin. This culture system may provide a useful model for studying the mechanism of transport of organic anions across the hepatocyte plasma membrane.