Characteristics of b-type cytochromes in brain microsomes: comparison with liver microsomes

Biochemical aspects of b-type cytochromes in swine cerebral microsomes were different from those of cytochrome b5 in liver microsomes, as well as the difference in absorption spectra. First, the kinetic constants, Km and Vmax, in rotenone-insensitive NADH-cytochrome c reductase activity were different from those of liver microsomes, and the activity of cerebral microsomes was higher than that of liver microsomes. Second, midpoint potentials (Em) of b-type cytochromes in cerebral microsomes were measured and compared with liver microsomal cytochrome b5. In cerebral microsomes two components of b-type cytochromes were resolved, and showed Em's of -30 and +50 mV, respectively, in the presence of 2 mM KCN. On the other hand, the Em of liver microsomal cytochrome b5 was -6 mV. The high-potential component of cerebral microsomal b-type cytochromes was identified as brain-b'5 [S. Yoshida, T. Yubisui, and M. Takeshita (1983) Biochem. Int. 7, 291-298] and the low-potential component as brain-b5. The significance of the difference between cerebral and liver microsomal b-type cytochromes was discussed.